Abstract

Receiver tests were conducted to compare the responses of downhole geophones and hydrophones. Commercial receiver tools use a maximum of eight geophone levels; however, we use hydrophones because we can record 48 levels simultaneously. For frequencies above 300 Hz, signal-to-background-noise ratios for hydrophones and geophones in a prototype tool were comparable. (This prototype tool is a light-weight, large-clamping-force device that can record higher frequencies than commercial geophone tools.) For frequencies below 300 Hz, signal-to-noise ratios were greater for the geophones than for the hydrophones. A commercial geophone tool had lower low-frequency signal-to-background-noise ratios than the prototype tool, but greater than those of the hydrophones.Further analysis was performed to determine why the signal-to-background-noise ratios for geophones were greater than those for hydrophones at low frequencies. The measured signal level for a hydrophone was 2.4 times that for a geophone, compared with a theoretical prediction of 1.8. Thus, the signal levels do not explain the difference in signal-to-background-noise ratios. The low-frequency background noise was attributed to coherent noise in the form of tube waves, a noise type to which hydrophones are much more susceptible than are geophones. Thus, the low signal-to-background-noise ratios at frequencies below 300 Hz for hydrophones resulted from ambient noise propagating as tube waves in the borehole. The high-frequency background noise was attributed to random seismic noise in the environment and not to instrument noise.These results show that hydrophones, which do not need to be clamped to the borehole wall, are preferable to geophones for high-frequency borehole seismic applications using first arrivals. Geophones are preferable to hydrophones for borehole seismic applications using reflector arrivals, because these later-arriving events are obscured by source-generated tube waves in hydrophone data. Development of a method to reduce both the source-generated and ambient tube-wave noise detected by hydrophones would result in high-quality borehole seismic data at a greatly reduced cost.

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