Cross-Hole Sonic Logging and Frequency Tomography Analysis of Drilled Shaft Foundations to Better Evaluate Anomalies Locations-Article

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The study presents frequency tomography analysis (FTA), a signal-processing method intended to improve the identification and localization of anomalies in drilled-shaft foundations after cross-hole sonic logging (CSL). Conventional CSL assesses concrete integrity using ultrasonic-wave travel time and velocity, but these measures may indicate a problem without pinpointing its exact location.<br /><br />FTA analyzes the received signal’s frequency amplitude. It converts CSL time-domain data to the frequency domain using a fast Fourier transform (FFT), then denoises and fits the results before displaying frequency-amplitude changes as a tomography plot. Reduced amplitude indicates a potential anomaly.<br /><br />The method was tested on two 20-inch-diameter, 4-foot-long concrete shaft specimens containing foam blocks that represented anomalies occupying about 20% of the shaft cross-section. One specimen had anomalies near the top and bottom; the other had one near the middle. CSL measurements were collected between access tubes inside the shafts.<br /><br />In both specimens, FTA identified the depth of the artificial anomalies more clearly than time-domain signal plots. The authors report that frequency-amplitude reductions helped reveal anomaly locations and approximate extent, including cases where conventional CSL data did not precisely define the affected area. They conclude that FTA could support construction quality control and help engineers assess the structural adequacy of drilled shafts. However, the reported validation was limited to two laboratory specimens with planned foam anomalies.

Keywords

frequency tomography analysis

cross-hole sonic logging

drilled-shaft foundations

anomaly localization

signal processing

fast Fourier transform

frequency amplitude

concrete integrity

tomography plot

structural quality control