Detection of sinkholes or anomalies using full seismic wave fields : phase II.
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Detection of sinkholes or anomalies using full seismic wave fields : phase II.

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      A new 2-D Full Waveform Inversion (FWI) software code was developed to characterize layering and anomalies beneath the ground surface using seismic testing. The software is capable of assessing the shear and compression wave velocities (Vs and Vp) for 1,200 cells (0.75 x 0.75 m resolution) in an 18 x 36 m (60 x 120 ft) region in 20 to 30 min on a standard laptop computer. The software, which includes a graphical software interface, allows the user to setup the test line (spacing, number of shots, and receiver), view the raw data, condition the data (filter, window, remove poor channels), preprocess the data (identify initial velocity profile), and analyze (invert) the conditioned data to obtain independent Vs and Vp results graphically or in file format (PDF). Next, a synthetic study was undertaken with the 2-D FWI software to investigate its capability of identifying 3-D voids (size, shape, and depth). Synthetic waveform data sets were generated using a 3-D finite difference model of a domain with a void of various sizes and depths, and surface-based test lines were both on the top of the void and away from the void. The synthetic waveform data sets were then analyzed by the 2-D FWI software as if they were collected from field experiments. The FWI results revealed that if the test line is on the top of a void, the maximum embedded depth at which the void can be detected is about three void diameters. In the case of test lines that are away from the void, the void can still be identified in inverted profile if the test line is near the void edge; however, the void becomes distorted and non-existent if the test line is at least one diameter from the void. The 2-D FWI software was then tested at five FDOT sites with unknown soil/rock layering and both known and unknown voids at time of seismic testing. Subsequent invasive testing (SPT and CPT) was performed near or at the anomaly for validation. A comparison of the seismic results with the SPT/CPT results revealed that FWI did an excellent job of identifying soil and rock layering (e.g., soil and limestone depths at Kanapaha and Newberry, FL), identifying unknown voids (Newberry and Tallahassee, FL), and the extent of existing voids (US 441 and Gainesville, FL). As with the synthetic study, field seismic results from test lines one diameter from the void did show void distortion, but it was not present further away. Finally, the software was demonstrated and used by FDOT field personnel on a test site. The seismic equipment used for this project included both a 24-channel coupled geophone array with sledge hammer point source,a new 24-channel land-streamer, and a propelled energy generator (PEG). The land streamer and PEG may be towed behind a vehicle and used to investigate the subsurface profiles beneath a roadway (e.g., US 441).
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