Investigation of the subsurface using geophysical methods

Abstract

The investigation of sub-surface geology is important especially for the planning of roads, buildings, and strategic structures (example a power station building), as well as for archeological investigation and evaluation of site effects due to earthquakes. The use of passive techniques, like the use of microtremors is becoming more popular in these type of investigations, due to the fact that it is low-cost, easy to use and non-invasive (no environmental impact). In this study, the Salini area (Burmarrad/Qawra area) was investigated. Thousands of years ago this area was a valley with a Lower Coralline Limestone base, according to the Maltese geological map. Through the ages until present date, this area was filled with layers of sediments which today make this area nearly a flat plain. The main goal of this project is to gain information about some physical properties of the sediments covering the area and to estimate their thickness. This allows for the generation of a 3D model of the sediment pack and the reconstruction of the shape of the paleo-valley beneath them. In order to retrieve the shear-wave velocity of the sediments covering the valley, 17-geophone arrays were deployed in two different locations. Furthermore, using a portable seismometer, ambient noise was collected at around 40 locations, processing the data using the H/V technique. The results show a very important change in the shape of H/V curves, reflecting the strong variation in the thickness of the deposits. Using the shear-wave velocity obtained from the geophone array measurements, synthetic H/V plots were constructed and an estimate of the thickness of the sediment at each location was obtained. The final results are in good agreement with the data gathered in the area from 5 boreholes drilled as part of the Coast Road project. The comparison showed that the sediment thickness obtained from ambient noise was similar to that measured by the boreholes. A 3D map of the sediment thickness clearly reveals the morphology of the buried valley. Finally, we inverted our data in order to obtain a thickness-frequency relationship valid for the study area. This was significantly different from similar relationships derived in other regions in Europe.