A dense array experiment for the observation of waveform perturbations

We analyze observed three-component seismograms recorded by a dense array o f 21 short-period receivers (f > 2 Hz) installed on a homogeneous Quaternar y plain with 1.5 x 3.5 km(2) horizontal dimensions. The experiment was cond ucted in the district of Caille, in SE France for 3 months. The observed wa veform data are related to diffraction and refraction of the direct wavefie ld by different geological structures and topographic morphologies surround ing the array. The origin of the coda energy following the energetic Pg or Sg arrivals is either influenced by heterogeneities close to and inside of the valley, for instance thrust of Jurassic limestones over Cretaceous marl s in the northern and southern limits of the valley, or by heterogeneities located outside of the valley such as the interface unconformity between cr ystalline basement and sedimentary cover in the northern border of the Maur es-Tanneron ranges. The main energetic arrivals of the seismograms arrive a t the array with a definite range of azimuths that is not correlated with t he event back-azimuth. Moreover, these arrivals do not appear with azimuths related to the characteristic NS and EW valley orientations, and thus may be related to heterogeneities located outside of the valley. (C) 1998 Elsev ier Science Ltd. All rights reserved.