Interferometric assessment of clamping quality of borehole geophones

Abstract

Borehole arrays are often preferred over surface installations for hydraulic-fracture monitoring of deep experiments due to their 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 and 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 determined that the characteristics of the retrieved crosscorrelation functions between a reference receiver and other receivers in an array are indicative of the clamping quality of the former geophone to the borehole wall. We have also defined the concept of separation frequency or emergence frequency as the frequency below which direct body waves propagating along the receiver line are clearly observed on the crosscorrelation gathers. The crosscorrelation gathers associated with poorly clamped geophones show predominantly tube waves or incoherent waveforms. Body waves only emerge below very low separation frequencies. The crosscorrelation gathers of relatively better coupled geophones, on the other hand, have higher separation frequencies. We have applied this method to four different borehole microseismic data sets, labeled here as A, B, C, and D, of which data set D was previously known to suffer from some clamping issues. Data sets B and C with inferred better coupling had separation frequencies of approximately 60 Hz, whereas the other two data sets are characterized by lower separation frequencies, 15 Hz for data set A and 20 Hz for data set D, suggesting relatively poorer coupling.