ASTM D6639-01(2008)

Concepts:

This guide summarizes the equipment, field procedures and interpretation methods used for the characterization of subsurface materials and geological structure as based on their properties to conduct, enhance or obstruct the flow of electrical currents as induced in the ground by an alternating electromagnetic field.

The frequency domain method requires a transmitter or energy source, a transmitter coil, receiver electronics, a receiver coil, and interconnect cables (Fig. 5).

The transmitter coil, when placed on or near the earth's surface and energized with an alternating current, induces small currents in the near earth material proportional to the conductivity of the material. These induced alternating currents generate a secondary magnetic field (H_s), which is sensed with the primary field (H_p) by the receiver coil.

Under a constraint known as the “low induction number approximation” (McNeill, 1980) and when the subsurface is nonmagnetic, the secondary magnetic field is fully out-of-phase with the primary field and is given by a function of these variables.

Perhaps the most important constraint is that the depth of penetration (skin depth, see section 6.5.3.1) of the electromagnetic wave generated by the transmitter be much greater than the intercoil spacing of the instrument. The depth of penetration is inversely proportional to the ground conductivity and instrument frequency. For example, an instrument with an intercoil spacing of 10 m (33 ft) and a frequency of 6400 Hz, using the vertical dipole, meets the low induction number assumption for earth conductivities less than 200 mS/m.

Multi-frequency domain instruments usually measure the two components of the secondary magnetic field: a component in-phase with the primary field and a component 90° out-of-phase (quadrature component) with the primary field (Kearey and Brook 1991). Generally, instruments do not display either the in-phase or out-of-phase (quadrature) components but do show either the apparent conductivity or the ratio of the secondary to primary magnetic fields.

When ground conditions are such that the low induction number approximation is valid, the in-phase component is much less than the quadrature phase component. If there is a relatively large in-phase component , the low induction number approximation is not valid and there is likely a very conductive buried body or layer, that is, ore body or man-made metal object.

The transmitter and receiver coils are almost always aligned in a plane either parallel to the earth's surface (axis of the coils vertical) and generally called the vertical dipole (VD) mode or aligned in a plane perpendicular to the earth surface (axis of the coils horizontal) generally called the horizontal dipole (HD) mode (Fig. 3).

The vertical and horizontal dipole orientations measure distinctly different responses to the subsurface material (Fig. 2). When these vertical and horizontal dipole mode measurements are made with several intercoil spacings or appropriate frequencies, they can be combined to resolve multiple earth layers of varying conductivities and thicknesses. This FDEM method is generally limited to only 2 or 3 layers with good resolution of depth and conductivity and only if there is a strong conductivity contrast between layers that are relatively thick and relatively shallow (in terms of the intercoil spacing).

The conductivity value obtained in 5.1.4 is referred to as the apparent conductivity σ_a. For a homogeneous and isotropic earth or half space (in which no layering is present), the apparent conductivity will be the same for both the measurements. Since the horizontal dipole (HD) is more sensitive to the near surface material than the vertical dipole (VD), these two measurements can be used together to tell whether the conductivity is increasing or decreasing with depth.

For instruments referred to as Ground Conductivity Meters (GCMs), the system parameters and constants in 5.1.4 are included in the measurement process, giving a calculated reading of σ_a, usually in mS/m. In some instruments, the ratio of the in-phase components of the secondary to primary magnetic fields (H_s/H_pp) is displayed in ppt (parts per thousand).

For other frequency domain instruments, the measurements for both the in-phase and quadrature phase of the secondary magnetic field are given as ratios.

For a homogeneous horizontally layered earth, the measured apparent conductivity calculated by the instrument is the sum of each layer's conductivity weighted by the appropriate HD or VD response function (Fig. 2).

When the subsurface is not homogeneous or horizontally layered (such as when there is a geologic anomaly or man-made object present), the apparent conductivity may not be representative of the bulk conductivity of the subsurface material. Some anomalous features can, because of their orientation relative to the instrument coils, produce a negative apparent conductivity. While this negative value is not valid as a conductivity measurement, it is an indication of the presence of a geologic anomaly or buried object.

Many common geologic features such as fracture zones, buried channels, dikes and faults, and man-made buried objects, can be detected and identified by relatively well-known anomalous survey signatures (Fig. 3).

Parameters Measured and Representative Values:

The FDEM method provides a measure of the apparent electrical conductivity of the subsurface materials. For ground conductivity meters (GCMs), this apparent conductivity is read or recorded directly. For instruments not using the “low induction number approximation” the measurement is given by the ratio of the secondary magnetic field to the primary magnetic field (H_s/H_p).

Some GCMs also give an in-phase measurement corresponding to the in-phase component of the secondary magnetic field in parts per thousand (ppt) of the primary field. The in-phase component is especially useful for mineral exploration, detecting buried man-made metallic objects, or for measuring the soil or rock magnetic susceptibility and verifying the assumption that the subsurface is nonmagnetic (McNeill, 1983).

Fig. 6 shows the electrical conductivities for typical earth materials varying over five decades from 0.01 mS/m to a few thousand mS/m. Even a specific earth material (Fig. 6) can have a large variation in conductivity, which is related to its temperature, particle size, porosity, pore fluid saturation, and pore fluid conductivity. Some of these variations, such as a conductive contaminant pore fluid, may be detected by the FDEM method.

Equipment:

The FDEM equipment consists of a transmitter electronics and transmitter coil, a receiver electronics and receiver coil, and interconnect cables. Generally these vary only from one instrument to another in transmitter power, coil size, intercoil separation and transmitter frequency.

Some instruments are designed with a rigid, fixed intercoil separation usually less than about 4 meters (13 ft) and are used for relatively shallow measurements of less than 6 meters (20 ft).

For deeper measurements of up to 100 meters (330 ft), depending on the instrument, the instrument consists of separate coils interconnected by cable, (Fig. 5) and generally operates at several intercoil spacings. Instruments using the “low induction number approximation” usually have a single frequency for each intercoil spacing and are generally referred to as Ground Conductivity Meters (GCMs). Measurements of apparent conductivity, σ_a, are calculated and displayed in millisiemens per meter (mS/m).

FDEM instruments taking multiple frequency measurements at a fixed intercoil separation usually give their results as a ratio of the secondary to primary magnetic fields (H_s/H_p). These instruments usually have some frequencies that satisfy the low induction number approximation from which the apparent conductivity is calculated. The larger multiple coil separation, multiple frequency instruments are mainly used for mineral exploration, whereas the smaller multiple frequency instruments are used for much the same applications as the GCMs.

Limitations and Interferences:

General Limitations Inherent to Geophysical Methods:

A fundamental limitation inherent to all geophysical methods lies in the fact that a given set of data cannot be associated with a unique set of subsurface conditions. In most situations, surface geophysical measurements alone cannot resolve all ambiguities, and some additional information, such as borehole data, is required. Because of this inherent limitation in geophysical methods, a frequency domain or ground conductivity survey alone can never be considered a complete assessment of subsurface conditions. It should be noted that multiple methods of measuring electrical conductivity in the earth (that is, FDEM, TDEM, DC Resistivity) will only produce the same answers for the ideal conditions of a nonmagnetic, frequency-independent, isotropic homogeneous half-space. The presence of heterogeneities (for example, layering, objects), anisotropy, magnetic materials, and frequency dependent mechanisms will result in varying geometric patterns of electrical current flow in the ground and consequent different values of measured apparent conductivity between the methods. Properly integrated with other information, conductivity surveying can be an effective method of obtaining subsurface information.

In addition, all surface geophysical methods are inherently limited by decreasing resolution with depth.

Limitations Specific to the FDEM Method:

The interpretation of subsurface conditions from frequency domain measurements assumes a nonmagnetic homogeneous horizontally layered earth. Any variation from this ideal results in variations in the interpretation from the actual subsurface. There are areas with soils that contain significant quantities of ferromagnetic or superparamagnetic minerals or metal fragments in which this assumption is no longer valid. This can be tested with electromagnetic instruments (see 5.2.2). If the assumption is incorrect, then the apparent conductivity will be higher than it should be.

Ground conductivity meters using a single frequency and one intercoil spacing are limited to detecting lateral variations. With two coil orientations, (horizontal and vertical dipole modes), a qualitative interpretation of whether the conductivity is increasing or decreasing with depth is available. Information as to the layering or vertical distribution of the subsurface conductivity can be derived from measurements at different heights above the surface.

For soundings, using both coil orientations and multiple intercoil separations, only two or three layers can be reasonably interpreted. There must still be a significant conductivity contrast between layers and layer thicknesses.

Equivalence problems occur when more than one layered model fits the data because combinations of layer conductivities and thicknesses produce the same sounding responses. For example, a thin highly conductive layer will look much like a thicker, less conductive layer of approximately the same conductivity thickness product. These problems are sometimes resolved by using borehole conductivity or resistivity data, knowing the general geology of the area, or by knowing what is being looked for and what response is expected. FDEM systems give the best results when searching for a conductive layer in a resistive medium. It is difficult to resolve resistive thin layers in a conductive medium even if the layers have a significant electrical contrast.

Frequency domain instruments are best used under relatively high electrical conductivity conditions (greater than 1 mS/m). For low conductivity materials (less than 1 mS/m), useful measurements are better obtained with resistivity methods (Guide D6431).

Ground conductivity meters (GCMs) have a straight-line (linear) relationship between the true bulk conductivity of a homogeneous half space and the apparent conductivity read by the instrument, provided that the true conductivity is within the region controlled by the low induction number approximation for the physical parameters of the particular instrument-intercoil separation and frequency. As the conductivity of the half space increases, making the approximation less and less valid, the apparent conductivity measured by the GCM or calculated using the low induction number approximation (5.1.4) deviates more and more from the true ground conductivity. Fig. 7 shows this nonlinearity for a short one-meter (3.3 ft) intercoil spaced instrument operating at 13 kHz, and shows that, for this spacing, nonlinearity of response is not a problem for most earth materials.

The deviation from linearity, however, can be quite significant for instruments with large intercoil spacings (upwards of 20 m [66 ft]) and relatively high frequency of operation. Here the nonlinearity can start at relatively low values of conductivity and can result in negative values at high values of the true conductivity (Fig. 8).

Natural and Cultural Sources of Noise (Interferences):

Sources of noise referred to here do not include those of a physical nature such as difficult terrain or man-made obstructions but rather those of a geologic, ambient, or cultural nature that adversely affect the measurements and hence the interpretation.

The project's objectives in many cases determine what is characterized as noise. If the survey is attempting to characterize geologic conditions, responses due to buried pipelines and man-made objects are considered noise. However, if the survey were attempting to locate such objects, variations in the measurements due to varying geologic conditions would be considered noise. In general, noise is any variation in the measured values not attributable to the object of the survey.

Natural Sources of Noise—The major natural source of noise in FDEM measurements is naturally occurring atmospheric electricity (spherics). This interference is caused by solar activity or electrical storms. Information about solar activity can be obtained on the Internet at the National Oceanic and Atmospheric Administration web site (http://www.noaa.gov). Electrical storms many miles away can still cause large variations in measurements. When these conditions exist, it is best to abandon the survey until a better time. Increasing the transmitter power can significantly reduce the effect of spherics. This increases the secondary field strength and hence the signal to noise ratio. Unfortunately such a process is at the expense of a larger and heavier transmitter coil.

Cultural Sources of Noise—Cultural sources of noise include interference from electrical power lines, communications equipment, nearby buildings, metal fences, surface or near surface metal, pipes, underground storage tanks, landfills and conductive leachates. Interference from power lines is directly proportional to the intercoil spacing and mainly only affects large intercoil spacings (greater than 15 or 20 m [50 or 66 ft]). Frequency domain instruments with small intercoil spacings are generally unaffected.

Surveys should not be made in close proximity to buildings, metal fences or buried metal pipelines that can be detected by frequency domain, unless detection of the buried pipeline, for example, is the object of the survey. It is sometimes difficult to predict the appropriate distance from potential noise sources. Measurements made on-site can quickly identify the magnitude of the problem and the survey design should incorporate this information (see 6.3.2.2).

Alternate Methods—In some instances, the preceding factors may prevent the effective use of the FDEM method. Other surface geophysical (see Guide D6429) or non-geophysical methods may be required to investigate the subsurface conditions. Alternate methods, such as DC Resistivity (Guide D6431) or TDEM, which may not be affected by the specific source of interference affecting the frequency domain method may be used to show an electrical contrast.

FIG. 5 Schematic of Frequency Domain Electromagnetic Instrument

FIG. 6 Earth Material Conductivity Ranges (Sheriff, 1991)

FIG. 7 Non-linearity for a Short-spaced Instrument

FIG. 8 Non-linearity for a Long-spaced Instrument