Karst Void Detection by Full-waveform Inversion of Borehole Sonic Zero Offset Data
Abstract
Drilling boreholes are commonly used to detect rock voids and fractures in underground construction projects like pile foundations, tunnel borings and mining. However, a void can be missed due to its limited size. In this study, borehole zero-offset sonic sounding was conducted using a new downhole tool that operated in the frequency range of 0.05 kHz to 8 kHz. We present time domain 2D full-waveform inversion (FWI) of the borehole sounding data for detection of karst voids in the subsurface. First, a new FWI workflow was developed and demonstrated using modeling waveform. To efficiently simulate many traces of zero-offset profile, high-density multiple sources were triggered simultaneously. We use an amplitude envelope-based objective function FWI method to provide a better initial velocity model for the correlation-based FWI. Modelled anomalies of a single void, multiple voids and fractures can be interpreted by the inverted vs velocity profile. The FWI method is also demonstrated using field data of a bridge foundation project in a karst area. Lastly, we compare the FWI result of the single borehole sonic zero-offset data with imaging from four surrounding cross-hole acoustic tomography borehole profiles. The inverted velocity profiles identify a void as a low-velocity anomaly at the correct depth and horizontal distance. The results demonstrate that FWI is capable of imaging the karst void near the drilling borehole.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMNS31B0772L
- Keywords:
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- 4302 Geological;
- NATURAL HAZARDS;
- 4306 Multihazards;
- NATURAL HAZARDS;
- 4325 Megacities and urban environment;
- NATURAL HAZARDS;
- 4327 Resilience;
- NATURAL HAZARDS