Application Of Full Waveform Borehole Sonic Logs To Environmental And Subsurface Engineering Investigations

Recent experience with small diameter borehole sonic probes and digital full waveform<br>acquisition and processing capabilities has demonstrated the practical value of full waveform<br>sonic (FWS) logs in environmental and engineering investigations. Applications of FWS include<br>evaluation of lithology and permeability, delineation of geologic contacts, fracture analysis,<br>and calculation of engineering properties.<br>Four field examples presented in this paper document these applications. The first example<br>illustrates the characterization of lithology in a 4” diameter borehole drilled in Tertiary fluvial<br>sediments and presents a permeability profile derived from the tube wave portion of the full<br>waveform that shows the qualitative permeability variation between the sandstones,<br>siltstones, and claystones. The second example highlights the full waveform display,<br>commonly referred to as the variable density log (or VDL), as a means to identify geologic<br>contacts. The third example compares tube wave derived permeability with field tests and<br>core data from a 4” borehole in fractured welded tuff. The last example examines the<br>engineering analysis from a 3” corehole in fractured granite that includes the calculation of<br>engineering properties (Young’s modulus, shear modulus, and Poisson’s ratio) from<br>compression and shear velocities.<br>The analyses and interpretations illustrated by these field examples are made possible by<br>the recent development of digital FWS acquisition software for small diameter sonic probes<br>and advanced interactive FWS post processing. The post processing includes options to<br>examine individual waveforms, interactively repick travel time, and calculate the amplitude of<br>any portion of the waveform. Also, the technique of plotting vertically stacked waveforms as<br>a variable density log (VDL) is utilized here to enhance and contribute to the interpretation<br>power of FWS logs.