A Joint TEM‐HLEM Geophysical Approach to Borehole Siting in Deeply Weathered Granitic Terrains

Abstract

The accurate location of aquiferous fracture zones in granite beneath a > 50 m thick weathered mantle in semi-arid regions is a major hydrogeological problem. It is expected that the zone of intensive fracturing will be more susceptible to weathering and thus be characterized by the thickest development of saprolite, a good electrically conductive target for deep-probing electromagnetic systems. The single-loop transient electromagnetic (TEM) technique is well known to have the capability for detecting concealed steep mineralized targets in mining environments and can be adapted to this hydrogeological problem. We propose that combining the conventional frequency-domain horizontal-loop electromagnetic (HLEM) and single-loop TEM is an effective practical approach to locating concealed aquiferous fracture zones. In the supporting case studies presented here, we deployed multifrequency HLEM profiling (with 50 m transmitter-receiver separation) and TEM soundings with contiguous 10 or 20 m sided loops along the survey lines in a granitic terrain affected by deep (> 50 m) weathering in northeast Brazil. A somewhat layered structure consisting of resistive hardpan/leached zone, conductive saprolite, and resistive basement is identifiable in the typical TEM depth sounding data. We obtained coincident HLEM and TEM anomalies at all the sites, enabling a relatively straightforward selection of potential drilling positions. Simple resistivity-depth transformation of the TEM data was done for each site, yielding an approximate section from which drilling depths were estimated. All of the boreholes located were successful. Although our results appear to indicate that the single-loop TEM method could be used independently for borehole sitting in deeply weathered granitic terrains and that the weathering profile over granite can be mapped using TEM depth soundings of appropriate observational bandwidth, we recommend a joint electromagnetic approach for optimal well sitting.

Return-flow prediction and buffer designation in two rainforest headwaters

“…These percolines may not have much surface expression, but may focus subsurface water to a point. (5) If fractures in the granite are oriented towards the wet zones (Meju et al, 2001), then these would bring more water than predicted by the surface-derived, topographic index. (6) Natural soil pipes are present with Ultisol soils at several sites throughout the tropics (Baillie, 1975;Jones, 1986Jones, , 1990Chappell and Sherlock, 2005).…”

“…Further field investigations, notably geophysical studies (e.g., Meju et al, 2001) would help identify which of these factors are locally the most important. The critical point being that the evaluation of the topographic index against measured soil moisture patterns should be seen as the starting point for further investigations into the nature of the soils and geology of a research catchment, rather than a final study.…”

“…The patchy nature of near-saturated soils within streamside areas, as seen in the Huai Pacha catchment, does, however, indicate that topography alone may be insufficient to characterise the spatial distribution of rainfallrunoff processes within micro-catchments. Indeed, where measurements of soil moisture (or capillary potential) within an experimental catchment indicate different patterns to those of a topographic index, then field campaigns of geophysical measurements (e.g., Meju et al, 2001) need to be utilised to map the changing depth to rock-head, soil permeability, fracture patterns, etc. Such information may then be incorporated into a revised soil-topographic index (Pellenq et al, 2003;Lyon et al, 2004).…”

Audiomagnetotelluric 3D imaging of the Regis kimberlite pipe, Minas Gerais, Brazil

“…Electromagnetic methods have a wide range of applications to several near-surface exploration issues, such as hydrocarbon (Menezes and Moraes, 2003), groundwater (Meju et al, 2001), environmental surveying (Vrbancich, 2011) and mining (Nabighian and Asten, 2002 and references therein). For kimberlite exploration, airborne EM surveys are intensively used as they can directly locate pipes and define optimal targets for ground follow-up surveys that are usually performed with TDEM (time-domain EM) or FDEM (frequency-domain EM), which are fast and cost-effective methods (Nabighian and Asten, 2002).…”

Near-Surface Controlled-Source Electromagnetic Induction

“…The CSEM method is also able to identify conductive fracture zones in crystalline bedrock aquifers for groundwater prospecting applications (Meju et al, 2001). In FDEM applications, fracture zones are typically identified by the presence of anomalously high apparent conductivity readings along a measurement profile.…”