Seismic reflection experiment with a drill-bit source
, by Clement Kostov
The "seismic reflection experiment with a drill-bit source" consists of recording ambient
noise during the drilling of a well by means of an array of geophones laid on the earth's
surface. Goals of the experiment are to estimate acoustic velocities, locate the drill bit
and reflectors, and image the subsurface similarly to conventional surface sesmic or VSP
surveys. Two challenges in processing data from this novel experiment are the separation
of weak drill-bit signal from strong, spatially coherent sources of noise, such as
mechanical equipment or refracted waves, and the signature deconvolution that compresses
a long wave-form into an impulse.
To obtain a section comparable to data from a VSP experiment, I apply a processing
sequence that includes the attenuation of coherent noise by dipfiltering, the estimation
of the drill-bit signature, the cross-correlation with the estimated signature, and the
stack along offset after normal-moveout (NMO) correction. A stacked section for one depth
interval shows an event that is continuous with depth and has the expected time-delay
and moveout for a signal reflected at about 200 meters below the drill-bit.
An alternate processing sequence, not yet tested on the field data, would replace
several of the stacking estimates with optimal least-squares estimates. In particular,
the drill-bit signature could be estimated by solving a constrained multichannel least-
squares problem. An efficient time-adaptive implementation of this algorithm, whereby the
signature is estimated from a sliding window of data, can be achieved in the time-offset
domain by applying Givens rotations.
STANFORD