INVERSE VERTICAL SEISMIC PROFILING USING A MEASUREMENT WHILE DRILLING TOOL AS A SEISMIC SOURCE

摘要

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 "Vi broseis"-type frequency sweep (preferably from 1 Hz to 50 Hz). With the bit of the MWD tool in conta ct 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 th e 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 sign als obtained by the geophones are processed using known signal processing techniques. The disclo sed downhole frequency-controllable seismic source provides many advantages including goo d resolution due to the ability to output a sinusoidal wave at desired frequencies and the fa ct that the location of the source signal is well-defined (at the bit), the ability to image formati ons ahead of the bit, and the ability to conduct reverse VSP processing without adding downhole hardwa re equipment and without requiring trips out of the well.