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Surface-Wave Analysis (SASW or MASW)

Overview

A hammer blow or mechanical device generates dispersive waves which travel along the surface of the earth analogous to ocean waves. The energy is measured at a small number of sensors to find the phase velocity vs. frequency of the waves. The depth of penetration and the mechanical properties (shear-wave velocities) of the materials directly below the observation point are sensed. If two-dimensional information is required, the active geophones are rolled along the surface to produce a shear-wave velocity cross-section.

Geologic Model

Layered geologic or manmade materials without the requirement that the velocities increase with depth. Thus, the velocity within a gravel base can be measured beneath pavement in some cases. The engineering model is the measurement of the layer-weighted average shear-wave velocity of the top 100 feet (or 30 meters). This engineering model is important because of the adoption of the International Building Code which requires this number (Vs100 or Vs30) to set the seismic site class for foundation design.

Requirements

A reasonable source such as a bulldozer, accelerated weight drop, interstate highway, busy airport, or gravel processing facility or even ambient noise in some urban areas.

Considerations

Interpretation is subject to ambiguity as velocities can be traded off for structure and higher-mode excitations may interfere. Very susceptible to lateral variations on the scale of the depth of investigation.

Logistics

Crew size is 1-2 persons, equipment is portable though sources may not be if significant depth is required.

Deliverables

Plan maps of station locations, amplitude vs. frequency plots, inversion for velocities vs. depth, ambiguity estimates, geologic interpretation, and a narrative description of the work. The IBC site class velocity is produced, but engineering judgement is necessary to insert Vs100 into foundation design.

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