Caracterización geosísmica de un sector de traza de la Ruta Nº 40: Provincia de Santa Cruz - Argentina

Abstract

With the purpose to characterize underground distinct quality terrain related to pavement project in Ruta Nacional Nº40, in Santa Cruz province, there were carried through a seismic refraction survey. The objective pursued was to calculate the rippability of subsoil beneath the road trace along 5000m extension, spanned in two sectors near among them. This determination permitted to assess blasting or ripper. The study area presents a typical steppe patagonian landscape with basaltic formations indicating old volcanism. At surface, fine to medium-coarse alluvial soil was observed detecting on occasions organic material and besides great basaltic rocks spreading somewhere. Fifty seismic arrays were performed, deployed linearly one after the other along the trace. At each one, five records were taken, symmetrically upon the array center. Geophones and shooting points were determined upon prior proofs tests and space availability. Generally array extension ranged among 25m and 100m, being 5m the geophone gap, allowing thus a great survey detail. The amount and distribution of shootings had for objective to calculate with utmost precision apparent velocities and their true ones derived; refractor inclinations and rock basement; assuring at least a 15m depth investigation. At the processing stage, first picking were determined, which were displayed versus geophones distances, i.e. time-distance or ‘dromocrone’ graphs. After calculating critical distances, apparent velocities, were estimated. Then, two interpretation methods were carried through: Intercept Time Method (ITM); and Generalized Reciprocal Method (GRM). With the latter, true velocities and depths beneath each geophone were calculated, and with the former, beneath each shot-point, extending so the determinations. The correlation with geologic data was performed using visual inspection and very scarce geologic information available. At last, seismic profiles were developed in separate form according to trace outreaching showing lateral and in-depth variations of compression wave velocity. In both survey areas, two well different layers were detected, being the first a thin sand-muddy alluvial fill-up and the inferior, altered rock (basalt) (Zone 1); or compact alluvial terrain, wet to saturated. Information supported by geoseismic refraction method using GRM with 5 emission points defined with confidence and resolution the layers beneath the RN Nº40 trace along the sectors considered. Besides, lateral resolution of this method made possible to detect with precision lateral velocitythickness changes, of utmost importance when determining rippability.