Borehole Tomography

Applications

• Bedrock mapping
• Location of fault/fracture zones
• Void detection
• Subsurface profiling
• Imaging velocity inversions (hard layer overlying softer layer)
• Deep profiling with limited space
• High accuracy geotechnical parameters at depth

Method

The borehole tomography method utilises existing or new boreholes to allow for seismic source and/or receivers to be placed at depth improving resolution which is typically lost from surface-only methods. Borehole tomography measures the direct arrival times from the source to the receiver to produce a 2D slice of P- or S-wave velocities which can be used to determine the subsurface structure. A number of configurations can be used depending on available boreholes and required outcomes. Common configurations include surface to borehole and crosshole (borehole to borehole). A modified borehole tomographic technique using a receiver array placed in a sewer can produce valuable information regarding the variations in compaction between the road surface and top of the sewer.

Data Analysis & Presentation

Direct travel times between source and receiver arrays calculate and processed to generate a 2D slice showing variations in either P- or S-wave velocity depending on the source type. Each dataset is processed using multiple inversions to reduce RMS error. By using multiple source/receiver locations over several boreholes or a sweep of surface sources recording from a single borehole a pseudo-3D model can be generated.

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