f Borehole coupling effects on clamped geophone responses: experiments and theory

Seismic waves detected by clamped geophones in a borehole can be affected by the borehole at high frequenties (Chang et al, 1989). A 20 cm diameter water filled borehole resonates in anti-syminetric mode at approximately 4 kHz and in symmetric mode at approximately 9 kHz. Below 2 kHz, the clamped geophone response is a combination of monopole, dipole and quadrapole vibirations in the borehole. To illustrate these effects, a, crosshole experiment was perfomed at the Imperial College borehole test site. Figure 1 Shows veldcity and gamma logs from the site and the source and receiver distribution in two, vertical holes. A three component clamped accelerometer was positianed at. 65m and 105m depth and detonators were fired at 5m intervals from 30m to 105m in the other Hole. The inter-hole spacing is 25m . The accelerometer was specifically designed to be free of resonances up to at least 2.5 kHz . Figure 2 (a.) and (b) show the recorded waveforms and the corresponding first arrival P wave hodograms from the radial and axial components of the accelerometer when positioned at 65m and 105m depth respectively. Depths in these figures refer to the positions of the source . The dominant frequency of these data. is at 1 kHz. Note that there is very little axial vibration when the source and the receiver are at approximately the same depth where as at, larger depth offsets, the hodograms are elliptical. It is conceivable that some hodogram ellipticity might result from interference between the direct and reflected or refracted waves within the multi layered medium. Full elastic wave finite difference modelling was perfomed with a model based on the borehole sonic and density logs. Hodograms calculated from these synthetic seismograms are shown in figure 2 (b) (labelled MODEL) and are seen to be essentially linear.