Field Test of a Low-Frequency Sparker Source for Acoustic Waveform Logging
Name
1984.9 Paillet.pdf
Size
578.41 KB
Format
Adobe PDF
Checksum (MD5)
6736e81d84abe1fbc1ef3b719238ce50
Author(s)
Paillet, Frederick L.
Date Issued
1984
Publisher
Massachusetts Institute of Technology. Earth Resources Laboratory
Series/Report no.
Earth Resources Laboratory Industry Consortia Annual Report;1984-09
Abstract
Low-frequency acoustic-energy sources for waveform logging have important applications in: 1) Verifying theoretical calculations; 2) generating tube waves in large-diameter boreholes; and 3) providing larger sample volumes in cases where borehole effects are important. A new low-frequency source was fabricated by modifying an existing acoustic-waveform logging system to discharge multiple capacitors in series with an automobile spark plug. The sparker source was tested in boreholes of 15- and 8-centimeter diameter in homogeneous granite containing isolated fractures. The sparker source produced repeatable waveforms with frequencies centered on 5 kilohertz in the 8-centimeter-diameter borehole, and 7 kilohertz in the 15-centimeter-diameter borehole, compared to frequencies near 15 kilohertz for the same system using a low-frequency magnetostrictive source. The lower-frequency sparker source excited consistently measurable tube waves, in agreement with theory. Test results also confirmed that lower-source frequencies greatly decreased sensitivity to borehole effects. Observed differences in frequency content and extent of shear-mode excitation in the two different diameter boreholes are
probably related to differences in mode-excitation functions. The data confirm theoretical predictions that optimum shear-mode excitation occurs for source frequencies near normal-mode cutoff. Reflection of low-frequency tube waves appears to be an effective means for distinguishing between isolated open fractures and intervals containing extensive alteration around nearly impermeable fractures.
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