ASTM D6820-02(2007)

Concepts:

This guide summarizes the equipment, field procedures, and interpretation methods for using the TDEM/TEM method for determination of those subsurface conditions that cause variations in subsurface resistivity. Personnel requirements are as discussed in Practice D3740.

All TDEM/TEM instruments are based on the concept that a time-varying magnetic field generated by a change in the current flowing in a large loop on the ground will cause current to flow in the earth below it (Fig. 3). In the typical TDEM/TEM system, these earth-induced currents are generated by abruptly terminating a steady current flowing in the transmitter loop (2). The currents induced in the earth material move downward and outward with time and, in a horizontally layered earth, the strength of the currents is directly related to the ground conductivity at that depth. These currents decay exponentially. The decay lasts microseconds, except in the cases of a highly conductive ore body or conductive layer when the decay can last up to a second. Hence, many measurements can be made in a short time period allowing the data quality to be improved by stacking.

Most TDEM/TEM systems use a square wave transmitter current with the measurements taken during the off-time (Fig. 2) with the total measurement period of less than a minute. Because the strength of the signal depends on the induced current strength and secondary magnetic field, the depth of investigation depends on the magnetic moment of the transmitter.

A typical transient response, or receiver voltage measured, for a homogeneous subsurface (half-space) is shown in Fig. 4. The resistivity of the subsurface is obtained from the late stage response. If there are two horizontal layers with different resistivities, the response or receiver output voltage is similar to the curves shown in Fig. 5.

Parameter Measured and Representative Values:

The TDEM/TEM technique is used to measure the resistivity of subsurface materials. Although the resistivity of materials can be a good indicator of the type of material, it is never a unique indicator. Fig. 6 shows resistivity values for various earth materials. Each soil or rock type has a wide range of resistivity values and many ranges overlap. It is the interpreter who, based on knowledge of the local geology and other conditions, must interpret the resistivity data and arrive at a reasonable geologic and hydrologic interpretation. Very often, it is the l shape of a resistivity anomaly that is diagnostic, rather than the actual values of interpreted resistivity.

In the TDEM/TEM technique, the measured quantity is the time-varying voltage induced in the receiver coil and generated by the time-varying magnetic flux (field) of the decaying currents as they move to successively greater depths in the earth. This time rate of change of magnetic flux, and thus the receiver output voltage, has units of volts per square meter of receiver coil area (which area is supplied by the equipment manufacturer). Since the voltage is usually extremely small it is measured in nanovolts (nV) per square meter of receiver coil, where 1 nV = 10^-9 volts.

The resistivity (usually designated in the geophysical literature by the symbol ρ) represents the absolute ability of a substance to prevent the flow of an electrical current. The reciprocal of resistivity is conductivity (usually designated by the symbol σ, where σ = 1/ρ), which represents the absolute ability of the same substance to allow the flow of electrical current. Resistive terrain has a low value of conductivity and vice versa. Throughout this guide, the term resistivity is used . The resistivity of a material depends on the physical properties of the material and is independent of the geometry. Units of resistivity are ohmmeters or ohm-ft (1 ohmmeter = 3.28 ohm ft). Units of conductivity are siemens/meter (S/m) or more commonly millisiemens/meter (mS/m), where 1 S/m = 1000 mS/m. Thus ρ (ohmmeters) = 1/ σ (siemens/meter) = 1000/σ (mS/m).

For most applications in engineering and hydrogeology, the pore fluid dominates the flow of electrical current and thus, the resistivity. As a general rule, materials that lack porosity show high resistivity (examples are massive limestone, most igneous and metamorphic rocks); materials whose pore space lacks water show high resistivity (examples are dry sand or gravel, ice); materials whose pore water is fresh show high resistivity (examples are clean gravel or sand, even when saturated); and materials whose pore water is saline show very low resistivity.

The relationship between resistivity and water saturation is not linear. The resistivity increases relatively slowly as saturation decreases from 100 % to 40 to 60 %, and then increases much more rapidly as the saturation continues to decrease.

Many geologic materials show medium or low resistivity if clay minerals are present (examples are clay soil, severely weathered rock). Clay minerals decrease the resistivity because they adsorb cations in an exchangeable state on their surfaces.

An empirical relationship known as Archie's Law describes an approximate relationship between the resistivity of a matrix material, its porosity and the resistivity of the pore fluid. For saturated sandstones and limestones and many other saturated substances, the resistivity, r, is given approximately by:

Variations in temperature above freezing will affect resistivity measurements as a result of the temperature dependence of the resistivity of the pore fluid, which is of the order of 2 % per degree Celsius. Thus, data from measurements made in winter can be quite different from those made in summer.

As the ground temperature decreases below freezing, the resistivity increases with decreasing temperature, slowly for fine materials (in which a significant portion of the water l remains unfrozen, even at quite low temperatures), and rapidly for coarse materials (in which the water freezes immediately).

Further information about factors that control the electrical resistivity or conductivity of different geological materials (7).

Because the TDEM/TEM technique measures subsurface resistivity, only geological or hydrological structures that cause spatial variations in resistivity are detected by this technique. If there is no resistivity contrast between the different geological materials or structures, if the resistivity contrast is too small to be detected by the instrument, or if the resistivity of the subsurface material is very high, the TDEM/TEM technique gives no useful information.

Equipment—Geophysical equipment used for the TDEM/TEM method includes a transmitter, a transmitter loop of wire, a transmitter power supply, a receiver and a receiver coil.

The transmitter may have power output ranging from a few watts to tens of kilowatts. Important parameters of the transmitter are that it transmits a clean square wave (Fig. 2), and that the “turn-off” characteristics are well known and extremely stable, because they influence the initial shape of the transient response.

The size of the transmitter power supply determines the depth of exploration, and can range from a few small batteries to a 10-kW, gasoline-driven generator.

The transmitter loop wire is usually insulated for safety. The size of the loop and the amount of current flowing through it (and thus the diameter of the wire) determines the desired depth of exploration. The weight of the loop, which is mounted on one or more reels, can be anywhere from a few kilograms to over 100 kg.

The receiver measures the time-varying characteristic of the receiver coil output voltage at a number of points along the decay curve and stores this data in memory. Because the voltage is small, and changes rapidly with time, the receiver must have excellent sensitivity, noise rejection, linearity, stability, and bandwidth. The transmitter/receiver combination must have some facility for synchronization so that the receiver accurately records the time of transmitter current termination. This synchronization is done either with an interconnecting timing cable or with high-stability quartz crystal oscillators mounted in each unit. The characteristics of a TDEM/TEM receiver are sufficiently specialized that use of a general-purpose receiver is not recommended.

The receiver coil must match the characteristics of the receiver itself. It contains a built-in preamplifier so that it can be located some distance from the receiver. The coil must be free from microphone noise, and it must be constructed so that the transient response from the metal of the coil and the coil shielding is negligible.

Limitations and Interferences:

General Limitations Inherent to Geophysical Methods:

A fundamental limitation of all geophysical methods is 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 additional information, such as borehole data, is required. Because of this inherent limitation in the geophysical methods, a TDEM/TEM survey alone is not considered a complete assessment of subsurface conditions. Properly integrated with other geologic information, TDEM/TEM surveying is a highly effective method of obtaining subsurface information.

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

Limitations Specific to the TDEM/TEM Method:

Subsurface layers are assumed horizontal within the area of measurement.

A sufficient resistivity contrast between the background conditions and the feature being mapped must exist for the feature to be detected. Some significant geologic or hydrogeologic boundaries may have no field-measurable resistivity contrast across them and consequently cannot be detected with this technique.

The TDEM/TEM method does not work well in highly resistive (very low conductivity) materials due to the difficulty in measuring low values of conductivity.

An interpretation of TDEM/TEM data alone does not yield a unique correlation between possible geologic models and a single set of field data. This ambiguity can be significantly reduced by doing an equivalence analysis as discussed in 6.11.3 and can be further resolved through the use of sufficient supporting geologic data and by an experienced interpreter.

Interferences Caused by Natural and Cultural Conditions:

The TDEM/TEM method is sensitive to noise from a variety of natural ambient and cultural sources. Spatial variations in resistivity caused by geologic factors may also produce noise.

Ambient Sources of Noise—Ambient sources of noise include radiated and induced responses from nearby metallic structures, and soil and rock electrochemical effects, including induced polarization. In TDEM/TEM soundings, the signal-to-noise ratio (SNR) is usually good over most of the measurement time range. However, at late times, the transient response from the ground decays extremely rapidly such that, towards the end of the transient, the signal deteriorates completely and the data become extremely noisy.

Radiated and Induced Noise—Radiated noise consists of signals generated by radio, radar transmitters, and lightning. The first two are not generally a problem. However, on summer days when there is extensive local thunderstorm activity, the electrical noise from lightning strikes can cause noise problems. It may be necessary to increase the integration (stacking) time or, in severe cases, to discontinue the survey until the storms have passed by or abated.

(1) The most important source of induced noise consists of intense magnetic fields arising from 50/60 Hz power lines. The large signals induced in the receiver from this source (the strength of which falls off more or less linearly with distance from the power line) can overload the receiver if the receiver gain is set too high, causing serious errors. The remedy is to reduce receiver gain to the point that overload does not occur. In some cases, this may result in less accurate measurement of the transient because the available dynamic range of the receiver is not fully utilized. Another alternative is to move the measurement array (particularly the receiver coil) further from the power line.

(2) It was mentioned above that one of the advantages of TDEM/TEM resistivity sounding was that measurement of the transient signal from the ground was made in the absence of the primary transmitter field, since measurement is made after transmitter current turnoff (Fig. 2). Modern transmitters use extremely effective electronic switches to terminate the large transmitter current. Nevertheless very sensitive receivers can still detect small currents that linger in the loop after turn-off. The magnitude of these currents and their time behavior are available from the equipment manufacturer, who can advise the user as to how closely the receiver coil can be placed to the actual transmitter loop wire.

(3) Another source of induced noise, common to ferrite or iron-cored receiver coils, is microphone noise arising from minute movements of the receiver coil in the earth's relatively strong magnetic field. Such movements are usually caused by the wind, and the coil must be shielded from the wind noise, or the measurements made at night when this source of noise is minimal. In extreme cases, it may be necessary to bury the coil.

Presence of Nearby Metallic Structures—TDEM/TEM systems are excellent metal detectors. Use of such systems for resistivity sounding demands that measurements are not made in the presence of metal. This requires removal of all metallic objects not part of the survey equipment (metallic chairs, toolboxes, etc.) from the area of the survey instruments. The recommendations of the manufacturer with regard to the location of the receiver case itself with respect to the receiver coil must be followed carefully.

(1) Power lines can often be detected as metallic targets as well as sources of induced noise. In this case, they exhibit an oscillatory response (the response from all other targets, including the earth, decays monotonically to zero without oscillation). Because the frequency of the oscillation is unrelated to the receiver base frequency, the effect of power line metallic response is to render the transient “noisy” (Fig. 7). Because these oscillations arise from response to eddy currents induced in the power line by the TDEM/TEM transmitter, repeating the measurement produces an identical response, which is one way that these oscillators are identified. Another way is to take a measurement with the transmitter turned off. If the noise disappears, it is a good indication that power line response is the problem. The only remedy is to move the transmitter loop further from the power line.

(2) Other metallic responses, such as those from buried metallic trash or pipes can present a problem. If the response is large, another sounding site must be selected. Use of a different geophysical instrument such as a metal detector or ground conductivity meter is helpful to quickly survey the sounding site for buried metal.

Geologic Sources of Noise—Geologic noise arises from the presence of unsuspected geological structures or materials, which cause variations in terrain resistivity. A rare effect that can occur in clayey soils, is induced polarization. Rapid termination of the transmitter current and thus primary magnetic field can charge up small electrical capacitors at soil particle interfaces. These capacitors subsequently discharge, producing current flow similar to that shown in Fig. 3, but reversed in direction. The net effect is to reduce the amplitude of the transient response (thus increasing the apparent resistivity) or, in severe situations, to cause the transient response to become negative over some portion of the measurement time range. Because these sources of reverse current are most significant in the vicinity of the transmitter loop, using the offset configuration (described in 6.6.1.1) usually reduces the induced polarization effect.

Summary—During the course of designing and carrying out a TDEM/TEM survey, the sources of ambient, geologic and cultural noise must be considered and the time of occurrence and location noted. The form of the interference is not always predictable, as it not only depends upon the type of noise and the magnitude of the noise but upon the distance from the source of noise and possibly the time of day.

Alternate Methods—In some cases, the factors discussed above may prevent the effective use of the TDEM/TEM method, and other surface geophysical methods such as conventional direct current (DC) resistivity sounding (Guide D6431), frequency domain electromagnetic surveying (Guide D6639)non-geophysical methods may be required to investigate subsurface conditions.

FIG. 4 Typical TEM Receiver Output Voltage Versus Time Plot (9)

FIG. 5 TDEM Receiver Output Voltage for Various Earth Models (9)

FIG. 6 Typical Ranges of Resistivities of Earth Materials (5)

FIG. 7 Oscillations Induced in Receiver Response by Power Lines (9)