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Cross-hole Seismic

Overview

A three-axis geophone is lowered into a specially-prepared, cased borehole and excited with a hammer source in a similar borehole nearby. This procedure is covered by an ASTM guideline but has several variants. Invariant are requirements to measure both P and S waves and to use a reversible source to aid in the identification of the S arrivals.

Geology

The geologic model is of rock materials which are homogeneous, at least laterally, on the scale of the test. Usually the boreholes are about ten feet apart. The standard calls for three holes (one source and two receivers) but if geologic variation is known to be small, two holes are an acceptable alternative. Another variant is a downhole test where the source is left at the surface and only one hole is used. Where homogeneity is geologically probable or results are not critical, this variation can be cheaper.

Logistics

A reversible source as mentioned above and a downhole survey of the boreholes are required. Most drillers can drill a straight hole when requested. A lateral deviation of a foot at depth inserts a large error into the spacing measurement of about ten feet. The completion of the borehole to the ASTM standard is probably the most important part of the procedure.

After the holes are drilled, a single crewperson can make the measurements. However, an untrained second person makes the work more efficient, especially where several sites need to be measured.

Considerations

Not measuring the borehole deviation makes the test almost meaningless. Bad completion (loose casing, grout poured in from the top, etc.) destroys the quality of the waveforms. Strong bedding with high-speed stringers may mask low-velocity zones of interest especially if the boreholes are placed far apart.

Deliverables

Deviation survey results, depth profiles of S and P wave velocities and tables of the elastic constants (Young’s modulus, shear modulus, Poisson’s ratio) based on client-supplied densities, and site maps with borehole locations.

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