Seismic surveys

SEISMIC REFRACTION

The seismic refraction technique is an established, proven method for mapping buried bedrock surfaces. The method is particularly well suited for measuring the depths to bedrock in connection with the construction of large buildings, dams, tunnels, and highways. It is a favoured method for mapping alluvial cover (i.e. buried bedrock valleys) and the depth to bedrock.

Limitations: Seismic velocities required to increase with depth; generally limited to a depth of 100 ft with standard equipment.

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SEISMIC REFLECTION

Seismic reflection detects subsurface boundaries, assisting in the mapping of geologic structures. It is a robust geophysical method that has been increasingly used in shallow applications, such as groundwater exploration and civil engineering evaluations. Typical applications include subsurface mapping and bedrock void detection.

Limitations: Vibrational noise car impair results; poor resolution for shallow reflections.

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SEISMIC ACQUISITION IN HOLES

Borehole seismic measurements provide an indirect means of estimating pile tip elevations in otherwise inaccessible deep foundation elements. Seismic energy can by delivered to the pile cap or exposed pile surfaces, and measured in a nearby borehole that is drilled parallel to the pile. The method is relatively robust, and can often be employed without interruption to traffic or activities within a structure.

Limitations: Vibrational noise can impair results; requires cased boreholes next to structure.

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MASW - MULTICHANNEL ANALYSIS OF SURFACE WAVES

Non-invasive surface seismic method is capable of measuring shear wave velocities in the upper 100 ft of soil for seismic site class applications. Testing is performed on the ground surface, allowing for less costly measurements and faster results than with traditional borehole methods. This method compares favourably with downhole and CPT measurement systems.

Limitations: Horizontal resolution can be poor; decreasing resolution of velocity structure with depth.