Shallow seismic survey, acoustic/seismic coupling, and surface impedance measurements on one site

Abstract

At low frequencies, ground surfaces can be modeled as a poroelastic layer above an elastic substate. Theory predicts that the presence of seismic resonances within the layer should be observed in surface impedance measurements, due to acoustic/seismic coupling. A number of different measurements have now been made on one site. Core samples show that this ground consisted of a uniform layer of silt of thickness (1.6 ± 0.3) m over bedrock. Seismic velocity measurements were consistent with this simple structure indicating a layer thickness (1.9 ± 0.3) m. Quarter‐wavelength related resonances in the silt layer should lead to maxima in the acoustic/seismic admittance spectrum at roughly 45, 135, and 225 Hz and minima at 90 and 180 Hz. Direct measurement of the acoustic/seismic coupling suggests maxima around 50, 135, and 225 Hz and a minimum around 90 Hz. The results also suggest other resonances [there are (a) peaks at about 65 and 185 Hz with a minimum at 130 Hz, and (b) a large peak at 105 Hz with a broad dip at 210 Hz] that are unrelated to the silt layer. Maxima in the acoustic reflection coefficient of the surface should be related to the minima of the acoustic/seismic admittance spectrum. Measurements [G. A. Daigle and M. R. Stinson, J. Acoust. Soc. Am. 81, 62–68 (1987)] of the reflection coefficient suggest peaks at 90–95 Hz, around 130 Hz, and at 200–210 Hz.