Microtremors as a geophysical exploration tool: Applications and limitations

The usefulness of microtremors as a geophysical exploration tool has been a nalyzed. This application is possible due to the relationship between the m ain resonance frequency of a given soil, obtained from the H/V spectral rat ios of microtremors, its thickness and average shear velocity. We first mea sured the ambient noise at 33 sites and determined their main resonance fre quency. Detailed geotechnical information was available for 23 of the sites , thereby allowing us to establish a quantitative relationship between the resonance frequency and the thickness of the soil, and indirectly between e ither of them and the shear velocity of the soil. The practical application of this relationship has revealed its usefulness in determining the surfac e structure of a valley with excellent accuracy, with an error of only 15% in the depths calculated. These errors are due to the simplification of the problem that this method implies: it requires that the shear velocity vary constantly with depth throughout the study region, which is evidently not always so, and that the input data themselves intrinsically have a certain degree of uncertainty. This method is therefore not valid when there is no mechanical contrast between the study soil and the underlying layer or when the shear velocity varies irregularly with depth in the study area.