Electrical resistivity is a geophysical technique for studying sub-surfaces structures by electrical measurements made at the surface, er by electrodes in one or more boreholes. It is closely related to the medical imaging technique called electrical impedance tomography (EIT), and mathematically it is the same inverse problem. In contrast to the medical EIT however ERTis essentially a direct current method. A related geophysical method, called induced polarization, measures the transient response. The technique evolved from techniques of electrical prospecting that predate digital computers where layers or anomalies were detected. As for other non-invasive surveys methods, the purpose of the geoelectric methods is to obtain information on the nature and structure of the subsurface through a series of measurements carried out on the surface.
The acquisition of a full 3D survey on a large area of investigation is difficult and ,from an economic point of view, very costly. In high resolution 3D surveys, the number of electrodes increases rapidly and the total number of electrode combinations become very large. We propose an innovative 3D acquisition procedure based on the roll along technique. It makes use of 2D parallel lines with additional cross-line measurements. However, in order to increase the number of directions represented in the data, we propose to use cross-line measurements in several directions. Those cross line measurements are based on dipole-dipole configurations as commonly used in cross-borehole surveys. We always use this method of investigation of the subsurface geometry in a karstic environment for a future wind turbine project. We first test our methodology with a numerical benchmark using a synthetic model. Then we validate it through a field case application to image the 3D geometry of karst features and the top of unaltered bedrock in limestone formations. We analyze the importance of cross -line measuring and we analyze their capability of accurate subsurface imaging. The comparison with standard parallel 2D surveys clearly highlighted the added value of the cross -lines measurements to detect those structures. It provides elements in subsurface geometry for the positioning of the future wind turbine foundation. The developed method can provide a useful tool in the design of 3D ERT survey to optimize the amount of information collected within a limited time frame.
The basic components of any acquisition system are: transmitter or current source; receiver which measures the resulting electrode potentials; multiplexer for connecting quickly and automatically the electrodes to the transmitter and receiver; and a computer for system control and data archival.
The instrument used for this method is a fast multi-channel resistivity meter 10-channel system Syscal-Pro produced by IRIS Instruments. It allows the acquisition of several thousand data points in few hours. Under typical conditions, for the Syscal-Pro operating at 4 Hz and stacking 4-5 times, it is possible to acquire data within about 1% error in the magnitude (as determined by comparing reciprocal measurements), at a rate of about 6-7000 measurements per hour.
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