Drill bit noise imaging without pilot trace, a near surface interferometry example
Abstract. Acoustic energy emitted by drill bit can be recorded by geophones on the surface and processed for an image of the subsurface using seismic interferometry methods. Pilot sensors record bit signal on the drill rig and play an important role in processing geophone traces for the image. When pilot traces are not available, traces of the nearest geophone to the rig may be used in deconvolution and crosscorrelation of data but extra signal processing efforts are required to reduce the effect of source signature on crosscorrelation results. In this study, we use seismic interferometry method to image the shallow subsurface beneath a 2D geophone array by converting geophones to virtual sources. As there is no pilot signal available for this survey, we use nearest geophone trace for pilot crosscorrelation and pilot deconvolution. We modify the spectrum of pilot crosscorrelation and deconvolution results so that the effect of source function on virtual data is minimized. We then migrate the virtual shots and compare the results of interferometric imaging with the available image from 3D (active source) survey and assess the efficiency of our approach. We show that drill bit noise data can be used to generate a reasonably accurate image of the subsurface even in the absence of pilot recordings but the results should be checked for appearance of virtual multiples and depth inconsistencies that are caused by errors in the migration velocity.