Shallow, high-resolution seismic reflection in the study of the Quaternary in foothill areas

Several trial shallow seismic reflexion surveys were carried out in distal alluvial fan settings in Salta Province, Argentina. The tests span two areas with contrasting subsurface characteristics. In the north-eastern sector of the province, the extensive Bermejo alluvial fan deposits are sandy and massive, with almost no clay intercalations; the water table in this area was located below a depth of 40 m at the time of the survey. The other survey area is located south of Salta City, where small, coalescing alluvial fans fill the Valle de Lerma with sandy and gravelly deposits that in the distal, axial reaches interfinger with thick clayey beds; the water table in this area was located below a depth of 20 m at the time of the survey. Seismic operation involved a Geometrics R24 seismograph with 24 active channels that were incremented to 48 using a roll-over switch, providing a maximum 1200% subsurface coverage. Two seismic source types were tested: a 5 kg sledge hammer and a 12-gauge down-hole seismic gun. Mainly due to operational considerations the hammer source was finally chosen. Tests on the Bermejo fan generally resulted in poor quality seismic records, likely due to rapid loss of energy through the unconsolidated, dry sands. Locally, however, three seismic units could be distinguished and the top of the Tertiary bedrock was possibly mapped. Tests along the axis of the Valle Lerma resulted in excellent quality seismic records. Four seismic units were distinguished. A weak reflector at about 220 msec marks a shift to velocities of around 2500 m/s and may correspond with the top of Tertiary bedrock. A thick clay unit at 25-50 m depth can be clearly mapped and acts as an aquitard. The results of this pilot study show that shallow seismic reflection can be applied profitably in NW Argentina for subsurface imaging in distal alluvial fan environments.