Frequency Domain Electromagnetics

Electromagnetic Methods
Frequency domain electromagnetic methods (abbreviated as EM for this discussion) are frequently used in the search for minerals and in shallow geophysical applications related to engineering and environmental investigations. There are a number of specific techniques for electromagnetic prospecting, but this discussion will focus on the most common techniques used for shallow studies. Parts of this discussion are taken from Keller and Frischknecht (1966), McNeill (1980), and Dobrin (1976).

Electrical Properties of Subsurface Materials
Conduction of electricity in materials takes place through electronic or ionic processes. Solid conductive materials can be divided into three classes: metals, electron semiconductors, and solid electrolytes. In the shallow groundwater environment, it is expected that the only metallic conductors are related to man-made objects such as pipes, tanks, and metallic landfill material rather than natural metallic bodies. Nearly all materials which are not true metal are electron semiconductors to some extent. The silicate rock-forming minerals in sedimentary formations are in the class of solid electrolytes.

Porosity, saturation, and pore fluid chemistry are much more important to the bulk electrical properties of a soil or rock than the electrical properties of the solid matrix. Most pore fluids contain some salts in solution and electrolytic conduction is the dominant conduction mechanism. The relative ability of a material to conduct electricity when a voltage is applied is expressed as conductivity in units of Seimens/meter (S/m).

Electromagnetic Induction
The EM method is based on the induction of electrical currents in subsurface conductors by electromagnetic waves which are generated on the surface. The EM source is commonly a closed loop (transmitter) in which a controlled alternating current produces a time-varying magnetic field . The time-variant magnetic field induces alternating currents (often called eddy currents) in subsurface conductors which produce a secondary time-variant magnetic field that is measured at the surface with another closed loop of wire (receiver). If the assumptions on which the EM instrument was designed are found to be true, the magnitude of part of the received signal is linearly related to terrain conductivity(see definition below).

The secondary field is often not in phase with the primary (transmitted) field . The secondary field is divided into the portion of the field that is in phase and the portion that is out of phase with the primary field. These quantities may be referred to using a variety of names; inphase and quadrature components, or real and imaginary components. The quadrature component is linearly related to terrain conductivity under normal subsurface conditions.

Electromagnetic measurements facilitate rapid determination of the average terrain conductivity because they do not require direct electrical contact with the ground. A disadvantage is that unless measurements are taken at different coil spacings, little vertical information is gained. However, EM profiling can be effective in investigations for locating lateral discontinuities such as landfill boundaries, changes in soil composition, or in the search for buried objects

Terrain conductivity is defined as the conductivity that the instrument would report if located over a homogenous half-space with exactly that conductivity. As the earth is seldom well characterized as a homogenous half-space, the instrument simply integrates the effects of all the subsurface variations and indicates an "apparent conductivity" as terrain conductivity. The units are milliseimens/meter or inverse ohm-meters times 1000.

Interpretation of Electromagnetic Data
The outputs of an EM-31 survey are the conductivity (quadrature) and inphase components of the secondary magnetic field. The secondary magnetic field is a complicated function of the intercoil spacing, the operating frequency, and the ground conductivity. The relationship is simplified when certain constraints, technically defined as "operation at low induction number", are met. When the low induction number constraints are not satisfied the measured quadrature and inphase responses deviate from expected values. In very conductive terrain, or in the presence of metal, (>300 mS/m) the quadrature component of the received magnetic field is not linearly proportional to the terrain conductivity, so conductivity readings are not accurate. Also at high conductivity, the inphase portion of the received magnetic field increases in magnitude and, due to the limited dynamic range of the EM-31, the inphase signal saturates the instrument's amplifiers causing the recorded data to be clipped.

To understand the depth of investigation of the EM-31 it is useful to consider a homogeneous halfspace with the addition of a thin layer at some depth. It is possible to calculate the secondary magnetic field that results from this thin layer as a function of depth. Material located at a depth of 0.4 times the coil spacing gives the most contribution to the response, however deeper layers still contribute a significant amount to the response (figures).



The geometry of an anomalous conductor can be inferred from the size and lateral extent of a feature. A strong inphase response is expected over highly conductive bodies, such as buried metal. Anisotropic subsurface conductors can often be detected by comparing EM measurements from orthogonal instrument orientations. For example, a conductivity value output by an EM-31 instrument with the boom parallel to a north-south azimuth will be different from the conductivity value obtained with the boom parallel to an east-west azimuth, if the subsurface consists of an anisotropic conductor. Taking the difference of the north-south measurement from the east-west measurement yields a non-zero number - a relative indication of the amount of anisotropy. Difference plots also help to enhance lateral conductor boundaries when the boundaries are are sharp transitions (landfill boundaries, for example).

It is necessary to integrate any possible external information into the EM interpretation, whether it is in the form of historical information or an interpretation from a different geophysical method.

It is important to separate anomalies caused by cultural features such as debris piles, pipes, and buildings from subsurface related anomalies. Field maps of cultural features enable the identification of cultural EM anomalies and distinguish known features from subsurface targets. One additional rule of thumb that is important in mapping objects is that the station spacing should be less (preferably 50% or so) than the coil spacing.

EM-61 High Sensitivity Metal Detector
The Geonics EM-61 is a high-sensitivity high-resolution time-domain metal detector which is used to detect both ferrous and non-ferrous metallic objects. It consists of a powerful transmitter that generates a pulsed primary magnetic field, which induces eddy currents in nearby metallic objects. The decay of these currents is measured by two receiver coils mounted on the coil assembly. The responses are recorded and displayed by an integrated data logger as two channels of information. For further processing and interpretation data can be transferred to PC computer.

Under ideal conditions, the EM-61 detects a single 200 litre (55 gal) drum at a depth of over 3 meters beneath the instrument, yet it is relatively insensitive to interference from nearby surface metal such as fences, buildings, cars, etc. By making the measurement at a relatively long time after termination of the primary pulse, the response is practically independent of the electrical conductivity of the ground.

Due to its unique coil arrangement, the response curve is a single well defined positive peak, greatly facilitating quick and accurate location of the target. The depth of the target can be estimated from the width of the response and/or from relative response from each of the two receiver coils.

The EM-61 consists of three major parts: coil assembly, backpack with battery and processing electronics and digital data recorder.

EM-61 Interpretation
The EM-61 is designed in such a way that it is possible not only to separate anomalies spatially but it is also possible under some conditions, to distinguish deeper targets from shallow ones. In addition, the unique two receiver coil system allows suppression of near surface targets that may mask response from deeper more important ones. This feature is very useful when the purpose of the survey is to locate deeper targets, like underground storage tanks or drums, in presence of shallow near surface metallic objects (manhole covers or metal scrap).

Because the amplitude of response is highly depended on the distance between the coil assembly and target, small near surface anomalies will very often produce a response orders of magnitude larger than much bigger but deeper targets. This masking effect from near surface material is drastically reduced by using output of two coils and processing them in the differential mode. In this case output from channel 1 is subtracted from channel 2. Channel 1 represents data from top receiver coil, whereas channel 2 is data from coil closer to the ground. The calculation is automatically performed by EM-61 DAt61 computer program.

The most common way of interpretation of EM-61 data is by using channel 2 and difference channel data.The difference channel is calculated in the following way:

D = k*CH1 - CH2

Where: D is differential output in mV

CH1 is output from top coil in mV

CH2 is output from bottom coil in mV

k is depth coefficient normally set to 1

It is possible to vary k, and adjust the depth at which the response will be suppressed the most. If k is selected to be 1, the response from targets right below the surface will be reduced the most. If the coefficient k is made smaller than 1, the deeper target will be suppressed more than shallow targets. In this case surface anomalies will have negative response in the difference channel.

It should be noted that the degree of cancellation will be affected by size, shape and depth of targets. The response from the targets shaped like balls, spheres or small plate-like targets parallel with the ground can be reduced more than response from larger 3- dimensional targets.

Note that the negative values on the differential channel map are often associated with the metallic objects located on or above the surface, assuming that the depth coefficient of 1 is used (normal practice).

EM-61 Calculation of Apparent Depth of Target
The user can estimate an approximate depth (apparent depth) of a target. This parameter is calculated on the basis of ratio of amplitude from channel 1 and channel 2 response. The apparent depth estimation is most accurate when the instrument is positioned over the center of buried target. (an additional reason from choosing the spacing between the survey lines). In order to determine position of an anomaly, the peak response of the channel 2 profile should be examined along the survey line as well as on the neighboring survey lines. By comparing responses of nearby lines and selecting anomaly maximum, it is normally easy to locate the position of the target. The apparent depth is determined at the highest point of the anomaly.

It should be noted that the calculation of depth is an approximation. The accuracy of estimation will depend on the relation between the line (station) and center of the target, the size and shape of target, as well as the quality of data.

Depth estimation for the small ball-shaped targets will be more accurate that the estimate for larger targets (like underground storage tanks or pipes). Depth for the larger targets will be normally overestimated, meaning that the anomaly will appear deeper than it actually is.

In order to improve depth estimation accuracy, especially for deeper targets with low response, it may be necessary to remove a small offset from the readings. Although each instrument prior to leaving the factory, has outputs of both channels adjusted to read zero, it is possible that with time a small offset of several millivolts appears at the output(s). This effect could be recognized as a small non-zero shift in readings over the portion of the survey line that has no visible anomaly response.

EM-61 Surface Metal Discrimination
The EM-61 has another very useful feature. For surveys carried out in areas where there is a large amount of near surface metal, a second coil is utilized. The design of this coil is such that this near surface metal response can be made virtually zero, greatly facilitating the detection of deeper targets such as buried drums. Conversely, it is also possible, using the coil, to make the response from near surface metal to have one polarity, while that from deeper metal (the actual depth can be adjusted) is of opposite polarity, so that distinguishing between the two is rendered very simple.

The interpretation of EM-61 data is often qualitative as the metal detector is an excellent "anomaly finder ". The EM-61 is manufactured by Geonics, Ltd.

MGC has completed EM studies at conductivities of 0.5 mS/m (successful) to 500mS/m (unsuccessful). We can estimate the resistivity of your geologic milieu and assist you in projecting the probable success of your survey. Just fax (303-424-0807) or E-mail (microgeo@aol.com). If you would like to talk to a real, live person we will pay for the call at 1-800-GEOPHYSics. Please furnish us with descriptions of your geology, your targets, and the cultural background and we will assist you in designing your survey.