CLU-IN.ORG | Remediation > Technology Descriptions > : Magnetics for Environmental Applications

Magnetics, as related to the environmental field, is a technology used for locating subsurface iron, nickel, cobalt and their alloys which are typically referred to as ferrous materials. The theory of magnetics has been adapted to specialized tools called magnetometers which are capable of measuring ambient magnetic fields emanating from terrestrial forces, natural ferrous minerals or ferrous alloys found in cultural objects. These fields or forces are imperceptible to human senses and are very similar to lines of force or flux which continuously loop around a magnet from one pole to another. The technology has been widely used for quickly locating buried or subsurface cultural ferrous objects that could pose a potential threat to the environment or by assisting remediation efforts. Locating ferrous materials is dependent on the strength of the object's associated magnetic force. The intensity of magnetic forces can be related, in general terms, to the amount ferrous mass present. In other words, the stronger the force, the greater amount of ferrous mass. Magnetometers will only detect ferrous metals. Other nonferrous metals cannot be detected.

Magnetometers should not be confused with metal detectors. Metal detectors will detect nonferrous metals (aluminum, brass, copper, stainless steel, titanium) as well as ferrous metals by applying an entirely different physical method of detection.

Since information on this site will only address ferrous detecting magnetometers capable of measuring ambient magnetic forces, other types of tools known as magnetic susceptibility instruments will not be presented. Magnetic susceptibility instruments are not considered passive ambient magnetic force measuring tools since they supply an electromagnetic signal to enhance fields around ferrous materials which are then measured within a limited area. Such susceptibility instruments are primarily used to evaluate soils and minerals for the mining industry and usually not applied for locating buried environmental ferrous objects.

There are several advantages to using magnetics in the field including fast data acquisition, ease of use and portability. A person with a general background in magnetics and field data acquisition techniques can easily learn the operating basics of a magnetometer in a day or less. However, proficiency in its use is obtained by mastering the selection of optimal intervals for data collection specific to the type of object(s) being investigated. Good data collection techniques are keyed to specifications related to the type of target of interest (size, shape, depth, mass, ferrous content, condition), thus optimizing the method. Most magnetometers are designed for ease of operation by the operator, although, a background in basic physics, environmental waste issues, mapping techniques, and interpolating X, Y (position coordinates) and Z (magnetic data) plots are essential to the operator.

Magnetics is a widely accepted technology for the location of ferrous masses that are either cultural or natural. Some examples of applications include: locating buried ferrous drums, tanks, pipes, ordnance, abandoned well casing, boundaries of landfills (if landfill contains ferrous metal), and mineralized iron ores. In addition to locating ferrous metal, magnetometers also provide some information as to the amount of ferrous mass present. Some potential problems that could be remedied using magnetics are listed as follows:

EPA has no standard methodology for use of magnetics at this time. Currently no ASTM standard exists for magnetics.

Magnetic objects, including the Earth, are analogous to a bar magnet or dipole having positive and negative ends with opposing forces that attract and repel within its area of influence. Magnetic lines of force, or flux, are strongest at the ends of a magnet or dipole. The Earth, for example, has its strongest flux at the poles and a weaker magnetic force as it nears the equator. Thus the Earth's background magnetic field is not the same throughout the globe and changes with latitude. The same principle holds true for a bar magnet, or any cultural ferrous object resembling a dipole configuration such as a pipe or drum. Magnetic forces of cultural objects vary dependent on orientation, shape, condition and other factors.

Magnetic materials, iron and steel for example, contain tiny subatomic regions of magnetism called domains. They are magnetic because the atoms inside of them behave like miniature magnets. Electrons within an atom spin around an internal axis as well as circling the nucleus producing transient electrical charges in their domains. When these domains align in a way unique to ferrous metals, the result is a magnetic field.

A ferrous drum, for example, can be approximated by a magnetic dipole and will have its own variations of magnetic lines of force. The magnetic forces from a drum will also have an influence on the Earth's background forces which causes a change in the Earth's ambient local magnetic field near the drum, this change is commonly known as an anomaly. A magnetic anomaly is caused by the superposition of a local anomaly on the geomagnetic (Earth's) field. Magnetic field anomalies can be measured with magnetometers. The amount of measurable change in an anomaly force will vary due to the amount and condition of magnetic mass present and its distance from the measuring point of the magnetometer.

Technological advances have provided several improved versions of magnetometers over the past several decades. It is possible to see one of two methodologies applied to magnetometers that are used in the field at environmental sites. Any of these magnetometer systems will work within certain limits, if they are applied correctly and the limitations of each instrument are understood.

The two magnetometer methods presented measure magnetic flux density, which is a vector unit, meaning that it has a directional component as well as a component of magnitude. Of the two magnetometer methods that will be discussed, each measure the magnitude component, which is a scalar measurement These methods specifically measure the magnitude of the Earth's field vector independent of its direction. Each individual sensor tends to measure in an omni directional range so there is no one directional component, when a single sensor is used. Directional components can be measured if two sensors are positioned in certain geometric configurations, but this topic will be discussed later.

The basic differences in the two types of magnetometers are their measurement efficiencies which can be broken down into two categories, instrument accuracy and data acquisition rates.

Instrument accuracy is usually measured in nanoTeslas (nT) or gammas (g) which are two commonly used magnetic units. NanoTeslas is the official International System (SI) unit, however some geophysicists tend to use the gamma as a unit (1 nT = 1 gamma). Magnetometers capable of measuring the smallest changes in nT or g units are indicative of more sensitive instruments that can detect smaller or deeply buried masses.

Data acquisition cycle rates are typically measured in seconds. Faster acquisition cycle times increase the data collection rate and thus reduces time in the field. Refer to the Table below for comparisons:

You can see from the above Table that the Proton Precession method (especially the conventional method) will not allow data to be collected at a consistent fast walking pace since it takes several seconds to obtain a measurement. However, optically pumped and proton Overhauser methods can be used to collect data at a walking pace with accuracy and speed variance, dependent on the method used.

Magnetometers do not use transmitted or propagating radio wave frequencies emanating externally from the detector to locate anomalies, such as those found in electromagnetic, resistivity or ground penetrating radar geophysical methods. Measurements are made through the detection of ambient magnetic forces near the magnetometer's sensors. Measurement techniques that detect ambient magnetic forces are beneficial since other methods using propagating radio wave techniques are limited by many phenomenon which can interfere, slow or impede their signals.

Although each of the two magnetic methods mentioned measure magnetic field forces, the principles used to obtain a measurement for each method are different. An explanation of each method follows:

During the 1950's a more accurate method of measuring magnetic fields was discovered to supersede a less precise method (fluxgate) that was used to locate submarines during World War II. This more accurate method involves measuring the reaction of subatomic particles in a sample volume to external magnetic forces. Although this sounds complicated, the method is simple to explain. A fluid, containing any hydrogen rich compound (water for example), could be used as a detector for sensing magnetic fields by manipulating and monitoring the reaction of protons within the fluid. To initiate the process for making measurements, electrical coils are placed around a container of hydrogen fluid and energized for a very short time interval. An electrical Direct Current (DC) causes the random natural spin of the protons to align themselves to the induced current. When the current is removed from the coil, the protons will want to precede (precession) back to their natural random state of spin. However, the rate at which this proton precession occurs is dependent on the ambient magnetic field near the container or sensor. Strong magnetic fields will force the protons to precess at a faster rate back to normal than in a weaker magnetic field. The rate at which the protons precess back to normal is proportional to the magnetic field strength and thus provides a measurable value. A benefit of this technique is greater accuracy over earlier magnetometers, but it does require several seconds to cycle through the entire process before obtaining a measurement. The most common fluid used in proton magnetometers is hexane or decane since, unlike water, these fluids will not freeze as easily in colder climates.

Proton precession data are usually collected in one of two ways over an area. One method is to obtain data tied to a sequential numbering system which increases each time a reading is recorded. This method works best if each increasing numeric value can be tied to some type of coordinate location. A more common method is to establish a grid system over the area to be surveyed and preprogram the magnetometer's internal data acquisition program to match the grid system. This method not only saves time for the operator by automatically advancing to the next grid point, it reduces the chances for errors in the field. Data values and grid information typically are visible to the operator on the console of the control unit, where the data are also stored.

Raw data consists of time stamped values, sequential numbering or X - Y positioning, sensor stability information and sensor measurement data. The X - Y positioning data can be pre-programed to match a specific data collection grid pattern. When this mode is engaged, positioning data will automatically advance to the next reading when data is collected. Maintenance typically consists of replenishing fluid when low and ensuring proper battery condition.

The units of measurement are commonly expressed either as nanoTeslas (nT), which is the International System Unit (SI), or gammas (g). Both units equate to each other.

An Overhauser proton precession magnetometer provides a slight technological improvement over the conventional proton precession method. This type of magnetometer is basically the same as the conventional proton precession magnetometer with the exception of differences in processing electronics, sensor fluid and type of current applied around the fluid. Rather than just having a proton rich fluid, the fluid has been "spiked" with free radicals to enhance the reactiveness of the protons in the fluid to an electrical stimulus. The other difference is non-application of a high power Direct Current (DC) around the sensor (as in the conventional systems), instead, a low power radio frequency magnetic field is applied for a very short time interval around the fluid. This type of system maximizes resolution and is more efficient since polarization and measurement of the protons occurs almost simultaneously.

A cautionary note is worth mentioning for this type of system since the sensors are sensitive to extreme heat (above 149 degrees F). It is recommended that if one is working in direct sun light when the temperature is above 100 degrees F, a light colored wet cloth be wrapped around each sensor to keep the sensor(s) cool. Damage can occur to the sensor(s) if they are subjected to heat above 149 degrees F. This type of tool should never be left in an unventilated vehicle on a hot day. Maintenance typically consists of ensuring proper battery condition.

Data collection times are slightly faster than conventional proton precession methods and may allow the operator to collect data at a slow walking pace. Raw data, data storage, data collection techniques and maintenance issues are very similar to that of the conventional proton precession method listed previously.

The units of measurement are also commonly expressed either as nanoTeslas (nT), which is the International System Unit (SI), or gammas (g). Both units equate to each other.

A faster and even more accurate method of obtaining magnetic measurements was discovered in the 1960's. This method uses an ionizing light beam to manipulate one of several elements from a specific chemical Group within a sample volume for the purpose of observing their reaction to external magnetic forces. By manipulating and monitoring the nuclei of any one of the Periodic Element Table Group 1 or alkali metals (Li, Na, K, Rb, Cs, Fr), measurements can be made of magnetic forces.

Alkali metals are very reactive to certain external forces and will easily lose an electron such as when ionizing light energy is applied. The term used for applying constant ionizing light energy for the purpose of ejecting an electron from its outer orbit, related to magnetics, is referred to as being optically pumped. However, magnetic forces have a stabilizing effect on alkali metals that have lost an electron and tend to force any losing electron back to its stable neutral state, thus counteracting the ionizing light energy or optically pumped energy. This battle between electrons gaining and losing energy can be monitored and measured within a confined sample volume. Stronger magnetic fields will tend to stabilize electrons at a faster rate than a weaker field. Energy gained by the electron when forced from its outer orbit (by "pumping in" ionizing light, for example) is lost when it is forced back to its neutral state by a repelling energy, such as a magnetic force. By monitoring the gain and loss of energy in a volume of alkali gas one can relate, proportionately, magnetic field strengths.

A tool which allows this to happen is the optically pumped magnetometer. An alkali vapor, such as cesium or potassium is sealed within a temperature controlled vacuum chamber where ionizing light is emitted or "pumped" into the chamber through various optical filters. The ionizing light energizes the molecules in the sample volume and ejects electrons from the outermost orbit of individual electrons. Ambient magnetic fields near the vacuum chamber will tend to force the electrons back to their stable state. During this process the loss of energy due to the electrons dropping down to their stable state must be released and is given off as a spark of light. A photomultiplier tube (a device that measures light intensity) at the other end of the vacuum chamber measures the amount of light given off. Greater light intensity means that a strong magnetic field is quickly forcing electrons back to a normal state within the sample volume. Weaker magnetic fields will not cause the electrons to return to normal as rapidly, thus producing less light in the sample volume. The rate at which electrons revert back to normal is proportional to the magnetic field strength and thus provides a measurable value.

Benefits of this technology are faster measuring cycles which can be obtained as often as 0.1 second and greater accuracy in measuring magnetic field strength. One disadvantage of this tool is fragility of the sensor due to the type of instrument components used, since it must be handled with care in the field. Optically pumped magnetometers have an inherent "dead zone" field of view in the sensor due to the required configuration of internal components. Properly positioning or orienting the sensors for the specific location or latitude (a relationship which determines angles of magnetic fields at a latitude) will reduce the "dead zone" effect and allow for an efficient measurement. Establishing proper sensor angles is easily obtained from published charts, tables or computer programs (typically supplied by the magnetometer vendor).

Data from optically pumped systems are usually collected in one of three ways. One method is to obtain data in a search mode where no positional data are recorded, only data values are shown on the instrument's control panel as the sensor is moved through an area. Another method is collecting data using a sequential numbering system which automatically advances each time the operator wants a reading to be recorded. This method works best if each increasing numeric value can be tied to some type of location. A more common method is to establish a grid system having lines and positions over the area to be surveyed. The lines are preprogrammed into the magnetometer to match the grid coordinate system and positions are obtained by starting and stopping constant data recording at the ends of each line. An internal program will automatically post a grid coordinate to each data position point. This data collection method requires and assumes that a constant walking pace is maintained between the start and finish of each line.

Raw data consists of time stamped values, sensor stability information, pre-programed grid line intervals (X axis) with start and end markers to indicate all (Y axis) data collected in each line and averaged data posted at an operator selected time interval. Newer systems also have inputs for global positioning systems (GPS). Maintenance typically consists ensuring proper battery condition.

The units of measurement are commonly expressed either as nanoTeslas (nT), which is the International System Unit (SI), or gammas (g). Both units equate to each other.

Optically pumped magnetometers are used most often for environmental field analysis since the technology is optimized for speed, sensitivity and compatibility with GPS tools.

A listing of advantages and disadvantages between the two methods are listed below. Note that both methods are compact and can be easily transported in a small case.

This type of magnetometer will not be fully addressed by this guide since it is rarely used in today's environmental investigations, however a short mention will be made since it has been used in some past environmental investigations. In previous decades, fluxgate magnetometers have been used at some environmental sites but their use has since waned and proton/optical magnetometers have taken their place. Fluxgate magnetometers was one of the first modern magnetometer methods to be developed for the purpose of locating submarines during World War II. It senses both the directional and magnitude components of the magnetic field and is therefore very sensitive to orientation errors. Fluxgate magnetometers usually measure along one axis, but can have up to three sensor axis for measurement. However, even these multiple axis sensor tools are still sensitive to orientation because it is almost impossible to make the three sensors completely orthogonally receptive.

The range of fluxgate tool options varies widely from inexpensive less precise ferromagnetic locators providing plus or minus unitless non-recordable onboard values, to more sophisticated digital fluxgate gradiometers providing internal storable values with accuracy's ranging from 1.0nT to as low as 0.1nT in some cases.

A fluxgate magnetometer operates by using two parallel cores of a magnetic material (which establishes the axis direction), each wrapped within several sets of wire, placed several inches apart and energized with a current. The current is sufficient to magnetize the each core, with one core oriented so that it is in opposite polarity of the other, thus essentially nulling any electronic response near the two cores. When an external magnetic field is introduced near the cores, the null is disrupted and an electronic response can be measured. The amount of variance from the null state is proportional to the strength of the external field near the cores. One deficiency in this type of design is a lack of sensitivity in the instrument.

Fluxgate magnetometers are usually less expensive than other magnetic methods and are commonly used for archaeological surveys to locate very near surface anomalies.

Most magnetometers will either have one or two sensors on a collapsible aluminum staff, a power supply (with external charger), and a control unit with processor. A typical system will include hardware cabling for transferring data to a computer and simplified software for processing data. A majority of magnetometers will have built-in data acquisition systems which are part of the control unit and processor. Systems are self contained and can be carried and operated by one person using a back or waist pack.

Optional support equipment for some magnetometers include an external serial connection port for a Global Positioning System (GPS). Other optional equipment could include parallel swathing guidance systems linked to a GPS which eliminates the need for pre-surveying a grid, provided the GPS antenna has an unobstructed view of the sky. Although most magnetometer systems include software for processing data, typically more sophisticated programming software is used to interpret data for final interpretations and hardcopy reports.

All systems operate using onboard battery systems with battery life ranging from 4 to 12 hours depending on the type of magnetometer and how it is configured (one or two sensors). Most units have rechargeable battery packs.

Some geophysical service companies have configured multiple off-the-shelf magnetometers with specialized data collection systems and towing equipment to acquire data for specific purposes. For example, connecting several magnetometers together in a perpendicular line to the direction traversed by a towing vehicle has been developed so that data can be collected over a large swath in an ordnance range to detect unexploded bombs. The duplication of magnetometers would allow the system to span more ground, thus limiting the number of swaths needed and saving time in the field. Other variations for specific purposes may also be available by geophysical service companies to fit a clients' needs.

It should also be mentioned that marine magnetometers are similar to land-based systems but designed in a special housing so that it can be towed underwater behind a boat for environmental applications in navigable waterways. Airborne systems are also available, but are only practical for use over very expansive areas (as measured in miles).

Optional support equipment, such as GPS systems, typically are not supplied with standard magnetometer systems. Support equipment usually can be rented or purchased from the manufacturer. Most manufacturers will rent all necessary equipment as a package. Generally, the more accurate and sensitive detector systems increase the purchase price of the equipment. If systems will not be used routinely, it would probably be more economical to access equipment through rental rather than purchase. Vender services that provide equipment and operators usually have fixed prices, per day or week for conducting magnetometer surveys.

The more sensitive magnetometers, such as proton precession and optically pumped systems, are susceptible to atmospheric changes in the Earth's magnetic field which can interfere with magnetic forces emanating from the ferrous objects one is trying to locate. It is important to be aware of this phenomenon and take proper precautions to neutralize its affect.

Atmospheric interference may occur due to the Interaction of the Earth's molten core with reactions occurring on the sun that influence and alter Earth's magnetic field which is always in flux. The degree and duration of these changes are undetectable by the human senses. However, space vehicles stationed between the Earth and sun are able to monitor and thus provide forecasts of solar events which could effect Earth's magnetic field. At those times when solar activity is high, changes in the Earth's magnetic field are also detectable and measurable by magnetometers, making it difficult to accurately measure local small anomalous ferrous features.

Some of these solar events can be significant as witnessed by the population in and around Montreal Canada February, 1995 when a blackout occurred due to a solar flare which altered the upper latitudes of Earth's magnetic field. Changes in the Earth's magnetic field in this region were strong enough to strain the natural flow of electricity through local power utility transformers. Increasing the ambient magnetic field around a transformer will cause it to overheat, beyond its engineered limits, eventually causing it to shut down or explode. Duration of these solar events can range from fractions of a second to several days. The episode which occurred near Montreal lasted several days.

Most magnetometer sensors can be assembled in a special configuration or mode to effectively counteract solar events, in most cases.

All magnetometer systems are designed for field use and manufacturers typically provide step-by-step instructions for equipment operation. They are designed to allow a novice to operate them adequately. However, knowing how to apply the instrument to a specific problem and interpret the results will require some training and expertise. Training can be obtained through the manufacturer, or through formal courses offered by public and private organizations. The basic steps in the application and use of magnetometers are described below.

Magnetometers can be configured and operated in several ways to meet the needs dictated by site conditions and in some cases, eliminate most unwanted atmospheric or solar disturbances.

There are three modes of operation, each categorized by how sensors are configured and data collected. Each mode has advantages and disadvantages, selection of the most applicable mode is determined by the type of problem one is trying to solve.

Usually in all modes and configurations, sensors are placed at the end of horizontal or vertical poles (depending on individual vender design or operator needs) away from the operator to reduce effects from small amounts of ferrous metal that typically cannot be removed (glasses, steel toe boots, etc.). Sensors can also be mounted at various heights from the ground to eliminate interference from minor surface debris or mounted close to the ground to obtain measurements from a very minor or small mass.

The operator carries a system that only uses one sensor. Data consists of one measurement in gammas or nanoTeslas. This mode is susceptible to atmospheric disturbances which could mask small anomalies during times of intense solar activity.

To counteract and minimize the effect of solar or atmospheric activity on a magnetometer system, a simple process can be applied. Two sensors are used to obtain a measurement at precisely the same time. These sensors can either be carried together separated by an established vertical distance or by placing one sensor at a fixed point while the other is used as a roving or mobile unit.

When two measurements are taken simultaneously over the same area, the readings are subtracted from each other to obtain a true value independent of any background solar activity. These are commonly known as gradient measurements. So no matter how the background magnetic field is responding, two instant measurements in time separated by a uniform vertical or horizontal distance and subtracted from each will essentially eliminate most atmospheric background interference. The type of measurement unit for this corrected value is gammas or nanoTeslas per the distance unit separating the two sensors; i.e. gammas per meter or nanoTeslas per meter. This procedure will not eliminate unwanted background interference from nearby non-terrestrial ferrous masses such as structures, automobiles, etc. adjacent to an area of investigation.

One or more operators each carry a single total field mobile system that has one sensor while one exclusive static, remote base station is programed to automatically collect data at very short intervals (ranging from several seconds to several minutes). After all data are collected by the operator(s), each of their systems are connected to the base station for an automatic data merge. Built-in programs will use a statistical method to segregate the base station data and operator system data into segments having data collected at or near the same moment in time. After segments are segregated into very similar moments in time, the base station data are individually subtracted from the operators' mobile data. This resulting positive or negative value will be the best statistical determination for correcting unwanted atmospheric changes. The data will be expressed in corrected gammas or corrected nanoTeslas.

The operator carries two mobile sensors separated by a vertical distance of usually one half or one meter perpendicular to the ground (for upper northern and southern latitudes). Typically the bottom sensor is referenced as the total field sensor and the top sensor is referenced as the gradient sensor. After data is collected a built-in program is used to subtract lower sensor data from the upper sensor. This resulting positive or negative value will be the most accurate method in eliminating most atmospheric noise. The data will be expressed in gammas or nanoTeslas per meter or half meter (dependent on the distance separating the sensors).

Magnetometer sensors can be configured one of two ways when used in the gradient mode. In the upper northern (generally between 20 degrees north latitude and 65 degrees north latitude) and southern (generally between 20 degrees south latitude and 65 degrees south latitude) latitudes, where magnetic flux angles are high, the most common configuration is vertical for most site investigations. The vertical configuration is when two sensors are aligned one over another separated by a vertical distance, typically 0.5 or 1 meter. The top sensor is typically noted as the gradient sensor while the bottom sensor is typically called the total field sensor. However, when in the upper-most and lower-most latitudes beyond those latitudes just mentioned, sensor configurations are usually adjusted to compensate for the angle of magnetic flux. The angles for compensation are taken from data tables usually provided by the magnetometer vendor. At the Earth's equator the sensor configuration is completely horizontal to compensate for the lowest angle of magnetic flux.

* Sensor configuration Table related to upper northern & southern latitudes. For latitudes near the equator, reverse sensor configuration, vertical to horizontal and horizontal to vertical

Correctly applying the magnetic method is key to a successful survey. As always in the science of geophysics, knowing the most information as possible about a problem allows a more precise, exacting and efficient solution to be applied.

Several key factors about locating ferrous items influence how the method will be applied and interpreted. The most important factors include:

It is to the benefit of the operator to obtain as much data as possible in the preceding four categories to allow for a successful interpretation. If specific information is not available about the target, one must provide a "best guess assumption" of conditions in order to apply the method successfully.

Composition of a mass references ferrous content of the target. Higher ferrous contents increases the range in which a mass can be detected.

Knowing the approximate mass of a target is important since larger masses will generally have magnetic fields which emanate much farther than those produced by a small mass. Thus locating larger masses would require less data and larger grid spacing intervals since it would be detectable from greater distances than that of a smaller mass.

Depth of the target is another issue that will factor into the delectability of a target. Smaller masses emanate weaker magnetic fields and are only detectable when they are near-surface. However, masses whether small or large, stacked or grouped together will usually emanate a stronger magnetic field which can be detected at deeper depths.

The matrix that is near or surrounding a target is only of concern if it contains ferrous material. For example, if one is trying to locate a small unexploded grenade several feet below ground on a firing range which has several millimeters of ferrous shrapnel lying on the surface - the chances of detecting the magnetic field emanating from the grenade is minimal. Since the shrapnel is extensive and nearer to the detector than the grenade, the shrapnel would tend to mask the magnetic field emanating from the grenade. Another difficult issue to address would be locating a drum buried in soil containing highly mineralized ferrous ores or slag. Surrounding mineralization could mask the tanks' magnetic field. Other unwanted magnetic influences can emanate from surface features such as nearby buildings, vehicles, powerlines or fences.

Orientation and condition of a target will directly effect the emanating magnetic field. Like a magnet, ferrous objects (a barrel for example) will have stronger and weaker fields emanating through the mass. Changing the orientation of a magnet, or a barrel, will cause the magnetic forces to move with the mass maintaining the geometry of the field since the strongest areas will generally be near the ends of the mass. Knowing the condition of a mass is relevant since ferrous materials once distorted, like a magnet or barrel, will emanate a weaker magnetic field due to the disruption of geometry and cohesiveness within the object.

The following Table lists some common items and their respective data values as measured close to the object and far away from the object to help comprehend the parameters previously mentioned. It should be noted that magnetic data values will not be influenced by soil cover (as long as it does not contain ferrous minerals) or water so, for example, data collected above of a buried object should have approximately the same magnetic force as those collected from underneath as if the same object was suspended in the air (although polarities may be reversed).

*Note: Anomalies are only representative and may vary by factor of 5 or even 10 depending upon certain factors

As one can infer from the Table, depths of investigation limitations are based on the amount of mass. The larger the mass, the deeper an emanating magnetic field can be detected. It should be noted that by increasing the amount of similar (or dissimilar ferrous objects) will increase the detectable limits of the mass. Note that many ferrous components overlap similar data values, therefore it would be difficult to link a specific object to a unique value range.

The graphic below provides some comparisons for anomalies typically encountered at environmental waste sites.

The most commonly applied surveys are conducted by using one of two methods. One method is to obtain enough magnetic data in a random pattern to fulfill statistical requirements which would be representative of the area. Another method, which is more frequently used, is incorporating a systematic grid pattern to collect magnetic data. In either case the size of the target will determine how many samples, or how small or large grid spacing intervals must be established. Smaller sized targets will require more sampling points than larger sized targets.

Use of survey grids are the most common method to obtain data since it is systematic and provides better coverage than randomly collecting data. Most vendors provide information on how to setup a grid and collect data. Some instruments have several options of how data can be collected, refer to vendor manuals for details.

Once as much background information as possible is collected concerning the ferrous target, one can start to develop a survey plan. The survey area should not be unlimited but constrained through available information, while eliminating the possibility of targets existing outside the area to be surveyed. Data are collected along individual lines of traverse either at specific intervals (in the case of proton precession magnetometers) or at nearly continuous intervals (in the case of proton Overhauser or optically pumped magnetometers). Lines of traverse are set up in the survey area separated by a distance based on the estimated size of the target. The traverse line interval should be separated no further than the estimated detectable range of the object.

Establishing a grid over an area can be done in one of many ways. Traditional engineering surveys and stakes can be used, measuring tapes laid out and followed, flagging set using measuring tapes or engineering surveys, or hip chains have traditionally been used in the past. However, as newer magnetometer tools are becoming available, some manufacturers are making provisions to readily adapt Global Positioning Systems to their systems. This would eliminate the need for pre-establishing detailed grids since locational data collected and merged to data points are recorded as one walks collecting data. Note that not all systems have made this adaptation for Global Positioning System synchronization at this time.

Data collected within each traverse line are referred to as positions and each individual traverse paths are noted as Lines. Documenting positions and lines are best visualized by relating them to X and Y axis coordinates referenced specifically by the distances in feet separating each line and position. Using alphabetic labeling sequences are usually harder to interpret since the alpha characters do not immediately relate to a quantifiable numeric reference. It is preferable to have traverse lines trending north-south, parallel with the Earth's magnetic field, but this is not a requirement. Traditionally the origin of the grid is preferable at the southwest corner of the grid, but having the corner elsewhere is not uncommon.

Data are collected along the first traverse line (walking north for example), when the line is completed the operator will then move the magnetometer to the next line and start collecting data again (this time walking south). This alternating pattern is continued until the entire survey area is traversed. Most magnetometers will have built-in software that will allow the operator to preprogram the system to automatically increment to the position and line intervals that have been pre-established. A press of a button will then increment the grid details automatically to the next position. During this process readouts from the instrument are visible to the operator who can monitor the operation of the equipment, such as battery condition, memory availability, sensor stability, data values, positioning information (some variables may not be available on all units).

Once all the data are collected, they are transferred (commonly referred to as "dumped") to a computer through a compatible data transfer cable and software usually supplied by the vendor. The data typically consists of a time and date stamp, X and Y positioning data, total field and gradiometer data values. Next (or after total field and base station data have been merged) data are typically imported into a commercially available contouring package for processing. Several options are provided for analyzing the data and the method to select will depend on the grid pattern, spacing and other information. Displaying the data can also involve various methods such as table format, line plots, contours, shaded relief and 3D. Most commonly seen presentations are those that use contouring plots which are similar to elevation contour lines found on U.S. Geological Survey topographic maps.

One of the most important details to remember whether one is conducting their own survey or having someone else conducting a survey is to have a permanent point of reference for the data. For example, if a grid is being used, one or more corners of the grid should be measured to permanent objects such as a street corner, building, power pole so that the grid can be reestablished to locate any anomalies indicated by the survey.

The Occupational Safety and Health Administration (OSHA) has established a standard for handling buried drums and containers. It requires that some type of detection system or device be used to estimate the location and depth of buried drums or containers prior to handling. Several geophysical methods could be used to comply with this standard, including magnetics which can provide an accurate location. Depth estimates could be determined from magnetic modeling programs or from other geophysical methods. The standard is 29 CFR Part 1910.120 (j) (1) (x) Revised as of July 1, 1998, and can be found using the following web page: http://www.osha.gov/.

Magnetic data consists of individual numeric values which are usually provided either as total field measurements or gradient measurements. Total field measurements (in the northern and southern latitudes) are values typically in the range of tens of thousands and are always positive since they are a direct value of the Earth's magnetic field plus or minus any values emanating from local fields. Gradient and corrected total field measurements are always smaller values since they consist of two total field measurements that are subtracted from each other which provide only the value of the emanating local field. These range from 0 (background) to several thousand and can either be positive of negative.

A group of numeric magnetic values are meaningless unless there is someway of referencing them to a specific location for communicating details to other people. In order to document and reference numeric magnetic data to specific locations, it is often collected over a pre-established grid pattern. A grid provides a X coordinate and Y coordinate which can define any location on a two dimensional surface. When magnetometers store data it can usually be referenced to an X, Y, Z format where X and Y are locational data and Z is the magnetic value.

There are several ways in which to present magnetic data. In its raw form it can be listed in a column of X, Y and Z data, but this is difficult to visualize anomalous data. Data could also be presented as line graphs plotted for each line traversed with the magnetometer. Although this does provide a graphic image of the data it is difficult to relate each line to adjacent lines. To provide a better perspective of the data as a whole, contour presentations are the most typical method of illustrating data. This is a simple method of visualizing all of the data as a whole. A modification of the contouring illustration is to eliminate the two dimensional contours and attach a third dimension to the data based on the value of magnetic data to provide a net diagram which gives a perspective of three dimensions to a two dimensional data set.

To see four methods of presenting magnetic data, each showing portions of the same data set click here.

An experienced person is needed to interpret magnetic data to ensure that an appropriate scale is used that will present all necessary details of the data. Whether it is accidental or intentional, incorrect manipulation of contour scales can easily "hide" important anomalies. The interpreter must also account for interference which must be removed from the data so that it will not lead to a false interpretation. But most of all a good interpreter of the data will provide an overall perspective which will list any limitations of the data or potential "data gaps" which the end user must the aware.

To help obtain a perspective between actual buried targets and the data recorded from them, several contour data plots with photographs of buried objects can be seen by clicking on 1, 2, or 3. Note that the contour plots footages are numbered away from the center of the target so that one can see the extent of the anomaly range.

Since most individual geophysical methods each have their own advantages and disadvantages no one method may provide the best answer. If more than one geophysical method is used, and incorporates a different theoretical approach, a better solution can usually be obtained by interpreting data these data results together. An example of this practice would be using electromagnetics to locate a ferrous/non-ferrous metal mass and then using the magnetic method to discriminate ferrous from nonferrous metal. This concept is referred to as synergy and can be adapted as a confirmatory process for magnetics.

Accurately and efficiently collecting magnetic data is only half the job of a magnetic survey. The remaining half of a magnetic survey, data analysis and final report, is just as important as the data collection procedures. Each data value is only significant if it can be accurately recorded and transferred to a precise location documented by the final report.

All final reports should have maps to help the reader to understand the data. Maps should be at a scale that illustrates all the details of the area such as buildings, utilities, roads, surface interference and other features. Such permanent features should be used as reference points (or at least back-up reference points for GPS locations) for survey grids. This is critical since most environmental remediation methods do not occur immediately after geophysical surveys are conducted and usually occur weeks or months after a survey has been completed. That is why it is important to document grid corners in detail to accurately re-locate the survey area if any drilling, digging or probing is to occur using magnetic data to direct any of these actions.

Maps that show permanent reference points are usually called base maps and also define the location of any ferrous (automobiles, cyclone fences) or electrical (power lines, transformers) objects visible on or above the ground surface that could cause magnetic interference. This is important since the magnetometer will be influenced by these surface objects and if not noted correctly could cause the interpreter to assume that these objects could be underground anomalies.

Other maps should be provided that show the traverses of each survey line so that it can be used to establish if the spacing between the lines were adequate to resolve the specific problem. As for displaying actual data, several methods are possible as we have mentioned previously. But no matter what method is used, using the correct scale is important. For example, if a magnetic gradient anomaly exists that has a peak value of 500 gammas per meter and a contour map is made using a scale of -5000 to +5000 gammas per meter with a contour level of 1000 gammas per meter, the anomaly will be very difficult to locate. When computer programs are used to help interpret data the name of the program, version, interpretation method (kriging, etc.) and contour levels should be provided. Remember that negative values are possible for gradient magnetic data and maps should reflect the full data range.

As for any map, several details must be present such as a north arrow, distance scale (at a scale illustrating appropriate level of detail) data scale, title, legend, date and site or location. If maps or reports have the potential of being reproduced using black and white photocopiers by other parties, use of color maps should be discouraged. The size of paper used to make the map should also be considered for any post-reproduction purposes.

Performance specifications include information about interference, detection limits, calibration, quality control, and precision and accuracy.

A number of factors can effect the detection and sensing elements. Some interferences can be inherent to the engineering limitations of the instrument, other interferences are caused by outside factors such as nearby ferrous objects. To obtain useful data, it is important that the analyst understand potential interferences. Some effects are described below.

External interferences: Electrical noise from AC power lines (proton precession magnetometers are also susceptible to DC voltage); transformers or other radiating transmitter sources; high magnetic gradients from underlying rocks/soil/minerals; nearby visible or hidden iron alloy objects (cars, railroad tracks, manhole covers, fence lines, grates, etc.). Whenever external interferences are visible and obvious to the operator which may influence data, good field technique establishes that field notes should reflect their specific location and an accurate description.

Inherent interferences: These interferences may not be easily observed by an inexperienced operator and are varied to the specific type of magnetometer used. Optically pumped magnetometers have a "dead zone" in each sensor due to the structure of internal components which limits how certain ambient magnetic field angles intercept the sensor. To optimize sensitivity around the "dead zone" most vendors provide a supplemental program to calculate the best angle to mount the sensor for the specific latitude that you are working, thus making the sensor more efficient. Some proton precession sensors typically are constructed in a manner which orientation of the sensor (usually due north or south) is an important factor to optimize magnetic field measurements.

Solar interferences: Atmospheric effects are mainly of concern when a magnetometer is used in the total field mode. Minimizing problems associated with this type of phenomenon can be resolved by using a gradiometer or obtaining total field measurements in conjunction with a properly setup base station.

Detection limits for magnetometers will vary according to the physical method used (proton precession or optically pumped). Generally speaking, older technologies will have larger (less effective) detection limits. For example, inexpensive fluxgate systems can have a detection limit of 10 gammas; proton precession tools will range around 0.1 or 0.2 gammas; optically pumped systems will have a detection limit near 0.01 gamma. It is important to note that any detection limit is only relevant if the magnetic field of the object being evaluated is within range of the sensor so that the field can be distinguished from background. If a magnetic field from a buried ferrous object does not extend beyond the ground surface (for buried objects), it will not be detectable no matter how small the detection limit of a particular method.

Generally no calibration is needed for optically pumped magnetometers, if handled properly and not subjected to shock. Most magnetometers have a built-in self test mechanism capable of evaluating its own working condition. Although most proton precession magnetometers have onboard monitoring systems, they may also require a minor adjustment if the magnetometer's total field range was previously set for a field intensity significantly different (thousands of gammas) from the current background location. Such an adjustment is made with through the instrument's onboard numeric key pad. The correct value can be checked by using a reference map showing the Earth's total magnetic field intensity and matching the general total field background value closest to your geographic location. Once an approximate value is entered for the geographic location, the instrument will be able to automatically fine tune the value after the gross value has been entered.

To ensure that the data generated are of a valid quantity, there are four procedures that can be done to monitor quality control. One is to evaluate and monitor solar activity by using information from the following web-page: http://www.sel.noaa.gov/today.html. This web-page will provide daily information and a forecast of solar activity concerning solar events which may disrupt magnetic measurements. Knowing this type of information will allow the operator to determine the optimal time window to obtain total field measurements or when a gradiometer should be used. Another quality control is to select a background area free of ferrous materials and establish this point as background, then average several measurements at this location. Several times during the survey the operator should return to the background point and resample. If the readings are similar, the instrument is performing properly. A third type of quality control is provided by some instrument manufacturers which will have built-in monitoring systems so that the operator can observe the functionality of the system during a survey. Finally, before each survey the operator should keep the instrument stationary and obtain data while walking an equidistant circle around the instrument. If the data remains similar during this test, the operator is assured that nothing on the person was detectable by the sensor(s) which could bias the data.

Precision is a measure of the reproducibility of data from measurement to measurement and is affected mainly by the analyst’s technique. Accuracy is a measure of how close the result of an analysis comes to the “true” locational estimation of an anomaly. There are several comments that need to be stated about the precision and accuracy of an anomaly's response.

When dealing with the higher sensitivity magnetometers such as the proton precession and optically pumped systems, precision of the tools are highly refined. Duplicating a measurement to an exact tenth of a gamma or nanoTesla would be difficult to accomplish. Any slight changes in sensor orientation, elevation, location or path over object and changes in path direction over an object will contribute very slight changes in the data. Even if all these parameters were constant, differences could still occur due to the internal statistical averaging that occurs before a value is displayed or posted within the system. However, none of these parameters are significant enough to render the values unacceptable since most of the time differences are in the single digit range.

Accuracy of data to locate the "true" location of an object is a variable that relies on the experience of the person interpreting the data. Typically an anomaly will have peaking positive and/or negative values due to the composition, orientation and how the sensor traversed over the target among other factors, of the mass. An experienced data analyst can accurately pinpoint the center of an anomaly. However, larger mass(es) have a more extensive magnetic field which emanates from the main body and thus can be detected before actually reaching the target. Thus, knowing the exact endpoints of a target may only be accurate within several feet. Smaller targets will not have large emanating fields and thus its extents can be established more accurately. Note that accuracy is mainly considered for defining lateral extents over a target. Depth estimates are difficult to determine unless details such as target shape, orientation and mass are known and can be applied to a modeling program.

Another factor which comes into play for accuracy in magnetics is the depth of burial and geographic location on the Earth. For example, an anomaly from a mass of drums lying underwater in a deep quarry will not be positioned exactly in the center of the anomaly as measured from the surface. It will be offset slightly (and geometrically determinable) due to the angle of Earth's magnetic field at a particular latitude. In general this offset is minimal and only becomes a concern when target depths are significantly deep.

Despite these differences in accuracy, magnetics has proven to be an important tool in locating buried ferrous anomalies. It provides data capable of sensing mass, something that most other geophysical methods cannot obtain.

A word about modeling programs. Methods are available to model potential targets, but these methods require knowledge of the mass or shape. Frequently the amount of mass and shape (or condition) of environmental targets are unknown and difficult to apply in every case. However, gross estimates could be made using the simple half-width method (or Naudy's Method--a definition can be found at http://www.igcworld.com/gm-glos.html where the half peak width is approximately the depth to the center of a spherical body).

There are numerous advantages for using magnetics in the field, speed, portability, ease of use, and relatively low cost are some advantages cited most commonly.

While there are many advantages to magnetics, it is important that the user understand its limitations, if the technology is to be used properly for generating data that meets the needs of a project.

Typically there are three options for cost estimating a magnetic survey, one can either purchase equipment, rent equipment, or hire a contractor. If one would rent or purchase equipment it is assumed that the operator is familiar with the tools and would know how to correctly apply the method. Rental and purchase costs do not include materials and tools needed to set up a grid, markers or other locational devices. A geophysical field service contractor would be able to provide an entire magnetometer survey from start to finish. Please note that when using a geophysical field services contractor be sure that interpretations of the data and a final report are part of the total cost. Some contractor's basic price option just provides for data collection, not a documented interpretation.

Equipment Rental (approximate costs for weekly rental including mobilization fees, spring 2001)

Allen, R.P. and B.A. Rogers. 1989. Geophysical Surveys in Support of Remedial Investigation/Feasibility Study at the Municipal Landfill in Metamora, Michigan. In: Proc. 3rd Hat. Outdoor Action Conf. on Aquifer Restoration, Ground Water Monitoring and Geophysical Methods, National Water Well Association, Dublin, OH, pp. 1007-1020.

Allen, R.P. and M.A. Seelen. 1992. The Use of Geophysics in the Detection of Buried Toxic Agents at a U.S. Military Installation. In: Current Practices in Ground Water and Vadose Zone Investigations, ASTM STP 1118, D.M. Nielsen and M.N. Sara (eds.), American Society for Testing and Materials, Philadelphia, PA, pp. 59-68.

Aller, L. 1984. Methods for Determining the Location of Abandoned Wells. EPA/600/2-83/123 (NTIS PB84-141530), 130 pp. Also published in NWWA/EPA Series, National Water Well Association, Dublin, OH.

Carr, III, J.L., C.S. Ulmer, C.K. Eger, and P. Mann. 1990. Delineation of a Suspected Drum and Hazardous Waste Disposal Site Utilizing Multiple Geophysical Methods: Shaver's Farm, Chickmauga, Walker County, Georgia. In: Proc. Fourth Nat. Outdoor Action Conf. on Aquifer Restoration, Ground Water Monitoring and Geophysical Methods. Ground Water Management 2:1097-1111.

Emilsson, G.R. and P.R. Morin. 1989. Using Vertical Electric Soundings to Accurately Map a Buried Channel in Coastal Plain Sediments. In: Proc. Focus Conf. On Eastern Regional Ground Water Issues, National Water Well Association, Dublin, OH, pp. 41-54.

Environmental Consulting and Technology (EC&T), Inc., Technos, Inc., and UXB International, Inc. 1990. Construction Site Environmental Survey and Clearance Procedures Manual. U.S. Army Toxic and Hazardous Materials Agency, Aberdeen Proving Ground, MD.

Evans, R.H. And G.E. Schweitzer. 1984. Assessing Hazardous Waste Problems. Envrion. Sci. Technol. 18(11):330A-339A.

Feld, R.H., Stammler, G.A. Sandness, and C.S. Kimball. 1983. Geophysical Investigations of Abandoned Waste Sites and Contaminated Industrial Areas in West Germany. In: Proc, (4th) Nat. Conf. On Management of Uncontrolled Hazardous Waste Sites, Hazardous Materials Control Research Institute, Silver Spring, MD, pp. 68-70.

Fowler, J.W. And A. Ayubcha. 1986. Selection of Appropriate Geophysical Techniques for the Characterization of Abandoned Waste Sites. In: Proc. Surface and Borehole Geophysical Methods and Ground Water Instrumentation Conf. And Exp., National Water Well Association, Dublin, OH, pp. 625-656.

Fowler, J.W. And D.L. Pasicznyk. 1985. Magnetic Survey Methods Used in the Initial Assessment of a Waste Disposal Site. In: NWWA Conference on Surface and Borehole Geophysical Methods and Ground Water Investigations (2nd, Fort Worth, TX), National Water Well Association, Dublin, OH, pp. 267-281.

Ghatge, S.L. and D.L. Pasicznyk. 1986 Integrated Geophysical Methods in the Determination of Bedrock Topography. In: Proc. Surface and Borehole Geophysical Methods and Ground Water Instrumentation Con. And Exp., National Water Well Association, Dublin, OH, pp. 601-624.

Gilmer, T.H. And M.P. Helbling. 1984. Geophysical Investigations of a Hazardous Waste Site in Massachusetts. In: NWWA/EPA Conf. On Surface and Borehole Geophysical Methods in Ground Water Investigations (1st San Antonio, TX), National Water Well Association, Dublin, OH, pp. 618-634.

Hager, J.L., E.K. Triegel and M.J. Stell. 1991. Use of Surface Geophysical Techniques to Locate Underground Storage Tanks at the New Castle County Airport, Delaware. In: Ground Water Management 5:1031-1044 (5th NOAC).

Hinze, W.J. 1988. Gravity and Magnetic Methods Applied to Engineering and Environmental Problems. In: Proc. Symp. On Application of Geophysics to Eng. and Environmental Problems, SOc. Eng. and Mineral Exploration Geophysicists, Golden, CO, pp. 1-107.

Hitchcock, A.S. and H.D. Harman, Jr. 1983. Application of Geophysical Techniques as a Site Screening Procedure at Hazardous Waste Sites. In: Proc. Third Nat. Symp. On Aquifer Restoration and Ground Water Monitoring, National Water Well Association, Dublin, OH, pp. 307-313.

Koerner, R.M., A.E. Lord, Jr., S. Tyagi, and J.E. Brugger. 1982. Use of NDT Methods to Detect Buried Containers in Saturated Silty Clay Soil. In: Proc. (3rd) Nat. Conf. On Management of Uncontrolled Hazardous Waste Sites, Hazardous Materials Control Research Institute, Silver Spring, MD, pp.12-16.

Manufacturer	Technology Trade Name
Geometrics	G-858 Portable Cesium Magnetometer - Gradiometer
Geometrics	G-856 Portable Proton Precession Magnetometer - Gradiometer
Gem Systems Incorporated	GSMP-30 Portable Potassium (Overhauser Type) Magnetometer-Gradiometer
Gem Systems Incorporated	GSM-19 Portable Proton Precession Magnetometer- Gradiometer
Scintrex Ltd.	SMARTMAG Cesium Vapor Magnetometer- Gradiometer

Most manufacturers will rent their own magnetometer system brands (see table above). Other vendors operate businesses specifically for rental of geophysical equipment and are listed below:

For those interested in the magnetic fluxgate method here are some additional resources:

Fisher Research Laboratory	FX-3 Differential Induction Magnetometer or Magnetic Locator
Geoscan Research (USA)	Fluxgate Gradiometer FM36 & FM256
IRF Observatory Handbook
Metrotech	880B Ferromagnetic Locator
Schonstedt Instrument Company	GA-52Cx Differential Induction Magnetometer or Magnetic Locator

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Page Last Modified: June 22, 2007