SURFACE WAVE ANALYSIS

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Shear wave velocity is an important parameter for estimating how the earth will behave during an earthquake. Shear wave velocities are difficult to measure directly due to low signal-to-noise ratio, the bulk of the noise coming from higher amplitude p-waves.

Surface waves, as the name suggests, travel at or near the surface of the ground. They are characterized by low velocity, low frequency, and relatively high amplitude. They are often referred to as “ground roll” and are a source of noise in shallow seismic reflection.

Surface waves are dispersive; different frequencies travel at different velocities in a manner similar to light. As it happens, surface wave phase velocities – the velocities of different frequencies – are a useful proxy for shear wave velocity, and because of their high amplitude, are relatively easy to measure. They are easily generated by both active (hammer and plate, weight drop) and passive (microtremors) sources.

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For historical reasons, the terminology used for active and passive surface wave surveys depends largely on the inversion software. Geometrics' SeisImager SW package uses "MASW" (Multichannel Analysis of Surface Waves) for active source, and "MAM" (Microtremor Array Measurement) for passive source. The math and physics behind both is essentially the same; the difference lies in the wavelengths, which determine the depth of investigation. Lower frequencies (longer wavelengths) see deeper,are obtained from microtremors, and are typically coupled with active source data to achieve the necessary depth of investigation.

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A second approach, used mainly for microzonation studies, is the "H/V"or "Nakamura Method". This technique uses a single 3-component geophone and microtremors to determine the fundamental period of the ground beneath the geophone. SeisImager SW includes an H/V module.

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COMMON APPLICATIONS

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• Vs30 and Vs100 site classification

• Void and sinkhole detection

• Foundation engineering

• Landfill investigations

• Depth to bedrock

• Levee inspections

• Microzonation studies (H/V method)

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CONSIDERATIONS

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• Depth of investigation is about ½ the spread length

• 4.5 Hz geophones and a polypropylene plate are recommended

• Surface wave surveys do not require striking the ground as hard as in refraction surveys, due to the relatively high amplitude of surface waves.

• Typical channel count is 24. Passive recording for deeper measurements requires 16k records, available standard with the Geode and StrataVisor and optional on the SmartSeis and ES-3000.

• Crew size is generally two.

• Surface wave surveying works best in unconsolidated sediments; the more competent the subsurface, the harder it is to do successful surface wave surveys.

• Not all sites are suitable for MAM; remote sites may not have enough microtremor activity.

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BENEFITS/LIMITATIONS

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• Unlike in seismic refraction, surface wave analysis does not require velocities to increase with depth -- low velocity zones can be mapped with surface waves.

• Vertical resolution in the shear wave velocity profile from surface wave analysis is highly variable with depth. Close to the ground surface, a detailed reconstruction of thin layers may be obtained, as typically dense information is available in the high-frequency range, especially if active-source data are collected. The resolution decreases markedly with increasing depth. As a consequence, relatively thin deep layers cannot be identified and the depth of deeper interfaces is poorly constrainged.

• Higher modes have to be taken into account in the inversion, especially in complex stratigraphic conditions, where the fundamental mode is typically non-dominant (for example in the presence of a stiff top layer and/or a sharp jump of stiffness). Several methods have been proposed in the literature to account for higher modes, but the procedures are still not standardized. In fact, the analyses for complex stratigraphies have to be tailored for the specific case and require very experienced analysts.

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DELIVERABLES

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The final product of a surface wave survey is a 1D velocity model (below)

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