Soil Dynamics and Earthquake Engineering 12 (1993) 149-157 The determination of soil attenuation by geophysical prospecting and the validity of measured Q values for numerical simulations D. Jongmans Laboratory of Engineering Geology and Geophysical Prospecting, Liege University, Bat.Bl9, 4000 Liege, Belgium & M. Campillo Laboratoire de Geophysique Interne et Tectonophysique, Universitd J. Fourier, BP 53)(, F38401 Grenoble, France (Received 25 June 1992; accepted 14 April 1993) Q values of shallow materials are derived from seismic experiments in different frequency ranges. Three main measurement techniques are used: the rise-time method, the spectral ratio method and the study of surface wave attenuation. Attenuation measurements are performed in different geological site conditions. The Q values obtained at high frequency with the rise-time and spectral ratio methods are found to be lower than the values inferred from surface waves at lower frequencies. These results suggest a frequency dependence of the quality factor in surficial materials. Two sites are thoroughly investigated by geophysical prospecting. On the basis of the geometry and the dynamic characteristics of the geological formations, numerical modeling (1D and 2D) is carded out in order to compute the site response to small earthquakes or other solicitations. Comparisons between theoretical and experimental transfer functions show a good agreement if the Q values used in the computations are those measured in the same frequency range as that of the solicitation. These results stress the importance and the need of careful investigations about the frequency dependence of Q. INTRODUCTION During the last few years, several attempts have been made to infer surficial ground attenuation character- istics from in-situ tests (see for example Refs 1-3). The problem is rather complex owing to the difficulty to separate intrinsic attenuation from other causes of amplitude decrease such as transmission loss or diffraction. Intrinsic Q determinations are to be limited to horizontally layered media. On the other hand, the quality factors QS and QP constitute prominent parameters having an important effect upon the amplitude and duration of ground motions during earthquakes. Their determination by Soil Dynamics and Earthquake Engineering 0267-7261/93/$06.00 © 1993 Elsevier Science Publishers Ltd. 149 field measurements appears to be a crucial point for the quantitative interpretation of the amplification effects often produced by surficial deposits. Many methods have been proposed to measure the ground attenuation from geophysical prospecting. ~ The results derived from different techniques usually show a large dispersion. On the other hand, several authors 4'5 have presented data indicating that the quality factor could vary with frequency. Such a frequency dependence has already been found at the scale of the lithosphere: To test if Q values actually vary with frequency in surlicial deposits, seismic experiments were carried out in various sites with the aim of measuring Q values in different frequency bandwidths. Numerical simulations are carried out in different cases in order to validate the Q values by comparing theoretical and experimental results.