Distributed acoustic sensing for wireline borehole acquisition: noise sources and suppression methods
Abstract
In the field of recent advances in seismic instrumentation the application of distributed acoustic sensing (DAS) especially in boreholes allows to determine reservoir characteristics with high resolution. Nevertheless, the number of experiments conducted with the wireline DAS acquisition method is limited due to not well controllable cable coupling conditions. Cable slapping inside the casing creates specific noise patterns in the data. Therefore, a detailed analysis of the noise nature and proper methods for S/N ratio improvement are required.
In early 2017 an extensive vertical seismic profiling (VSP) survey using wireline DAS technology was conducted at the Groß Schönebeck geothermal research site, NE German Basin. Two wireline cables were installed in two 4.3 km deep wells. Data were recorded with a depth sampling of 5 m along the boreholes. Overall, the survey consisted of 61 source positions distributed in a spiral pattern around the target area covering offsets from 0.2 to 2 km with proper azimuth distribution. The acquired dataset was used for ringing noise problem analysis and revision of different approaches of noise elimination. Studying the noise in the raw strain data and spectrum behaviour led to the conclusion that these noise patterns represent a standing wave phenomenon, formed in areas where the cable has no continuous contact with the borehole wall and moves freely. In such depth ranges a resonance occurs at a fundamental frequency, followed by higher overtones in the amplitude spectrum. After testing and comparing of several existing methods for ringing noise suppression, we propose the application of a matching pursuit decomposition (MPD) method with Gabor atoms. This approach aims to decompose the input signal as a weighted sum of Gabor functions. Through analysis of the atoms' parameters such as frequency, amplitude, and position in time we identified the signal describing the ringing noise. Subtracting this from the recorded seismic trace allows to significantly reduce noise in poorly coupled depth intervals. The change from nearly horizontal to inclined reflectors and the absence of faults with larger vertical offsets especially in the reservoir section can be noted. In summary, the presented approach resolves the data with a higher resolution and better images distinct structural features.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMS014...02M
- Keywords:
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- 3094 Instruments and techniques;
- MARINE GEOLOGY AND GEOPHYSICS;
- 5494 Instruments and techniques;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS;
- 7280 Volcano seismology;
- SEISMOLOGY;
- 7294 Seismic instruments and networks;
- SEISMOLOGY