Mapping and predicting electrical conductivity variations across southern England using airborne electromagnetic data

Abstract

Electrical conductivity of is one of the fundamental geophysical properties of rock formations and can be measured at field and laboratory scales. A recent airborne geophysical survey of the Isle of Wight and part of the adjacent mainland has provided an assessment of the nearsurface (close to outcrop) electrical conductivities associated with Palaeogene and Cretaceous formations. This study examines the degree to which the high resolution survey data contain distinctive geological and lithological signatures. The geostatistical nature of the conductivity distributions are examined in relation to two existing sedimentary bedrock schemes involving lithostratigraphical and simpler lithological descriptions. A close association between conductivity and bedrock geology is evident. It is then demonstrated how the central moments and dispersion statistics of the distributions may be used to predict the continuous, bedrock conductivity distribution across a large area of southern England, containing, as it does, a high population density and extensive infrastructure. The effectiveness of the method is demonstrated using the survey data obtained on the mainland and an existing database of Vertical Electric Soundings. Page 3 of 34The electrical properties of UK soils and rocks were investigated during the 1930s in both the laboratory and using geophysical field measurements. These early measurements were intended to provide a framework for understanding the potential impact of ground conductivity on the then developing radio and telephone transmission systems. The laboratory experiments, typically using 'soil' samples to depths of about 3 m, are described by Smith-Rose (1933, 1935 During the latter half of the last century more routine electrical and electromagnetic geophysical measurements provided a wealth of localised survey information for a variety of applications across the UK. Many of the measurements comprised vertical electric soundings (VES) for groundwater applications. The National Resistivity Sounding Database (Barker et al., 1996) was developed from over 8,000 such soundings. The database information, comprising raw resistances, continues to be used in a geological context (Cuthbert et al., 2009;Busby et al., 2011). Electromagnetic measurements have also been used to guide geological mapping, usually in the near-surface (e.g. Zalasiewicz et al., 1985;Cornwell & Carruthers, 1986) Having established the statistical behaviour of the near-surface conductivity of the geological units across the IoW, the ability of these data to perform predictive mapping across a large central area of southern England is considered. The effectiveness of this procedure is demonstrated using both survey data obtained on the mainland and an existing database of Vertical Electric Soundings. The extended bedrock conductivity map has the potential to allow assessments of the degree to which localised measurements are consistent with, or represent departures from, the norm. The GIS-based predictive mapping of geophysical inf...

A lithological assessment of the resistivity data acquired during the airborne geophysical survey of Anglesey, North Wales

“…These sets of lithologies represent the youngest rocks under investigation, principally deposited in the Carboniferous, and the results point to an increase in resistivity with geological age. A similar conclusion was drawn from the younger Palaeogene and Cretaceous rocks on the Isle of Wight (Beamish & White, 2012). The most resistive lithology, at the highest frequency, is associated with the poorly sampled quartzite (QZITE).…”

“…This is a common feature of many large geophysical data sets (Beamish & White, 2012). The linear distributions of the lithologically distinct data sets display a long tail (sometimes two) and are highly peaked.…”

“…HiRES data have previously been used to investigate the extent to which conductivity data acquired over the relatively young rocks of southern England relate to bedrock characteristics (Beamish & White, 2011c;Beamish & White, 2012). Here, the same procedure is extended to investigate the correlation of lithology with electrical resistivity over the older, hard-rock geology found on the Island of Anglesey.…”

“…Here, the same procedure is extended to investigate the correlation of lithology with electrical resistivity over the older, hard-rock geology found on the Island of Anglesey. Adopting a generic lithological description of the rocks increases the portability of the technique and is most appropriate for geophysical attribution (Beamish & White, 2012). However, whilst Anglesey is a Precambrian and Cambrian domain overlain by a spatially incomplete Palaeozoic succession, its landscape and near-surface features were reshaped during the last ice age.…”

“…Schist describes fine grained foliated rocks with varying degrees of schistosity. However, this generic description of rock lithology is assumed to be the most appropriate scheme for geophysical attribution at medium to large spatial extents (Beamish & White, 2012). At this scale, maps highlighting the statistical nature of the geophysical data can be used to infer the degree to which individual measurements are consistent with, or depart from, the norm.…”