Abstract
Abstract. Acoustic energy emitted by drill bits can be recorded by geophones on the surface and processed for an image of the subsurface using seismic interferometry methods. Pilot sensors record bit signals 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 cross-correlation of data, but extra signal processing efforts are required to reduce the effect of source signature on cross-correlation results. In this study, we use the seismic interferometry method to image the shallow subsurface beneath a 2-D geophone array by converting geophones to virtual sources. As there is no pilot signal available for this survey, we use the nearest geophone trace for pilot cross-correlation and pilot deconvolution. We modify the spectrum of pilot cross-correlation 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 3-D (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 the appearance of virtual multiples and depth inconsistencies that are caused by errors in the migration velocity.
Highlights
A significant level of acoustic energy generated by the drill bit hammering action at the bottom of a borehole is transferred into the geological formations surrounding the borehole and can be recorded by an array of geophones on the surface and processed for a subsurface image under the receiver array
The objective was to check the accuracy of interferometric migration by comparing the results with the results of active source migration
We only used while drilling data recorded along line 5 for well BH001 to produce interferometric shots on the surface
Summary
A significant level of acoustic energy generated by the drill bit hammering action at the bottom of a borehole is transferred into the geological formations surrounding the borehole and can be recorded by an array of geophones on the surface and processed for a subsurface image under the receiver array. Yu and Schuster (2006) present a workflow for migrating virtual source data (ghost and direct wave migration) generated by cross-correlation and stacking of IVSP data which can be applied to drill bit passive measurements on the surface. Make the matter even worse, windowing out the ghost arrivals from the direct P-wave event may not be an option in drill bit imaging if the pilot deconvolution has not been effective in converting the bit signal to an impulse In such cases, full trace recording is cross-correlated with the traces of other geophones to produce virtual shot records, but the migration results must be scrutinized for the existence of coherent virtual multiples. We will discuss the results of this study and propose our recommendations and conclusions at the end of this paper
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