Mineral Resources Program
Borehole seismic and radar methods characterize fractured bedrock
The contamination of ground water caused by the leachate from mines and their tailings is a significant environmental issue for the minerals industry. Of particular importance is the behavior of leachate in fractured bedrock because the leachate in fractures may migrate at a high velocity. Thus, the first step in the isolation and remediation of contaminated ground water in bedrock is finding those fractures that have a high hydraulic conductivity. Although invaluable information about fractures can be obtained from data collected in monitoring wells (that is, core, geophysical logs, packer tests, and tracer tests), the orientation and spatial connection of the fractures between the wells also are needed. Some of this crucial information may be obtained with borehole seismic and radar methods.
For the borehole seismic method, a source in a well (or on the surface) is used to generate a seismic wave, which propagates through the ground. As the wave passes a second well, it is detected by an array of receivers in that well. The source is then moved to another location, and more data are acquired. This process is repeated many times so that the ground between the wells is thoroughly sampled along many different paths. To process these data, the time required for the wave to travel from each source to each receiver must be measured; a parameter estimation algorithm is then used to determine the speed of propagation (velocity) in the region between the wells. The result is a map or image showing spatial variations in speed (fig. 1). To interpret the image, the speeds adjacent to the well must be correlated with information from the well (for example, core, geophysical logs, and so on). After this correlation is established, the image can be used to interpolate the properties of the region between the wells. This borehole seismic method is often called seismic velocity tomography.
There are three common borehole radar methods. One method is radar velocity tomography in which radar waves are used to probe the ground between two wells. These radar waves are generated by a transmitting antenna in one well and are detected by a receiving antenna in the other well. The procedures for processing the propagation times and interpreting the velocity images are similar to those used for seismic velocity tomography. The second borehole radar method is radar attenuation tomography. Instead of processing the propagation times, this method processes the amplitudes of the waves. The result is an image showing the attenuation of the radar wave between the wells. This attenuation image is interpreted by correlating it with information from the wells. In the third borehole radar method, a transmitting antenna and a receiving antenna are placed within the same well. Radar waves from the transmitting antenna are reflected back to the well and detected by the receiving antenna. The method is often called single-well radar and is especially good for finding heterogeneities like fractures.
U.S. Geological Survey scientists have applied borehole seismic and radar methods in hydrologic investigations. Seismic velocity tomography can be used to differentiate the region between the wells into zones with low or moderate probabilities of a high hydraulic conductivity. Radar attenuation tomography and brine traces can be used to map fractures having high hydraulic conductivity. Single-well radar can determine the orientation of fractures near a well.
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