Abstract

The island of Gotland in the Baltic Sea exhibits rock types dominated by limestone and marlstone, deposited in a Silurian carbonate platform environment. The strata are extensively exposed in outcrops, quarries, and coastal cliff sections. Low-lying marlstone-dominated areas are separated by highs dominated by limestone. The surface bedrock is well known, but the subsurface composition down to a 100-m depth has, until recent deployment of modern geophysical measurements, been largely unknown. Our geophysical surveys and methods were performed with the aim to evaluate their usefulness in characterizing the upper 100 m of the bedrock in a carbonate-dominated bedrock setting. The surveys were performed in connection to a bedrock mapping project including geologic and geophysical investigations. The airborne techniques complemented by ground measurements and correlation with outcrops and boreholes resulted in a definition of bedrock boundaries and especially subsurface characterization of bedding sequences. The airborne electromagnetic (very low frequency) survey was accompanied by radio magnetotelluric (RMT) measurement on the ground along several profiles. The difference in resistivity between high-resistive limestone overlaying a more conductive marlstone gave favorable conditions for this type of survey. The results have so far given new and valuable information on the thickness of the limestone as well as the distinction of lateral changes of the bedrock related to the depositional setting. The RMT measurements have made it possible in a new way to remotely map out bedding sequences in carbonates and to detect saline groundwater and fracture zones. In addition, a recently performed airborne transient electromagnetic survey contributed to the interpretation of the RMT measurements that we evaluated. The applied geophysical methodology and correlation with outcrops revealed carbonate bedrock with geophysical properties, enabling a better understanding concerning the groundwater conditions, limestone resources, and radon situation on the island of Gotland.

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