Field Test of a Low-Frequency Sparker Source for Acoustic Waveform Logging

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.