In the above list, resistivity and conductivity measurements are separated because they involve different instruments, and there are important differences in the application of the techniques.

Methods

Since all of these techniques, except VLF surveys, have been discussed in detail earlier in this section, only the important factors for locating fractures and weak zones will be discussed in this section.

Resistivity

Basic Concept:  The resistivity method can be used to locate fracture zones and weak zones if they have a resistivity contrast with the host rocks.  There are two common resistivity methods: soundings and traverses.  Resistivity traverses are used to map changes in the bedrock and overburden resistivity. If the overburden resistivity does not change laterally, the method can be used to map variations in the elevation of the bedrock surface. Resistivity traverses are often done using only one electrode spacing and, therefore, do not provide the variation of resistivity with depth.  Resistivity soundings provide the resistivities and depths of the layers under the sounding site.  Lateral variations in resistivity, or conductivity, can be obtained much more efficiently using electromagnetic methods with instruments such as the EM31 and EM34. Hence, resistivity traverses using one electrode spacing are rarely used and will not be discussed further. However, automated resistivity systems are now available that record data from several different electrode spacings very efficiently, combining both traverse and sounding data.

These newer instruments, called automated resistivity systems, use electrodes that are addressable by a central control unit.  This means that a large number of electrodes can be placed in the ground prior to starting the survey and connected to the central control unit.  The data recording parameters and the electrode array to use are input to the central control unit.  Once the electrodes are all connected, the measurements are automatically recorded by the central control unit.  Resistivity soundings provide useful information and will be discussed in this section.  Resistivity traverses have now largely been replaced by conductivity measurements, since these are much easier to acquire.

The resistivity of the ground is measured using a number of electrode arrangements, some of which are illustrated in figure 2.  In all of these arrays, current is injected into the ground using two electrodes and the resulting voltage is measured using the remaining two electrodes. 

Figure 2. Electrode arrays used to measure resistivity.

The electrode arrays are used for different types of resistivity surveys.  The Schlumberger array is often used for resistivity soundings, as is the Wenner array. The Pole-pole array provides the best signal, but is cumbersome because of the long wires required for the remote electrodes and is rarely used. The Dipole-dipole array was originally used mostly by the mining industry for induced polarization surveys.  Readings were taken using several different separations of the voltage and current dipoles providing measurements of the variation of resistivity with depth.  Long lines of data were recorded requiring many readings. This array has now become common for resistivity surveys using the automated resistivity systems. If more signal (voltage) is needed than can be provided with the Dipole-dipole array, the Pole-dipole array can be used.

Resistivity Soundings

Figure 3 shows one of these electrode arrangements, called the Schlumberger array, and illustrates its use for obtaining a resistivity sounding.

Figure 3.  Electrode array for (a) measureing the resisivity of the ground, and (b) a resisitivity sounding curve.

Data Acquisition:  In figure 3a, current is passed into the ground using the two electrodes labeled A and B.  The voltage that results from this current is then measured using electrodes M and N.  Using the amount of current passed into the ground along with the voltage and a geometric factor for the electrode layout, the resistivity of the ground is calculated.  The electrode array is then expanded, making the current penetrate deeper into the ground, and another reading is taken. This procedure is repeated for many electrode spacings providing a set of resistivity values for each of these spacings.  These values are plotted on a graph of resistivity against electrode spacing, as illustrated in figure 3b.  This graph shows that, at small electrode spacings, the measured resistivity approaches that of the overburden, whereas at large electrode spacings, the measured resistivity approaches that of the bedrock.  The resistivity curve is interpreted using software that provides a resistivity model (depths and resistivities) whose resistivity calculations match the field data. 

Resistivity soundings are used to measure the vertical distribution of resistivity in the ground, and hence can be used to map the existence of a conductive shale layer that may occur beneath a more competent layer. Resistivity sounds alone are probably not a useful method for locating fractures. When conducting the survey, it is advisable to plot the measured resistivity versus electrode spacing in the field to ascertain that the deeper layers of interest have been observed.

Data Processing:  Usually little processing is needed although bad data points may be removed.

Data Interpretation:  Resistivity data are interpreted using software that calculates the sounding curve from a resistivity model.  The interpreter inputs a preliminary model, and the computer calculates the sounding curve from this model and evaluates the fit of this model data to the field data.  It then changes the model to provide an improved fit and recalculates the resistivity curve.  The process, called inversion, is repeated until the curve from the model matches that from the field data. 

Advantages:  Resistivity soundings are probably more useful than seismic refraction method for locating weak zones, since weak zones are likely to have a lower velocity than the overlaying rocks and seismic refraction will not see the top of the lower velocity layer.

Limitations:  Since electrodes have to be planted in the ground, this can be a problem is dry or rocky areas.  In such conditions, water may need to be poured onto the electrodes in order to lower the resistance between the electrode and the ground.

Automated Resistivity Systems

Data Acquisition:  As mentioned previously, newer resistivity systems are now available that make taking resistivity measurements much more efficient, and by taking many measurements at different electrode spacings along a traverse, these systems are able to build a comprehensive picture of the subsurface, combining both lateral and vertical variations in resistivity.  Figure 4 shows the Sting/Swift automated resistivity measuring system.  To take resistivity measurements, the electrodes along a traverse are inserted into the ground and connected to the wires leading the controller.  The controller is programmed with the desired electrode array to use along with other factors and then instructed to take the measurements.

Figure 4. String-Swift Resistivity Measuring System. (Advanced Geosciences, Inc.)

Data Processing:  Usually little processing is applied to the data, apart from removing any bad data points.

Data Interpretation:  Data from these systems are interpreted by using software that calculates the theoretical resistivity data from a resistivity/depth model, thus enabling a model to be developed that produces data that matches the field data, a process called inversion.  However, much information can be gained visually.  An example of the results from such a survey is presented in figure 5.  These data show two low-resistivity zones, one of which was shown to be a water- filled fracture zone.  The data have been inverted showing the resistivity variation, both laterally and vertically, against depth. 

Fracture zones can also be located using a method called Azimuthal Resistivity surveys.  In this method, resistivity is measured using an appropriate electrode array and spacing, say the Schlumberger array.  The array is then rotated about its center a few degrees, and another reading taken.  This process is repeated until readings have been taken as the array rotates through 360 degrees. Variations in resistivity as the array rotates are used to locate fracture zones.  However, the method assumes that the bedrock surface is fairly flat, and that there are no resistivity changes in the overburden.  Many azimuthal soundings will be required to adequately cover an area.  Data recorded with the automated resistivity system probably provide data that offer a better interpretation, and the system is probably more efficient.

Advantages:  Using the Automated Resistivity systems, fairly comprehensive data sets can be efficiently recorded and interpreted.  Provided the geology is appropriate these systems can be used to locate fractures and weak zones.

Limitations:  Three different methods (traverses, soundings, and automated resistivity systems) of using the resistivity method have been discussed above.  All of them require electrodes be placed in the ground; thus, the method is difficult to use in areas where the surface of the ground is hard, such as concrete or asphalt-covered areas. In addition, if the ground is dry, water may need to be poured onto the electrodes to improve the electrical contact between the electrode and the ground.

Figure 5.  Resistivity data over a fracture zone.  (Advanced Geoscience, Inc.)

Resistivity soundings are a useful tool and can provide the resistivities and depths of layers under the sounding site.  However, the interpretation assumes horizontal layers with no lateral variations in resistivity.  If this is not the case, the interpretation will be incorrect.  Generally, with the Schlumberger array, the separation between the current electrodes will need to reach a maximum of about three times the investigation depth.  If the bedrock is 15 m deep, the current electrodes will need to be spaced up to 45 m apart. 

Clearly, the automated resistivity system provides the best interpretation, although it also requires the most effort.  With this system, lateral variations in resistivity are recognized and resolved in the inversion process.  Since the data are recorded along a line, the interpretation necessarily assumes that there are no variations in ground resistivity normal to the line, which is probably not true.  Thus, errors in the interpretation will occur, depending on the resistivity variations normal to the survey line.  This problem can be minimized by conducting parallel traverses or by conducting three-dimensional surveys.

Time Domain Electromagnetic Soundings

Basic Concept:  Time Domain Electromagnetic soundings (TDEM) are used to obtain the vertical distribution of resistivity.  This method is particularly well suited to mapping conductive layers.  To a significant degree, this method has now superseded the resistivity sounding method since it requires less work for a given investigation depth and generally provides more precise depth estimates.  However, resistivity soundings are still useful for shallow investigations or when resistive targets are sought.  

TDEM soundings are an electromagnetic method used to provide the vertical distribution of resistivity within the ground.  A square loop of wire is laid on the ground surface.  The side length of this loop is about half of the desired depth of investigation.  A receiver coil is placed in the center of the transmitter loop.  Electrical current is passed through the transmitter loop and then quickly turned off.  This sudden change in the transmitter current causes secondary currents to be generated in the ground.  These decaying currents have associated decaying electromagnetic fields that produce a voltage in the receiver coil on the ground surface. The TDEM field layout, transmitter current, and received voltage are illustrated in figure 6.

Figure 6. Time Domain Electromagnetic Soundings.

The currents in a conductive layer decay at a slower rate than those in a resistive layer. The relationship between the time after the current has turned off (delay time) and the depth and resistivity of the layers is complex, although longer delay times generally correspond to greater depths. 

The voltage measured by the receiver coil does not decay instantly to zero when the current is turned off, but continues to decay for some time.  This decaying voltage is caused by the decaying secondary electrical currents and the associated electromagnetic fields in the ground.  The voltage measured by the receiver is then converted to resistivity. A plot is made of the measured resistivity against the delay time, as illustrated in figure 7.

Figure 7. A Time Domain Electromagnetic Sounding curve.

The resistivity sounding curve illustrates the curve that would be obtained over three-layered ground.  The near-surface layer has a higher resistivity than the middle layer.  The final layer again has a higher resistivity.  Figure 7 illustrates the type of curve expected over a resistive, competent limestone resting on a less competent shale.

The Protem receiver and transmitter, which are used to record TDEM data, are manufactured by Geonics Ltd of Canada and are illustrated in figure 8.

(a)                                                                                                                (b)

Figure 8.  Protem transmitter and receiver for TDEM measurements: (a) Protem transmitter, and (b) Protem receiver.  (Geonics, Ltd.)

Data Acquisition:  The field layout of the system for TDEM soundings is described above and is shown in figure 6.  Switched current is passed through the transmitter loop and the resulting voltage is measured by the receiver coil.  The switching and measuring procedure is repeated many times allowing the resulting voltages to be stacked and improving the signal-to-noise ratio.  Soundings are conducted at different locations until the area of interest has been covered. A sounding curve is plotted for each location showing the measured resistivity against delay time.

Data Processing: The only processing required is the possible removal of bad data points.

Data Interpretation: The sounding curves are interpreted using computer software that modifies a preliminary model input by the interpreter until the data from this model match that from the field. This process is called inversion.

Advantages:  The TDEM method does not require electrodes to be inserted into the ground, as is the case with the resistivity method, and hence is an efficient method for obtaining resistivity soundings.

Limitations:  Metal objects in the vicinity of the sounding site will create electromagnetic fields that will be detected by the receiver coil. This will distort the data from the ground and may produce data that are not interpretable.  In addition, the interpretation assumes that the geologic layers are horizontal and have resistivities that do not change laterally.  If this is not the case, the interpreted resistivities and depths may be incorrect.

Conductivity Measurements

Basic Concept:  Conductivity methods can be used to map the lateral variations in bedrock conductivity (or its inverse, resistivity).  Both the EM31 and EM34 can be used to record conductivity. They are designed to operate at particular frequencies and with defined transmitter and receiver coil separations such that the out-of-phase component of the received signal can be converted to apparent conductivity. These instruments use a transmitter and receiver coil, which are coplanar.  Sinusoidal current, oscillating up to about 10 kHz (depending on the instrument) is passed through the transmitter coil, generating a sinusoidal electromagnetic field.  This field then generates secondary sinusoidal currents in the ground, which, in turn, produce secondary electromagnetic fields that the receiver coil detects.  Conductivity measurements can be recorded with the plane of the coils either parallel to the ground surface (vertical dipole mode) or orthogonal to the ground surface (horizontal dipole mode). The anomaly measured in the vertical dipole mode over a vertical, or sub-vertical conductive feature is quite distinctive and is diagnostic of that feature.  Figure 9 shows the anomaly over a conductive feature (fracture zone) using an EM31 or EM34 in vertical dipole mode.  In the horizontal dipole mode, only a weak anomaly would be observed, and this mode is not recommended for locating vertical conductive features.

Figure 9. Anomaly from an EM31/34 used in vertical dipole mode over a conductive fracture zone.

As the conductive feature is approached, the measured conductivity increases.  When the transmitter and receiver coils straddle the conductive zone, the measured conductivity values decrease, reaching a minimum when they are equispaced about the conductive zone.  This distinctive anomaly shape provides a good method for locating conductive fractures.  However, this method does not provide the depth to the bedrock, and a resistivity sounding will need to be done if this is required. 

Figure 10 shows the EM31 being used in the vertical dipole mode, and figure 11 shows the EM34 being used in the horizontal dipole mode.

Figure 10. The EM31-MK2 Instrument. (Geonics, Ltd.)

Figue 11. The EM34 being used in horizontal dipole mode. (Geonics, Ltd.)

Since, with the EM34, a cable connects the transmitter and receiver coils, their separation can be changed, thereby altering the depth of investigation.  Three separations are available, 10, 20, and 40 m, providing investigation depths in the vertical dipole mode of 15, 30and 60 m.

Data Acquisition:  Conductivity measurements are taken along traverses crossing the area of interest. Probably the most important decision is the particular electromagnetic system to use, since this determines the depth of investigation.  The EM31 has its coils mounted in a rigid boom, and its depth of investigation in the vertical dipole mode is about 6 m.  However, the EM34, as discussed above, has three discrete depths of investigation in the vertical dipole mode.

The EM31 can record data either in timed mode, i.e., every second, or when a button is pressed on the instrument (discrete mode).  In timed mode, readings are taken while walking at a moderate speed.  With the EM34, readings are taken while the instrument is stationary.  Both the EM31 and EM34 require the station spacing be sufficient to accurately portray the shape of the resistivity curve over a conductive feature. This generally means that the station spacing should be one-fourth to one-third of the coil spacing.

Data Processing:  The data are usually plotted as conductivity values against distance, and usually no processing is required. 

Data Interpretation:  Interpretation is mostly visual, searching for the distinctive pattern shown in figure 9.

Advantages:  Surveys with the EM31 and EM34 are generally efficient and large areas can be covered much more rapidly than with other methods, including resistivity systems.

Limitations:  The EM31 provides a rapid method of measuring the ground conductivity, but only to a depth of about 6 m.  The EM34 provides a much greater depth of investigation, but requires two people to operate and only takes discrete measurements.  However, in spite of these limitations, the method still provides one of the most cost-effective methods of measuring the ground conductivity.  Neither method provides much information regarding the depth of investigation, beyond the general depths estimated for the coil separations. 

These instruments (EM31 and EM34) are also influenced by both buried and aboveground metal; thus, data collected near fences and other metal objects cannot be reliably interpreted.

Rayleigh Waves Recorded with a Common Offset Array

Basic Concept:  This method uses Rayleigh waves (surface waves) to detect fracture zones.  Rayleigh waves have a particle motion that is counterclockwise with respect to the direction of travel.  Figure 12 illustrates the particle motion for Rayleigh waves traveling in the positive X direction.  In addition, the particle displacement is greatest at the ground surface near the Rayleigh wave source and decreases with depth.  The effective depth of penetration is approximately one-third to one-half of the wavelength of the Rayleigh wave.

Three shot points are shown in figure 12, labeled A, B, and C.  The particle motion and displacement are shown for five depths under each shot point.  For shot B, over the fracture zone, the amplitude of the Rayleigh waves is smaller than that for the other shots due to attenuation caused by the fractures and affects the measured Rayleigh waves recorded by the geophones over the fracture zone.  Three parameters are usually observed.  The first is an increase in the travel time of the Rayleigh waves at the fracture zone.  The second is a decrease in the amplitude of the Rayleigh waves.  The third parameter is reverberations (sometimes called ringing) as the fracture zone is crossed.  Rayleigh wave data are recorded using a standard seismograph and geophone system.

Figure 12. Rayleigh wave particle motion and displacement over a void.

Data Acquisition:  Rayleigh waves are created by any impact source.  For shallow investigations, a hammer is all that is needed.  Data are recorded at regular intervals along a traverse using one geophone and one shot point for each measurement, with the shot geophone distance remaining constant. The distance between the shot and geophone depends on the depth of investigation and is usually about twice the expected target depth.  The interval between stations depends on the expected size of the fracture zone and the desired resolution.  Generally, in order to clearly see the fracture zone, it is desirable to have at least several stations that cross this area.  Data from a common offset Rayleigh wave survey over a void/fracture zone in an alluvial basin is presented in figure 100.  The geophone traces are drawn horizontally with the vertical axis being distance (shot stations).

Figure 13. Data from a Rayleigh wave survey over a void/fracture zone.

Data Processing: The data may be filtered to highlight the Rayleigh wave frequencies and are then plotted as shown on figure 13.  Spectral analysis of the individual traces can be performed that may show the lower frequencies over the fracture zone. 

Data Interpretation:  The data shown in figure 13 illustrate many of the features expected over a void/fracture.  The travel time to the first arrival of the Rayleigh waves is greater across the void/fracture and, in this case, is wider than the actual fractured zone.  The amplitudes of the Rayleigh waves decrease as the zone is crossed.  Since the records are not long enough, the ringing effect is not presented in this data.

Advantages:  The field data recording is simple and efficient and requires much less effort for a given line length than seismic refraction.

Limitations:  The method responds to the bulk seismic properties of the rocks and soil, which are influenced by other factors other than voids.  It has a limited depth of penetration and resolution. Penetration depth is limited by the wavelengths generated by the seismic source. However, this method is faster and less costly than most other seismic methods.

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