초록 ▼

A reverse VSP system is provided and uses a frequency-controllable modulator of an MWD tool as a downhole seismic source. The modulator has a rotor and stator arranged to provide substantially sinusoidal outputs, and is controlled to provide a "Vibroseis"-type frequency sweep (preferably from 1 Hz t

A reverse VSP system is provided and uses a frequency-controllable modulator of an MWD tool as a downhole seismic source. The modulator has a rotor and stator arranged to provide substantially sinusoidal outputs, and is controlled to provide a "Vibroseis"-type frequency sweep (preferably from 1 Hz to 50 Hz). With the bit of the MWD tool in contact with the bottom of the borehole, the force generated by the frequency-controllable modulator will generate measurable axial compressional (P-) waves. These waves which move through the formation are detected by an array of geophones on the formation surface, and a reference signal is obtained by a kelly-mounted accelerometer or strain gauge. The reference signal and signals obtained by the geophones are processed using known signal processing techniques. The disclosed downhole frequency-controllable seismic source provides many advantages including: good resolution due to the ability to output a sinusoidal wave at desired frequencies and the fact that the location of the source signal is well-defined (at the bit), the ability to image formations ahead of the bit, and the ability to conduct reverse VSP processing without adding downhole hardware equipment and without requiring trips out of the well.

대표청구항 ▼

[ We claim:] [1.] A vertical seismic profiling system for obtaining information regarding a formation traversed by a borehole having borehole fluid therein, said system comprising:a frequency controllable downhole seismic course comprising a drill string tool, said tool having formation contact mean

[ We claim:] [1.] A vertical seismic profiling system for obtaining information regarding a formation traversed by a borehole having borehole fluid therein, said system comprising:a frequency controllable downhole seismic course comprising a drill string tool, said tool having formation contact means for causing said tool to be coupled to the formation, a motor with a rotating drive shaft, a rotor coupled to said drive shaft, a stator which is stationary relative to said rotor, with said rotor and stator arranged to produce substantially sinusoidal signals in the borehole fluid as said rotor turns relative to said stator, and a microprocessor means coupled to said motor, said microprocessor means for causing said motor to turn said rotor such that said substantially sinusoidal signals change in frequency in a unidirectional frequency sweep over a predetermined frequency range while said tool is coupled to the formation;a plurality of geophone means located on a surface of said formation, said geophone means for receiving and recording seismic waveform information relating to said sinusoidal signals produced by said frequency controllable downhole seismic source;reference signal means located at said surface for obtaining a reference signal related to said sinusoidal signals produced by said frequency controllable downhole seismic source; andsignal processing means for receiving and processing said seismic waveform information recorded by said plurality of geophone means and said reference signal obtained by said reference signal means.