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A two-lane tunnel on a busy highway will be expanded. Reflected seismic waves were used to map structural features in the surrounding rock from the inside of the tunnel. The results of seismic imaging, largely in accord with the geological investigation, provided 3D characterization of the rock mass useful for the expansion project. Parts of the concrete liner of the tunnel had separated from damaged rock. This posed a challenge to conducting the seismic ground characterization from inside the tunnel. The uniqueness of the seismic survey approach was not the use of a small, hand-held source driving a swept frequency signals into the rock which allowed conducting the survey without stopping the traffic; nor was it the use of high sensitivity, broad-band accelerometers for detecting seismic waves in the rock. It was the fiberglass waveguides (1 meter long), anchored to the rock via holes drilled through the liner that introduced the uniqueness. These waveguides provided reliable coupling to the rock mass for sources and receivers mounted inside the tunnel near the inner surface of the liner. The sources and receivers, in interchanging arrays, formed 14 sets of "directional seismic antennae" along the tunnel, each deployed around the liner circumference within the limits of having to leave one lane open to traffic. Each "antenna" was used for imaging its own section of the rock mass around the tunnel. Overlapping ground images were then joined to successfully map the whole length of the tunnel.
A decision was made to widen the east-bound (EB) tunnel for one of the short double tunnels in Colorado. To assure an optimal design and a safe and economical construction, a comprehensive geotechnical investigation of the rock mass around and south of this tunnel was launched. The investigation included using reflected seismic waves for imaging anomalous rock mass conditions associated with possible fractures, faults, weathered rock etc.
The major obstacles for this investigation were:
– Elevated attenuation levels particularly for shorter/ higher frequency seismic waves due to often disturbed and fractured rock mass;
– Disruption caused by the traffic in the tunnel; and,
– Concrete liner blocking access for coupling seismic sources and receivers to the competent parts of the native rock over 1 meter behind the inner surface of the liner.
Using very sensitive accelerometers significantly improved detection of the weakest seismic waves by the digital data acquisition system.
The noise problem associated with the traffic was overcome by using a swept frequency seismic source. At the same time the controlled wide spectrum of the source signal enhanced detection of shorter reflected waves, improving the resolution of seismic imaging. And the access to the native ground for sources and receivers was provided through fiberglass rods (waveguides) anchored to the rock through holes in the liner.
The frequencies chosen for this study is 0.3 and 0.7 Hz. Fast Finite-Difference Time Domain Modelling (FDTM) of seabed logging (SBL) is used to study the effect of the spacing between receivers positioned in a regular grid on the seabed. A relatively simple model is used as an example to demonstrate the effect of receiver grid spacing on detectability of high resistive subsurface anomalies. We also show that the results can be refined by extracting azimuth data for large source-receivers distances. Introduction SBL exploration data, introduced by Eidesmo et al. (2002) and Ellingsrud et al. (2002), are usually collected along lines of receivers typically positioned 1 km apart. The electromagnetic (EM) source is towed along the line and subsurface resistive anomalies can be detected by studying the EM response of each receiver relative to a reference. The survey lines are carefully chosen based on geological data previously acquired in the area of interest.
Zhou, Q. (Department of Material Science and Mineral Engineering, University of California) | Becker, A. (Department of Material Science and Mineral Engineering, University of California) | Morrison, H.F. (Department of Material Science and Mineral Engineering, University of California) | Goldstein, N.E. (Earth Sciences Division, Lawrence Berkeley Laboratory) | Lee, K.H. (Earth Sciences Division, Lawrence Berkeley Laboratory)
Audio frequency subsurface electromagnetic (EM) techniques using cross-hole and in-hole arrays for fracture detection are evaluated numerically. The fracture zone is represented by a thin rectangular conductor with finite dimensions, embedded in a conductive host rock. Because of its practical advantages, the EM source considered in this study is a grounded vertical electrical dipole (G.V.E.D.) placed in a vertical bore hole. Three source-receiver configurations are considered. The first is the cross-hole configuration with the source and receiver moving parallel to each other in separate holes. The second configuration is a fixed source in one hole and a moving receiver in the other. Finally, we also treat the case of a tandem source and receiver at fixed separation traversing a single hole. In all cases the conductive fracture zone is not intersected by either hole. Comparisons between the grounded electric dipole and the vertical magnetic dipole indicate clear advantages for the former.
A practical problem of site characterization that occurs in waste repository or geothermal reservoir evaluation, is the detection of any major fracture zone that lies close to but is missed by the drill holes inplace. These holes provide the opportunity to use subsurface electromagnetic techniques for detecting any nearby fracture. A number of geophysical techniques have been used to address the fracture detection problem and each has its own advantages and limitations. To date, however, most of the work centered on the use of conventional geophysical bore hole logs to detect and characterize fractures intersected by the drill (Nelson et at., 1980; Jones et at., 1985). Because most holes are drilled close to vertical, conventional logging techniques are mainly sensitive to flat-dipping structures. Thus to detect the possible presence of a major, steeply-dipping shear or fracture zone that is not intersected by a bore hole, newer geophysical techniques have been studied. Among these is cross-hole seismic/acoustic tomography to map anomalies in the velocity distribution (Peterson, 1986). Electromagnetic techniques have also been studied in this regard because clay-rich shear zones or an extensive open but water filled fracture zones in crystalline rock have a much lower resistivity than the host medium (Green and Mair, 1983). Deadrick et at (1982) and Ramirez et at (1983) used cross-hole electromagnetic geotomography in map- ping fractures. Lytte et at (1979) studied the cross bore hole electromagnetic probing to locate high-contrast anomalies, and Chang et at (1984) developed a down-hole VHF radar apparatus with directional source and receiver antennas in the bore hole. Because of the high attenuation at radar frequencies, lower frequencies may be more suitable for fracture detection between widely spaced holes or where the host rock is more conductive than a low permeability granite. In this paper we consider a down-hole grounded vertical electric dipole (G.V.E.D.) source. The implied assumptions are always that the fracture is a good scatterer of electromagnetic waves, and that it is embedded in a less conductive but otherwise homogeneous region. The G.V.E.D. source has its unique advantages in these conditions.