Hydrophone VSP imaging at a shallow site
- Lawrence Berkeley National Lab., CA (United States)
- Univ. of California, Berkeley, CA (United States). Dept. of Materials, Science and Mineral Engineering
- Lawrence Livermore National Lab., CA (United States). Environmental Restoration Div.
The authors evaluated the capabilities of vertical seismic profiling (VSP) for imaging the complex heterogeneous unconsolidated sedimentary structures at a shallow site. They deployed a 24-level hydrophone array with 0.5-m level spacing down a preexisting poly vinyl chloride (PVC) cased well. 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 well, 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. The authors 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. To combat the tube wave interference, the authors 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. Reflections from depths as shallow as 10 m were easy to identify in the baffled VSP data.
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 514750
- Journal Information:
- Geophysics, Journal Name: Geophysics Journal Issue: 3 Vol. 62; ISSN GPYSA7; ISSN 0016-8033
- Country of Publication:
- United States
- Language:
- English
Similar Records
Comparison between surface and VSP reflection imaging of shallow sites
Comparison of vertical seismic profiling techniques