A geophysical survey to estimate the thickness of Upper Coralline Limestone and Blue Clay strata in the Mgarr-Wardija (Malta) locality

Abstract

The current identification of strata in the Maltese Islands is dependent on them being the surface stratum for the Maltese geological map or dependent on being associated with particular geomorphological features. The method opted for this dissertation, Spectral H/V ratio, can be used in most types of environments and hence does not depend on geomorphological features to be utilized; instead it is based on the seismic noise wave fields which are found throughout the crust. The aim of this dissertation is to apply the seismic noise wave fields via Spectral H/V ratio method and Grilla ® software to determine the estimated thickness value of the first two common stratigraphic layers of Malta, being Upper Coralline Limestone and Blue Clay. Geologically wise, these two strata have different properties and can be used for different reasons; particularly the presence of Blue Clay gives rise to the presence of the perched water aquifer, allowing access to one of the most important resources which Malta requires. The boundary between Upper Coralline Limestone and Blue Clay leads to a high velocity inversion which is observed in the gathered data. Utilizing the AutoCAD software, illustrations were produced to show the strata thickness and continuity along horizontal and vertical planes, according to how the sections A to F were connected. Overall, the illustrations and values of thickness show that Upper Coralline Limestone typically has thickness value of 35 metres but at some sites have 20 metres and decreasing in value until inexistence. Upper Coralline Limestone is always found as the surface stratum with exceptions for site MG10, MG13 and MG15 which have Blue Clay as topmost stratum layer. Blue Clay overall a thickness value of 45 metres with some sites having a larger or smaller thickness value according to where the sites are located. The overall significance of the results is that firstly the thicknesses of Upper Coralline Limestone and Blue Clay are now known for the Mgarr-Wardija zone and approximations can be made for the surrounding regions. Average strata thickness values were not available for any region in the Maltese Islands, however now there is. The thickness values give certain specific applications in the Mgarr-Wardija area but the main service that this dissertation offers is that it acts as a baseline study for countless other fields.