Seismic characterization of the Quaternary sediments at Llancanelo-Lake Area, Argentina

Abstract

A preliminary near-surface seismic survey was carried out at LlancaneloLake (Argentina) with the aim of mapping the quaternary layers, composed of sedi-mentary layers overlying basalt. A geological model has been obtained by interpretingthe events of a common-shot gather. Expressions of the traveltimes of the direct andrefracted waves versus o set provide means to evaluate the velocities of the layers andthickness of the upper layer. Moreover, the standard processing sequence is appliedto the data to obtain a stacked section. This procedure reveals the location of deeperinterfaces, although it is highly a ected by the ltering of the direct and refractedevents, which have the same slope of the medium- and far off set reflection signals.The fact that the direct arrival has not a zero intercept time indicates the pres-ence of a very thin layer at the surface (shallow salt deposits). Further evidence ofthe near-surface layer is that the refracted and reflection events are ringing and thereare strongly dispersive Rayleigh waves. An interface has been found approximately be-tween 54 and 57 m depth. The P- and S-wave velocities of the shallow layer are 2000and 550 m/s, according to the slopes of the direct and Rayleigh waves, respectively,and rock-physics empirical relations, while the lower layer has a P-wave velocity of2800 m/s, obtained from the refracted event. A tentative interpretation of the apexof a reflection event locates the top of the basalt layer at 190 m depth approximately.A viscoelastic model has been de ned, with quality factors of typical volcaniclasticsediments. We then compute a synthetic common-shot gather and snapshots by meansof full-wave pseudospectral methods. The synthetic gather resembles the real one, pro-viding additional support to the interpretation.