1. Sensors and Acquisition Technologies

Virtual Reverse VSP (VRVSP) and Crosshole Seismic Interferometry Application

Flavio Poletto and Lorenzo Petronio

Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Italy.

Abstract

Seismic interferometry is the process of obtaining seismic signals from correlation of traces recorded at different locations. The method makes it possible to simulate virtual sources where only receivers are used, thus providing virtual seismograms, with the advantage of enlarging the coverage in seismic exploration. We apply seismic interferometry to obtain multioffset virtual reverse VSP (VRVSP) and virtual-crosshole data. The analysis is performed by processing the data acquired in a mixed surface-borehole field test, where a seismic vibrator was used at the surface as source. Borehole receivers (3-C geophones) were positioned into two wells of 300 and 400 m located at a relative distance of 30 m. Surface receivers (vertical geophones) were deployed along four orthogonal seismic lines oriented in radial directions from the wells. Conventional multioffset VSP and surface seismograms were acquired by using about 10 energization positions along each surface line, with a maximum offset from the wells of approximately 300 m.

We process the borehole and surface signals to obtain the virtual crosshole and the VRVSP data. Processing is based on cross-correlation and summation of the traces measured in the borehole and at the surface receivers. The processed seismograms contain events with complex patterns, where the signal of the virtual borehole sources is present together with other coherent events with hyperbolic moveouts. We interpret these events as noise due to wavefields which are not effectively reduced after the summation of the data in the domain of the sources. The analysis of the signal and of the noise patterns, and the analysis of the imaging conditions in function of the distribution of the real sources at the surface, are performed by using the stationary-phase method. In this application, we compare the interferometry signals of the orthogonal surface lines, with the support of synthetic data. The interpretation of the noise wavefields and of the virtual signals is performed by splitting the virtual-source gathers before summation and, in particular, by analyzing the upgoing and downgoing wavefields in the borehole. Data processing includes deconvolution of the virtual source seismograms.

The results show that the virtual borehole signals obtained by interferometry are consistent with the conventional VSP signals, as well as with RVSP and crosshole signals acquired during the same test by using a standard drill-bit seismic method.

Last modified: Sat May 13 01:05:30 2006