Ground Penetrating Radar (GPR) can be used to locate fractures and fracture zones.  Reflections obtained over a fracture zone are more likely to be scattered that those emanating from the bedrock.  This may allow fractured areas to be recognized.  Figure 1 shows a GPR section over a fracture zone.  In this case, the fracture is sub-horizontal and is seen as a reflector on the GPR section. Basic Concept, Data Acquisition, Data Processing, Data Interpretation, Advantages and Limitations have been discussed in a previous section.

Figure 1.  Ground Penetrating Radar data illustrating a section over a fracture zone.  (Lane, 2000).

Rayleigh Waves Recorded with a Common Offset Array to Map Fractures

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 2 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.  Three shot points are shown, 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.  This affects the measured Rayleigh wave recorded by the geophones over the fracture zone.  The effective depth of penetration is approximately one-third to one-half of the wavelength of the Rayleigh wave.

For interpretation, three parameters are usually observed.  The first is an increase in the travel time of the Rayleigh wave as the fracture zone.  The second parameter is a decrease in the amplitude of the Rayleigh wave.  The third parameter is reverberations (sometimes called ringing) as the fracture zone is crossed.  The data are recorded using a seismograph and geophone.

Figure 2.  Rayleigh wave particle motion and displacement over a fracture zone.

Data Acquisition:  Rayleigh waves are created by any impact source.  For shallow investigations, a hammer is all that is needed.  Data are recorded using one geophone and one shot point. The distance between the shot and geophone depends on the depth of investigation and is usually about twice the expected target depth.  Data are recorded at regular intervals across the traverse while maintaining the same shot geophone separation.  The interval between stations depends on the expected size of the fracture zone and the desired resolution.  Generally, to clearly see the fracture zone, it is desirable to have at least several stations that cross this area.  Figure 3 presents data from a common offset Rayleigh wave survey over a void/fracture zone in an alluvial basin.  The geophone traces are drawn horizontally with the vertical axis being distance (shot stations).

Data Processing:  The data may be filtered to highlight the Rayleigh wave frequencies and is then plotted as shown on figure 3.  Spectral analysis can be performed on the individual traces and may show the lower frequencies over the fracture zone. 

Figure 3. Data from Rayleigh wave survey over a void/fracture zone.

Data Interpretation:  The data shown in figure 3 illustrates many of the features expected over a void/fracture.  The travel time to the first arrival of the Rayleigh wave is greater across the void/fracture and 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 these 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 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.

Seismic Refraction to Map Fractures

Basic Concept:  The seismic refraction method can be used to locate fracture zones.  Surveys for fracture zones are somewhat different from the more conventional application of the seismic refraction method described earlier, which is mapping bedrock topography.  Bedrock fracture zones are recognized from the acter of the first signal arrivals at the geophones, which can attenuate the signal and cause time shifts, as is illustrated in figure 3.

Figure 4.  Seismic Refraction for locating  fracture zones.

In addition to compressional wave surveys for fracture zones, shear waves can also be used.  If a shear wave is oscillating at right angles to its direction of travel, it will be severely attenuated over a fracture zone.  If, however, the shear wave is oscillating along the direction of travel, it will be relatively unaffected by the fracture zone.

For compressional wave surveys, surveys are conducted by traversing across the area of interest and evaluating the acter of the first arrival waveform.

Data Acquisition:  Data is recorded by placing a line of geophones across the area of interest and recording data from shots placed at the ends of the geophone spread.

Data Processing:  No processing is usually required.  The data is plotted so as to allow the acter and amplitude of the first waves to arrive to be viewed.

Data Interpretation:  The data is interpreted by viewing the first wave arrivals and looking for waves whose amplitude is diminished, indicating that a fracture zone has been crossed.

Advantages:  Field recording for this method is quite fast and the data can usually be viewed on the recorder screen as the survey progresses.

Limitations:  The velocity of the bedrock has to be greater than that of the overburden.  If the water table is near the bedrock then this may also provide refractions.  These refractions may interfere with refractions from the bedrock, or the bedrock refractions may not be visible.

Shear Wave Seismic Reflection to Map Fractures

Basic Concept:  Fracture zones can be detected using seismic shear wave reflection surveys.  These surveys are conducted in much the same manner as compressional wave seismic reflection surveys, except that a shear wave source has to be used.  One such source is called the Microvib and is shown in figure 95.  The data are recorded using a standard seismograph (figure 114b) using shear wave geophones.

Figure 5. The Microvib shear wave generator.  (Bay Geophysical)

Figure 6. Seismic Refraction: field set up and data recorder.

Data Acquisition:  Probably the most important difference between shear wave and compressional wave surveys is that, for shear wave surveys, the geophone spacing will be smaller for an equivalent depth of investigation, since the velocity of shear waves is only about 0.6 times that of compressional waves.  The geophone spacing and spread length are also dependent on the expected depth and size of the fracture zone.  Another important quantity to consider is the frequency and wavelength of the shear waves.  Higher frequencies provide better resolution of the detected structures.  However, higher frequencies also attenuate faster and therefore have less depth penetration. 

Data Processing:  Data processing is essentially the same as for reflection seismic surveys.  Since the data are recorded in shot mode, the records are sorted to gathers with a common midpoint.  Various kinds of filtering and other processes are then used to refine the data after which the traces for each gather are summed to produce a single trace whose signal-to-noise ratio is much greater than the unprocessed traces.  Once this procedure has been done for all traces, a plot can be produced showing the seismic reflectors.  Additional processing, such as migration, can also be applied to these data. 

Data Interpretation:  Data interpretation consists mostly of visual observation of the processed seismic records.  Fracture zones produce scattering of the seismic energy, resulting in less energy being reflected back to the geophones on the ground surface.  Areas where the amplitude of the bedrock reflector decreases or fades out will be of interest.  In addition, an increase in travel time to the bedrock reflector may also occur.  Figure 98 shows a shear wave seismic section, although in this case the target was voids.  However, it does illustrate the presentation of the data.

Advantages:  Providing the seismic source can produce high frequencies, the method can provide good resolution.

Limitations:  As with compressional wave seismic reflection the method is quite labor intensive and can require significant data processing.

Resistivity Measurements to Map Fractures

Basic Concept:  Resistivity measurements can also be used to locate fracture zones, since these areas often have a lower resistivity than unfractured bedrock. The method and instruments used are described earlier in this section and will not be described here.

Resistivity measurements can be taken with a simple four-electrode array or with the more sophisticated systems that use addressable electrodes, called Automated Resistivity systems. These systems provide an efficient method for measuring the lateral and vertical variations in resistivity and are the preferred system for locating fracture zones.  Figure 7 shows the Sting/Swift instrument for taking these measurements.

Figure 7. Sting-Swift Resistivity Measuring System. (Advanced Geosciences, Inc.)

Data Acquisition:  With the automated resistivity system, the electrodes and wire connections are made before data recording begins.  Once this is done, the data recording parameters and electrode array to be used are entered into the controller, which is then instructed to record the data. Since this system measures resistivity along the profile at a number of electrode spacings, it provides both lateral and vertical variations in resistivity. If long lines of data are recorded, the system is incrementally moved along the line as the survey progresses.

Fracture zones are also located using Azimuthal Resistivity measurements. With this method the electrode array is rotated about its center while taking resistivity readings. However, the method assumes that the bedrock surface is fairly flat and that no resistivity changes occur within the overburden.

Data Processing:  The data is transferred from the field data recorder to a computer. Generally no processing is required and the data is loaded into interpretation software.

Data Interpretation:  The data can be interpreted using computer software that provides a model whose calculated data fit the field data, thereby providing the variation of ground resistivity both laterally and vertically. A preliminary model is input by the interpreter, and the software then modifies this model until the resulting pseudosection matches that from the field data. The process of obtaining the model whose data fit the field data is called inversion.

Figure 8.  Resistivity measurements used for locating water-bearing fractures. (Quantum Geophysics, Inc.)

Figure 8 shows resistivity data obtained with the Sting/Swift resistivity system that has been inverted to provide a resistivity model showing a depth-versus-distance plot.  As can be seen, the water-bearing fracture zone has a significantly lower resistivity than the host rocks.

Advantages:  Compared to using a single electrode spacing to measure resistivity along a traverse the Automated Resistivity System provides much more data and allows for a much better interpretation.

Limitations:  A resistivity contrast with the host rocks must exist at the fracture zone. Generally, because fracture zones have a higher porosity than unfractured rock, these areas will contain more moisture and have a lower resistivity then unfractured areas.  Since electrodes have to be planted in the ground, areas with hard surfaces require more time to place these electrodes.  Any grounded structure, e.g., metal fences, will influence the data if they are close to the electrode array.  Likewise, buried metal features, e.g., pipes, will also create anomalies.

Geophysical Methods to Map Faults.

Since faults often do not often have clear ground or bedrock surface expressions, techniques used to map the locations of faults need to reach depths beyond the bedrock surface and, therefore, generally need to penetrate to depths greater than about 100 m.

For geophysical methods to be able to locate faults, there must, in general, be some difference in the physical properties of the rocks on either side of the fault.  However, the seismic reflection method is able to map faults that do not have significant physical property differences on either side.  This method provides detailed mapping of reflectors, and when a fault is present, the continuity of these reflectors is often disrupted.  Figure 9 shows a seismic reflection section where faults are imaged.

Figure 9.  Seismic Reflection section showing faults.

If the fault has placed rocks having different densities next to each other, then the gravitational method may be useful. 

A gravity profile across a faulted area will show higher values over more dense rocks and lower values over the rocks with lower densities.  In addition to the rocks having different densities on either side of the fault, it is possible that they will also have different resistivities, in which case electrical soundings may be effective.  Although resistivity soundings can be used, it is more likely that Time Domain Electromagnetic (TDEM) soundings will be conducted since the fieldwork for these soundings is more efficient than that for resistivity soundings.  In addition, the TDEM method provides more depth penetration with less influence from lateral variations in resistivity.