Geophysical Prospecting, 2010, 58, 257–265 doi: 10.1111/j.1365-2478.2009.00832.x Drill-bit seismic interferometry with and without pilot signals ∗ Flavio Poletto † , Piero Corubolo and Paolo Comelli Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/c, 34010 Sgonico (TS), Italy Received February 2009, revision accepted August 2009 ABSTRACT We use different interferometry approaches to process the seismic signals generated by a drill-bit source in one well and recorded by seismic receivers located both in a second borehole and at the surface near the source well. We compare the stan- dard interferometry results, obtained by using the raw drill-bit data without a pilot signal, with the new interferometry results obtained by using the drill-bit seismo- grams correlated with a reference pilot signal. The analysis of the stationary phase shows that the final results have different S/N levels and are affected by the coher- ent noise in the form of rig arrivals. The interferometry methods are compared by using different deconvolution approaches. The analysis shows that the results agree with the conventional drill-bit seismograms and that using the reference pilot sig- nal improves the quality of the drill-bit wavefields redatumed by the interferometry method. INTRODUCTION The seismic interferometry method synthesizes wavefields from virtual sources by combining seismic signals recorded using a plurality of sources and receivers. The method con- sists in cross-correlating the signals that are produced by one source and recorded at different receivers and the subsequent summing of these cross-correlations over a suitable number of sources (Bakulin and Calvert 2004). With the sources com- pletely surrounding the receivers, the method makes it possi- ble to simulate virtual sources at the position of the receivers (Calvert 2004) and to retrieve the Green’s function between different receivers (Wapenaar and Fokkema 2006). One ad- vantage of this method is that no knowledge is required of the formation or the source (either impulsive or random) proper- ties. Interferometry has important applications with borehole data: for redatuming virtual seismic sources at the location of buried receivers in exploration or passive seismic applica- tions and for vertical seismic profiling (VSP) purposes. Among the seismic borehole techniques, seismic while drilling drill-bit ∗ This paper is based on the extended abstract P275 presented at the 70 th EAGE Conference & Exhibition Incorporating SPE EUROPEC 2008, 9–12 June 2008 in Rome, Italy. † E-mail: fpoletto@ogs.trieste.it interferometry is a method that uses the unknown drill-bit vi- bration signal as a while drilling source. Drill-bit seismic while drilling interferometry is of potential interest for seismic ex- ploration, especially in areas where poor surface reflection seismic data and limited coverage for conventional VSP do not allow the targets to be properly illuminated at depth by using only surface sources. A noteworthy scenario for bore- hole interferometry is the possibility to image the flank of a salt dome (Mateeva et al. 2007). Different approaches and solutions, including deconvolution interferometry, have been proposed for reducing the typical interferometry artefacts, re- lated to limited illumination and to compensate for non-ideal source bandwidth (Vasconcelos and Snieder 2007). A beam- steering focusing method has been introduced for seismic while drilling processing purposes, to estimate the signature of the actual unknown drill-bit source from an array of recorded seismic signals and to design the optimum deconvolution filter (Haldorsen, Miller and Walsh 1995). The disadvantage of this multi-channel approach is that it requires knowledge (estima- tion) of the relative delays of the propagated waves, which, conversely, are not needed for recovering the interferometry signals. One drawback of using seismic interferometry, with the signal of the drill-bit source recorded by geophones located around the well, is that relevant additional noise wavefields C  2009 European Association of Geoscientists & Engineers 257