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:
Contaminated Soil, Surface or Ground Water
Possibly locate source – provided contamination is leaking from subsurface ferrous tanks, drums or pipes
Undocumented or Illegally Buried Metal Containers/Pipes
Locate lateral area of buried ferrous containers (drums, tanks, pipes)
Imperceptible Abandoned Well Casing Cutoff Below Grade
Locate below grade well by detecting iron or steel in casing
Unknown Lateral Extent of Landfills or Trenches Define lateral extent of landfills/trenches – must contain waste which includes ferrous metal
Possible Metal Under Area to be Excavated
Survey area to verify that no ferrous masses exist under proposed excavation zone
Imperceptible Abandoned Foundations Below Grade
Locate foundations below grade – must be steel reinforced
Define lateral extent of slag areas by detecting ferrous minerals
Submersed Metal in Ponds, Lakes, Rivers, and Quarries
Magnetic method is not affected by a volume of water – ferrous masses are detectable through water
Metal Mass Found Using Electromagnetic Geophysical Method (i.e. metal detector) – Is it Ferrous or Nonferrous Metal?
Segregate ferrous metal masses from nonferrous metal masses by comparing electromagnetic and magnetic results
Unknown Quantity of Subsurface Ferrous Mass
Generally, for near surface ferrous masses, a relative comparison could be interpreted between a large or small quantity or mass
Complying with OSHA Standard for Handling Buried Drums & Containers Magnetic method could be used to assist in complying with 29 CFR Part 1910.120 (j) (1) (x) Revised as of July 1, 1998
Subsurface Valve Boxes, Manhole Covers & Railroads Nonvisable features lying beneath the subsurface that contain significant amounts of iron can be easily detected
Property Marker Stakes Property stakes are often iron rods which are detectable by magnetics
Subsurface Ordnance Ferrous ordnance or shrapnel debris are detectable by magnetics
Iron Ore Mineralization Detection dependent on iron content, size of mass and depth of mass
EPA has no standard methodology for use of magnetics at this time. Currently no ASTM standard exists for magnetics.
Theory of Operation
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.
There are three mechanisms that effect magnetic fields on Earth:
The main field caused by electric currents induced in the outer core by convective movements within.
An external field from electrical currents in the ionosphere caused by sunspot activity (solar wind), and to a minor extent, Earth’s moon.
Local anomalies caused by magnetized bodies, either natural or cultural. Most magnetometers can usually detect all three mechanisms, although some instruments are more accurate than others. It is important for the operator and data analyst to be aware of these differences when interpreting magnetic data.
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.
Two most common magnetometers used in environmental investigations are:
Proton Precession Magnetometer; two types: (a) conventional – free precession; and (b) Overhauser (other common names: proton, precession, nuclear). Click here to see Precession Magnetometers.
Optically Pumped Magnetometer (other common names: cesium, potassium, cesium vapor, potassium vapor, alkali vapor, optical). Click here to see Optically Pumped Magnetometer.
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:
Data Acquisition Cycle Rates
Conventional: 3 to 5 seconds
Overhauser: 0.5 to 3 seconds
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:
Proton Precession Magnetometers (Conventional – Free Precession Type):
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.
Proton Precession Magnetometers (Overhauser Type):
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.
Optically Pumped Magnetometers (Cesium vapor or Potassium vapor):
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.