SEISMIC CHARACTERIZATION OF THE QUATERNARY SEDIMENTS AT LLANCANELO-LAKE AREA, ARGENTINA

摘要

Carcione, J.M., de la Vega, M., Gei, D., Osella, A., Picotti, S., Tassone, A. and Poscolieri, M., 2013. Seismic characterization of the Quaternary sediments at Llancanelo-Lake area, Argentina. Journal of Seismic Exploration, 22: 1-18. A preliminary near-surface seismic survey was carried out at Llancanelo Lake (Argentina) with the aim of mapping the quaternary layers, composed of sedimentary layers overlying basalt. A geological model has been obtained by interpreting the events of a common-shot gather. Expressions of the traveltimes of the direct and refracted waves versus offset provide means to evaluate the velocities of the layers and thickness of the upper layer. Moreover, the standard processing sequence is applied to the data to obtain a stacked section. This procedure reveals the location of deeper interfaces, although it is highly affected by the filtering of the direct and refracted events, which have the same slope of the medium- and far-offset reflection signals. The fact that the direct arrival has not a zero intercept time indicates the presence of a very thin layer at the surface(shallow salt deposits). Further evidence of the near-surface layer is that the refracted and reflection events are ringing and there are strongly dispersive Rayleigh waves. An interface has been found approximately between 54 and 57 m depth. The P- and S-wave velocities of the shallow layer are 2000 and 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 of 2800 m/s, obtained from the refracted event. A tentative interpretation of the apex of a reflection event locates the top of the basalt layer at 190 m depth approximately. A viscoelastic model has been defined, with quality factors of typical volcaniclastic sediments. We then compute a synthetic common-shot gather and snapshots by means of full-wave pseudospectral methods. The synthetic gather resembles the real one, providing additional support to the interpretation.