Detection of Fault Zones at Depth Using Low Frequency Induced Sources
Abstract
The locations and properties of small fault zones and fractures are of interest to industries including Radioactive Waste Disposal and Deep Underground Mining. At present there is limited knowledge regarding imaging of fault zones with sizes 1 to 100m. We explore the potential of seismic noise, such as that present in tunnels due to excavation processes, to image fractures at depth. Microseismic monitoring is a powerful tool but resolution depends on the characteristics e.g. wavelength and frequency, of the recorded seismic signals. In this study we investigate the role of those characteristics in the identification of fault zones at depth. We use finite element analysis to model a small fault zone within a crystalline host rock; a potential host rock for geological disposal. After an optimization analysis, a 25Hz short duration pulse was used to simulate a seismic source in a rock mass of dimensions 500m x 500m. The thickness and pressure wave speed (Vp) of the fault zone, its orientation, and the location of the pulse were varied. The fault core thickness was varied from <1m to 5m, Vp from 500m/s to 1500m/s while the host rock Vp remained constant at 5000m/s. Two orientations were considered for the fault zone: 1) horizontal and 2) vertical allowing two extremities to be evaluated. The location of the source was considered, 1) directly below the fault zone and 2) at some distance away from the fault zone (100m). Our analysis shows that the frequency of the pulse changes as the wave reflects and refracts due to material property changes as it propagates. The peak wave velocity on arrival at predefined monitoring points demonstrates reduction, giving an indication of attenuation. We show that the original frequency converges to a certain threshold value e.g. approximately 11Hz for a Vp of 5000m/s and 4Hz for a Vp of 500m/s. This threshold is characteristic of the material and the thickness of the layer through which the pulse is propagating. There is a strong linear relationship (R2 > 0.99) between wave propagation velocity in the rock, frequency threshold value and the distance from the seismic source at which this frequency threshold is achieved. Our results suggest that we are able to detect the presence of materials with very different properties, e.g. a fault zone within a rock mass. For example, a 1 m wide fault zone is detectable if there is at least a 30% contrast between the Vp values of the fault zone and the host rock. This requirement is not constant i.e. the minimum difference in Vp reduces as the thickness of the fault zone increases. Our results have direct implications for the novel application of seismic monitoring systems for field detection of sub-seismic scale fracture and fault zones.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMNS33B..04M
- Keywords:
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- 0935 EXPLORATION GEOPHYSICS Seismic methods;
- 0994 EXPLORATION GEOPHYSICS Instruments and techniques;
- 0902 EXPLORATION GEOPHYSICS Computational methods: seismic;
- 0910 EXPLORATION GEOPHYSICS Data processing