Multicomponent multioffset VSP processing

Processing of multicomponent, multioffset vertical seismic profile (VSP) data for shear-wave birefringence requires care and special procedures to preserve amplitude and phase necessary to identify the natural polarization direction and determine fast and SLOW shear-splitting for velocity anisotropy measurements. Presented in this paper arc preconditioning procedures that were used prior to four-component rotation to provide optimal results from a multicomponent, multioffset VSP survey. The preprocessing flow is applicable for most multicomponent, multioffset VSP data sets. Introduced in this paper is a threecomponent (3-C) deconvolution operator to be applied before tool rotation and P-SV separation that flattened the spectrum and reduced multiples while preserving shear-wave birefringence information. Introduction: As part of the Colorado School of Mines Reservoir Characterization Project, Phase III, multicomponent multioffset VSPs were recorded into a 3-C receiver sonde in the UNOCAL 10-6-3 well, South Casper Creek Field, Natrona County. Wyoming. The purpose of this study was to image zones of heterogeneity and identify anisotropy in a structurally complex reservoir. Interpretation of the birefringence results are presented in a second paper entitled: Multicomponent Vertical Seismic Profiles for Reservoir Characterization, South Casper Creek Field, Natrona County, Wyoming. Vertical seismic profiles (VSPs) allow recording of the downward travelling wave front which provides a much stronger signal than reflected energy and allows in situ measurement of the seismic wavelet. Procedures to computer process compressional wave VSPs were presented by Lee and Balch (1983), however, multicomponent and multioffset VSPs require additional procedures to preserve relative amplitude and phase information ecessary to obtain accurate shear-wave polarization and splitting. Acquisition: From June 26 to June 30, 1989 a multicomponent VSP was recorded in the UNOCAL 10-6-3 well, from 2700 ft to 300 ft, within the South Casper Creek Oil Field. Sources were located at one 300 ft near-offset (position 1) and two orthogonal 1092 ft far-offsets (positions 2 and 3). Each source location had one vertically polarized (P-wave) and two orthogonal, horizontally polarized (S-wave) vibrators (Figure 1). The S-wave sources were oriented with their polarization directions oblique to the anticipated axis of fracturing so that shear-wave birefringence might be observed from each source. Accelerometers attached to each S-wave vibrator base plate were used to determine the starting polarity direction. The S-wave sources were then aligned to vibrate either radial (SR) or transverse (ST) relative to a line connecting the source and the well so that all positive polarities conformed to a right-hand coordinate system with positive “z” downward. Data was recorded on a 3-C sonde with elements oriented radial (x) and transverse (y) to the locking arm with the third element (z) parallel to the sonde axis, Nomenclature for the recorded ata is first the source type, (P, SR or ST) then the offset position, (1, 2, or 3) and finally the recording element (x, y, or z); ie. SRZY is the S-wave source oriented to vibrate radial to the borehole from the second offset position and recorded on tbe transverse r ceiver element. Vibroseis sweep tests were made at each offset location at depths of 600 ft, 1210 ft and 2400 ft as the recording sonde was lowered in the hole. Tests were performed to determine the number of sweeps to stack per recording level and the frequency range of the sweep signal. The final sweep parameters used for the vibrators were as follows: P-wave: lo122 Hz, 14-s sweep, 5OO-ms taper, 7 sweeps/level, S-wave: 6-83 Hz, 14-s sweep, 5OO-ms taper, 7 sweeps/level. With the sonde anchored at 2400 fi dynamite, in shot holes at each of the offset locations, were also recorded to validate the polarity of each vibroseis ource and to compare tool-orientation, estimated from hodogram plots. Processing: Figure 2 is a processing flow diagram that was used for the South Casper Creek VSP data. A four-component rotation technique, as described by Alford (1986), was used to determine the natural polarization (fracture) direction. The amount of shear-wave splitting was then determined by cross-correlating the fast and slow shear-wave components. The processing steps before four-component rotation, although unconventional, help improve multicomponent data for shear-wave birefringence measurements. First, the data were sorted into nine distinct source files then correlated with the appropriate vibroseis weep (P or S). The correlated ata were then separated into files based upon source position and recording component. This procedure yielded 27 separate VSPs (three source locations by three source types by three receiver components). Source and receiver geometries were entered into the trace headers and the data were edited before being stacked at each depth level. An impulse response test of the sonde indicated a variation in the gain response between geophone lements. Scaling factors were applied to the xand y-data files to normalize tbe gain with respect o the zrecording element. 3-C Deconvolution Significant amounts of compressional energy were recorded from the far-offset horizontally polarized (S-wave) sources. High amplitude converted P to S-wave energy preceded the source generated S-waves making identification of the firstarrival shear-wave difficult. The problem becomes more complicated when anisotropy and multiples are included. The combined result is a far-offset S-wave data set with a ringy appearance (Figure 3). In an attempt o reduce the ringiness, several decon operators were tested. Spiking deconvolution,