Hydrological outlook UK - December 2018

Earth Sciences

British Geological Survey

关键词: Magnetotelluric;    resistivity;    Southern Uplands;   

学科分类：地球科学(综合)

英国|英语

来源: NERC Open Research Archive

【 摘 要 】

This report describes geoelectric modelling using data obtained from a reconnaissance MagnetoTe1luric (MT) survey carried out as part of the Southern Upland mapping programme. The 8 site survey, along a l4 km NW-SE pro铿乴e, was centred on the Ae forest area to the east of Thornhill and to the SW of Moffatt. The measurements were undertaken across one of the fault-bounded tract sequences of greywackes in the Central Belt. The purpose of the limited survey was to investigate the possibility of 鈥榣ocally鈥?detecting concealed tectonostratigraphic structure by virtue of its (possible) resistivity expression.The MT method attempts to provide a resistivity cross-section, often through the whole crust.The sensors currently available restrict information to depths greater than several hundred metres.The present survey was carried out using a new 7-channel 铿乪ld instrument developed in-house by the BGS.The present report provides information on the modelling of the survey data and presents bothone-dimensional (1-D) vertical pro铿乴es and a 铿乶al 2-D whole crustal geoelectric section. l-D inversion of data from two sites within the Palaeozoic Lochmaben basin provide estimates of basin depth comparable with estimates obtained from gravity modelling. 2-D inversion of the pro铿乴e data is undertaken using a new algorithm (OCCAM 2D) to provide a resistivity cross-section through the whole crust. Resistivities greater than 1000 ohm.m, tentatively associated with Silurian greywackes, extend toa depth of about 12 km. A relatively conducting (resistivities < 100 ohm.m) lower crust occurs atdepths greater than l5 km in keeping with previous, low frequency MT observations across the Southern Uplands. A major set of lateral resistivity gradients are found, centred on a depth of about 2.5 km, to the NW of the northern end of the pro铿乴e. Underlying this anomaly a modi铿乧ation in lower crustal resistivity is apparent at depths greater than l5 km. The resistivity cross-section suggests that a form of whole crustal modi铿乧ation occurs to the NW of the survey pro铿乴e. The location of the main anomaly remains speculative since the reconnaissance survey was of limited length.