Summary

Two types of commercial downhole 3-C geophones were modified to improve signal-to-noise ratio, equalize the spectral characteristics of the signal recorded by horizontal sensors, and calculate accurate tool orientation as evidenced by successful numerical rotation of horizontal signals. Common practice to determine azimuth of a downhole geophone with no automatic orientation system is to compute the angle that results in the maximum power of compressional-wave energy recorded by the horizontal components. This method assumes that the signal response of the two horizontal sensors is identical. Downhole seismic data were acquired in a shallow borehole with unoriented downhole geophones. The signals recorded with the horizontal sensors had different spectral and temporal characteristics related to the physical position of the sensors within the tool. Multiple tool modifications and repeat downhole tests were performed to assess possible causes. Physically rotating the horizontal sensors to orient each ±45° to the contact between the geophone and borehole wall equalized the signal response, and the calculated orientation angles resulted in successful numerical rotation.

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