Borehole Effects On Downhole Seismic Measurements

Abstract

An exact formulation for borehole coupling, which is valid for all frequencies and all azimuthally symmetric and nonsymrnetric components, is given in this paper. The borehole effects on downhole measurements are studied in detail as functions of frequency, incidence angle and polarization of an incident wave as well as geophone orientation. We found that correction of the borehole effect for downhole measurements should be made for frequencies above 500 Hz in a hard formation. In a soft formation, if the incidence angle is well away from the resonance angle for a SV incidence, no borehole correction is needed for frequencies below 300 Hz; while for frequencies above 300 Hz, the borehole can cause severe problems on downhole measurements. The borehole can also significantly alter the particle motion direction such that horizontal components rotation from data itself is unreliable for experiments with frequencies above 1 kHz in the hard formation and around 500 Hz in the soft formation.