Hydrophone VSP imaging at a shallow site

We evaluated the capabilities of vertical seismic profiling (VSP) for imaging the complex heterogeneous unconsolidated sedimentary structures at a shallow site. We deployed a 24-level hydrophone array with 0.5-m level spacing down a preexisting poly vinyl chloride (PVC) cased well. Data acquisition time was quick. Only 15 multioffset shot points using a hammer-on-plate source were needed to acquire reflection data between the water table at 3 m and the bedrock at 35 m to produce a depth section image. This image extended 9 m from the receiver will, yielding resolutions between fresh-water-bearing sands and impermeable muds and clays of better than 1 m. Depth accuracy of the image was confirmed by good correlation with cone penetrometer logs. We used conventional wavefield separation and VSP-CDP mapping techniques to image the data. Tube waves, created by seismic arrivals at cross-sectional area changes in the borehole fluid column, were the primary source of coherent noise in the data. The tube-wave arrival structure was complicated by the hydrophone array, which generated and scattered tube waves at each hydrophone pod. To combat the tube wave interference, we inserted closed-cell-foam baffles between elements. The baffles attenuated and slowed the tube waves, and reduced generation and scattering. A comparison between unbaffled and baffled VSP data showed that baffling increased the maximum useful frequency from 300 Hz to over 900 Hz. By contrast, surface shot data recorded at the same site, using buried 40-Hz vertical geophones, exhibited useful frequencies of less than 250 Hz. In addition, coherent noise in surface shot records caused by air waves and first arrivals made it very difficult to identify shallow reflections above 25 m. Reflections from depths as shallow as 10 m were easy to identify in the baffled VSP data.