Seismic Reflection - Methods

Near Surface P- and S-Wave Velocity Measurements

Methods

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Field setup at site VMF located on artificial fill about 2 km south of the Kingdome. Skyscapers of downtown Seattle are visible behind the Kingdome. Orange geophones at 3-m intervals are visible in the left foreground. S-wave generation using a 10-lb sledgehammer to hit a wooden timber under the wheels of a car.

The seismic profiles were acquired within about 100 m of the aftershock recording site and were located on the paved streets or city parks in residential areas, or railyards and any open ground in industrial areas (see photo above left). We interpreted the data using the slope-intercept method of analysis (Mooney, 1984). see Velocity Results. Recording parameters are:

Reversed seismic S-wave profiles ranged in length from 87 to 168 m. These S-wave profile lengths resulted in a maximum survey depth range of about 30 m. Because no additional S-wave layers were detected below about 20 m on most profiles, the maximum depth was approximated by assuming that a higher velocity layer would have been detected on the next geophone beyond the end of the profile (Mooney, 1984). see Seismic Reflection-Resonance Correspondence.

The S-wave seismic source consisted of a wooden timber placed on the pavement beneath the wheels of the vehicle at right angles to the direction of the profile (see photo above right). Reversed polarity seismic energy was produced by striking opposite ends of the timber with a 4-kg sledgehammer. We picked first-arrival phases assumed to be refracted from the same interface, calculated the velocity from the slope of the line connecting these phases, and then extended the line connecting these phases back to the zero offset point. We determined that the slopes were accurate to within about 10 percent. Thus the calculated layer thicknesses had roughly the same accuracy. There are two limitations underlying this technique: (1) an assumption that layer velocity is constant across the length of the profile, and (2) low-velocity layers underlying a high-velocity layer cannot be detected. In spite of these assumptions and level of accuracy, this approach has been shown to generally agree with seismic velocity downhole profiles determined from shallow boreholes in the Los Angeles area (Williams et al., 1996, 1997). Similar studies of near-surface materials for earthquake site response via surface seismic methods have been conducted by Street et al. (1995) and Harris et al. (1994).

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