Analysis of resolution and sensitivity of boat-towed RMT dataover Lake Mälaren, Stockholm
(English)In: Interpretation, ISSN 0020-9635, E-ISSN 2169-4494Article in journal, Editorial material (Refereed) Submitted
We examine the resolution and sensitivity of water-borne boat-towed multi-frequencyradio magnetotelluric (RMT) data for delineating weak zones in crystalline environmentunder a lake or a river. 2D modeling of RMT data in joint transverse electric (TE) andtransverse magnetic (TM) and determinant mode of 40 profiles over two water passages fromLake Mälaren near the city of Stockholm was used for this purpose. The 2D modeling revealsthat the estimated models have responses that fit the observed data reasonably well. Themodels show an overall agreement when compared with existing marine seismic results,bathymetry data and other available geological knowledge from the site. A low-resistivityzone observed in the middle of all profiles is suggested to be from a fracture/fault zonestriking in the direction of the water passages. Because RMT is an inductive-based method,the presence of conductive lake sediments shields scanning the underlying fractured bedrock.To reliably interpret the RMT data, synthetic modeling using a three-layered generic modelwith a conductive layer of varying thickness sandwiched between resistive water andfractured crystalline bedrock was performed. The modeling results suggest that lowerfrequency signal is required to have greater depth of penetration that would better resolve thebedrock and fracture zone although some evidence of fractured bedrock can still be inferredfrom the data. The study also shows the advantage of having a water layer that acts as a nearhomogeneous medium that eliminates near surface static shift effects. We propose that theboat-towed RMT acquisition system should be upgraded to enable controlled-source acquisition to improve the depth penetration and to overcome the shortcomings of usingradio-frequencies as signal.
IdentifiersURN: urn:nbn:se:uu:diva-267996OAI: oai:DiVA.org:uu-267996DiVA: diva2:875250