Influence of a narrow strait connecting a large ocean and a small sea on magnetotelluric data: Gibraltar Strait

Abstract

Long period magnetotelluric (LMT) data are generally used as the prime tool to shed light on the conductivity of the deep crust and upper mantle structure. Naturally, LMT data are sensitive to the influence of highly conductive bodies even at far distances. Sea water constitutes the main widespread electrical conductor at shallow levels of the Earth's crust. Its low resistivity values, around 0.25 Ωm, contrast sharply with most geological structures of solid earth. Consequently, the presence of irregular continental borders and varying bathymetries significantly influences LMT parameter, such as phase tensor and tipper vectors. This effect is especially important in areas where large and small oceans are connected by narrow gateways. The Gibraltar Strait (southern Spain) connects the vast Atlantic Ocean with the irregular and relatively small Mediterranean Sea. Several 3D models have been developed in the region nearby the Strait, some featuring a roughly simplified geological structure, to show the influence of seawater on tipper and phase tensor. Here, detailed bathymetry (General Bathymetric Chart of the Oceans, GEBCO) including the coastline is considered in a model that enables us to analyze the influence of the Strait of Gibraltar on LMT data. We stress the significance of the Sea for LMT data interpretation and the development of 3D conductivity models, especially in regions involving complex coastline geometries and bathymetry near narrow straits. Forward modeling studies are essential to accurately appraise the sea influence, which may mask 3D geological structures.