VSP over deep coal using vibrator and explosive

Abstract

A detailed comparison is presented of results for Vertical Seismic Profiling using a surface vibrator source and a buried explosive charge. The geological setting was the southern region of the Sydney basin near Appin where the Bulli coal seam is at a burial depth of approximately 500 m, and where overlying material is sandstone with interbedded shale. Borehole geophones were spaced at 8 m, commencing at 28 m below the surface and continuing down nearly to the Bulli coal seam. The vibrator operated at a location offset 60 m from the borehole, where 50 linear vibrator sweeps of 40 Hz to 360 Hz and 3 s duration were used. The vibrator performance is compared to results of using 0.5 kg of explosive at the same offset location, and buried beneath the weathering layer generally of depth 13 m. The source bandwidth of the buried explosive charge was 40 Hz to beyond 350 Hz, determined by spectral analysis of the unfiltered record from the downhole geophone closest to the surface. Results of VSP sections showing downgoing waves, and an upgoing wave (Bulli coal-seam reflection) are presented, together with processing details. An examination of vibrator and explosive signal bandwidth attenuation versus depth is presented. These results were determined from the unfiltered downhole geophone records of the downgoing waves. Digitally recorded vibrator ground force was obtained using the accepted practice of accelerometers mounted on the vibrator ground contact plate and reaction mass, and was also obtained using an accelerometer buried one metre beneath the ground contact plate. These signals were analysed to establish the quality of the ground excitation; results are presented graphically. VSP sections, both filtered band pass 60?250 Hz, clearly show downgoing waves of good continuity and comparable wavelet width. However, the explosive downgoing wave showed evident broadening of wavelet width at geophone depths greater than 276 m. This observation was supported by spectral analysis of the unfiltered explosive downgoing wave, which showed a marked reduction of the explosive energy bandwidth with depth, compared with the bandwidth of the wavelet energy produced by the vibrator. The vibrator output however, suffered substantial bandwidth attenuation in passing through the weathered layer, which effectively filtered out vibrator frequencies above 250 Hz. The lower energy output of the vibrator compared with the explosive showed up in the Bulli coal-seam reflection, which in the case of the vibrator became buried in noise at downhole geophone locations shallower than 188 m. On the other hand, the explosive reflection was much stronger, and still evident in the record from the geophone located at a depth of 116 m.