Schlumberger Technology Corporation

Peter C Masak

David L Malone

Daniel Lerner

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.

Inverse vertical seismic profiling using a measurement while drilling

An integrated modulator and turbine-generator includes a turbine impeller which is directly coupled by a drive shaft to a modulator rotor downstream from the impeller. The modulator rotor is further coupled by a drive shaft and a gear train to a three phase alternator downstream of the modulator rotor. The modulator stator blades are arranged downstream of and adjacent to the modulator rotor and the alternator is provided with a Hall effect tachometer. The turbine impeller directly drives the modulator rotor and the alternator generates power. The speed of rotation of the modulator rotor is adjusted by reference to the speed of rotation of the alternator as indicated by the tachometer and to a reference frequency. A control circuit including an electromagnetic braking circuit coupled to the tachometer and the stator windings of the alternator stabilizes the alternator speed and thus the rotor speed and modulates the rotor to obtain the desired frequency of the mudborne pressure wave by selectively shorting the stator windings of the alternator. During periods when braking is not applied, the alternator generates power for control and sensor electronics.

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Missouri City, TX, United States of America

Peter C Masak