Interferometric Assessment of Borehole-geophone Clamping Quality

Abstract

Borehole arrays are often preferred over surface installations for hydraulic fracture monitoring of deep experiments due to proximity to the treatment zone. Borehole geophone strings are typically clamped to the observation wellbore wall using electromechanical or magnetic devices in order for them to be in close contact with the surrounding formations so as to record the background noise and propagating wavefields related to the microseismic experiments. This contact needs to be maintained throughout the recording time. We have used seismic interferometry to assess the clamping quality of borehole geophone arrays. We suggest that the characteristics of the reconstructed crosscorrelation functions between a reference receiver and other receivers in an array are indicative of the quality of clamping. The dominant contribution of tube waves in correlation functions and emergence of incoherent and fluctuating crosscorrelation functions indicate inadequate coupling while dominant appearance of clean body waves in the correlation gathers suggest a properly maintained coupling. We have applied this method to two different borehole microseismic datasets.