Borehole Geophysical Methods for Near-Surface Characterization

Soo-Kyung Miong

ABSTRACT

This thesis examines the application of VSP techniques for near-surface characterization. First analyzed are multi-offset VSP datasets which were acquired in August 2006, using geophone and hydrophone sensors over depths of 4 m to 40 m, in the West Castle River area of southern Alberta. The hydrophone VSP shows superior data quality compared to the geophone VSP. P-wave velocity models obtained from the VSP analysis give velocities that range from 670 m/s in the unconsolidated near-surface material to 3500 m/s in the deeper competent shales. The position of the seismic reflectors in the VSPCDP stack of the data agree reasonably well with position of the lithologic boundaries identified by the driller; especially the tops of three water-bearing gravel layers that are resolved by the VSPCDP stack.

A more extensive set of field tests was carried out in July 2007, near Priddis, Alberta. Shallow well logs, VSP, and 2D and 3D seismic reflection survey data were collected at the geophysical test site near the Rothney Astrophysical Observatory. According to well log analyses, a major fracture and four porous sandstone units were identified at depths of 28 m, 39 m, 50 m, 61 m, and 120 m with porosities ranging from 0.34 to 0.58. The respective P-wave velocities of clean sandstone and shale zones are 3200 m/s and 2300 m/s after calibrating the sonic velocities using the VSP data. Five zones of interest are interpreted in the VSP images and they correlate with the 3D and 2D seismic results; especially the fracture zone at 28 m - the largest source of groundwater in the test well - and the thick, possibly water-bearing, sandstone unit at 60 m. The correlation of the various borehole and seismic data demonstrate the efficacy of these techniques for near-surface characterization.

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