Orientation Method Applied to onshore 3-Component VSP’s Recorded with a Relative Bearing Sensor in Partially Low Deviated Boreholes

Many Iranian rig-source VSP datasets are recorded with the multilevel versatile seismic imager (VSI) VSP tool. Each shuttle of the VSI toolstring contains a Relative Bearing (RB) sensor measuring the roll angle made by the VSP tool arm with the vertical plane locally tangent to the deviated borehole trajectory. The 3C VSP data actually look correctly oriented using the roll angle provided by the RB sensor where the hole vertical inclination is over 8°. The whole orientation into geographical system was done using the hole inclination and azimuth angles provided by the borehole survey recorded during the wireline logging operations. A method was designed at the preprocessing stage to orientate the VSP data recorded below the 8° hole inclination threshold, by finding the RB angle differences level to level so as to render the downgoing Shear VSP wavetrain coherent on adjacent VSP stations. This was followed by calibrating the true RB angle on the VSP levels located in the hole intervals with inclination over 8°, whether the hole is cased or not. The present paper focuses on the principles of this orientation method, successfully tested on a couple of 3 component onshore VSP datasets made available by NIOC.