Experimental techniques in VSP recording and tube wave suppression as applied in the Northern Mount Isa Basin

Abstract

This paper reports the first implementation of a downhole airgun and tube wave baffle to VSP recording in Australia. A downhole airgun, offset from the wells, was recently applied to the acquisition of VSPs in petroleum exploration in the northern Mount Isa Basin. Despite excellent source-to-ground coupling, good bandwidth and repeatability of shots, the recorded data were of poor quality. This is considered to be a consequence of both interference from tube wave energy and geology, rather than VSP source characteristics. In one of the wells, recording of the full VSP wavelet could not be reliably achieved for receiver depths greater than 1000 m, despite using a high powered airgun with a volume of 120 cubic inches. Field displays showed that the data were severely contaminated by tube wave energy which, in some cases, was 60 dB above reflected signal strength. Attempts were made to reduce the tube wave in the field by lowering the mud column in the well and by attaching a tube wave baffle above the receiver. In both cases there was little improvement in relative signal strength. However, the lowered mud column did attenuate the tube wave at high frequencies. A significant improvement in data quality was achieved by aligning, enhancing and subtracting the tube wave in processing. Unfortunately the requirement for pre-drilled pilot holes negated offsetting the source further from the well to delay and attenuate tube waves during recording. This would obviously have been a better approach to tube wave reduction so that the dynamic range of the recording system was tuned principally to signal. Extensive fracturing above targets and multiple velocity inversions within the sequence drilled also contributed to the relatively poor signal strength.