BOREHOLE EFFECTS ON DOWNHOLE SEISMIC MEASUREMENTS

An exact formulation for borehole coupling, which is valid for all fre quencies and all azimuthally symmetric and non-symmetric components, i s presented. The borehole effects on downhole seismic measurements are studied in detail as functions of frequency, angle of incidence and p olarization of an incident wave as well as geophone orientation. We fo und that correction for the borehole effect on downhole measurements s hould be made for frequencies above 500 Hz in a hard formation. In a s oft formation, if the angle of incidence is well away from the resonan ce angle for SV incidence, no borehole correction is needed for freque ncies below 300 Hz, while for frequencies above 300 Hz, the borehole c an cause severe problems in downhole measurements. The borehole can al so significantly alter the particle motion direction which implies tha t horizontal component rotation from data itself is unreliable for exp eriments with frequencies above 1 kHz in the hard formation and around 500 Hz in the soft formation.