Generation and Measurement of Shear Waves In Situ

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Abstract
Field procedures used to measure in situ shear wave velocity by cross-hole and downhole seismic methods are presented along with typical travel time records. Identification of the initial shear wave arrival is enhanced by use of a reversible torsional source in the crosshole method and a reversible embedded source in the downhole method. Correct triggering of recording equipment is critical in these measurements. Characteristics of three triggering systems—a velocity transducer, a resistance-capacitance (RC) circuit, and an electrical step trigger—are presented. Incorrect triggering can cause errors greater than 50 percent in field measurement of shear wave velocity. Other variables such as borehole casing, borehole disturbance, and source and sensor configuration also affect velocity measurement. The effects of many of the variables can be minimized by basing wave velocity computations on interval travel times of the initial arrival. It is recommended that any ASTM standards for crosshole and downhole seismic methods include field check procedures for correct timing and triggering of recording equipment and field measurement of borehole verticality.

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