Airborne EM applied to environmental geoscience in the UK

The British Geological Survey (BGS) has been highlighting the need for modern, multi-sensor airborne geophysical data in the UK. Here David Beamish, geophysicist with the BGS, describes the first trial airborne electromagnetic data acquired and its relevance to environmental geoscience. The lack of modern, multi-sensor (magnetic, radiometric and electromagnetic) data represents one of the most serious gaps in the geoscience knowledge base of the UK, and a national, high resolution airborne survey has been a stated corporate objective for many years. In 1999, the fixed-wing, frequency domain, airborne EM (AEM) system developed and operated by the Geological Survey of Finland was used in a series of trials to acquire detailed EM data sets in addition to magnetic gradiometer (wing-tip) and radiometric information. The purpose of the trials was, in part, to assess the case for the inclusion of AEM in future strategic airborne geophysical surveying. The limited data acquired (3324 line km in 5 days’ flying) constitute the first high resolution AEM survey information to address specific environmental issues in the UK. It was anticipated that the AEM data would provide pathfinder information for the general assessment of land quality issues such as planning and pollution control and water supply/resource protection

A deep geoelectric survey of the Carnmenellis granite

A magnetotelluric survey across the Carnmenellis granite, part of the Cornubian batholith of SW England, is described. The granite is being investigated with reference to the extraction of Hot Dry Rock geothermal energy. The sounding data are found to be influenced by near-surface static distortion which can be minimized by the construction of a granite-average sounding tensor. The anisotropy in the average tensor can be attributed to the spatial form of the granite. This is verified by 2-D modelling and the model results are used as control in the assessment of the vertical resistivity profile. The resistivity values obtained by the survey are found to correspond to a saturated ‘wet’ granite down to at least 10 km. Despite the high geothermal gradient the anticipated decrease of resistivity with increasing temperature does not take place. Below a depth of just over 1 km, resistivity values are found to increase to a depth of about 6 km. The results are compared with the laboratory-observed response of granite to increasing applied pressure and stress. The resistivity/depth section appears consistent with the completion of joint and crack closure by a depth of 7 km and a transfer to a pore-dominated resistivity mechanism below this depth. An electrical base to the granite is resolved at a depth of 14 km in accord with the depth to base adopted by regional gravity models

The time-dependence of electromagnetic response functions

Dynamic processes within the Earth are capable of modifying many of its physical properties. This review considers the modification of electrical resistivity mainly by time-changes in the stress­ field experienced by crustal materials. Laboratory experiments indicate that certain rock types, whose resistivity is controlled by the volume and geometry of microcracks, can undergo large resistivity variations in response to the typical strain accumulation and release that accompanies major earth­ quakes. Electromagnetic response functions are reviewed with regard to their ability to monitor time­ changes of resistivity in active regions. Some examples of the different experimental techniques used to investigate precursory changes in response functions prior to earthquakes are noted. The problems and successes of the subsequent correlation and interpretation of the derived response as a function of time are then discussed. Only relatively few controlled and integrated studies of time-dependent electromagnetic response functions exist. Those which are available, however, emphasise that changing stress-differentials occurring before major earthquakes can be monitored using suitable electromagnetic techniques

The tilt derivative applied to AEM conductivity data

Various procedures are used in the processing of potential field data to provide enhanced detection and definition of structural information. The majority of such procedures use the spatial derivatives of the data either individually or in combination. The ability of the tilt angle (or derivative) to provide enhanced mapping of electromagnetic (EM)/conductivity structure is considered here. Although this study considers airborne EM survey data, the concept can equally be applied to ground-based conductivity/resistivity data sets. Following a description of the principles of the tilt derivative (TDR) method, forward modelling studies of concealed EM/conductivity structure are presented. Case studies of the practical application of the procedures to survey data are then performed. The tilt function embodies Automatic Gain Control that normalises the detection and definition of both weak and strong conductivity gradients across an appropriate subsurface depth range. Noise amplification is an inevitable outcome of the procedure. Filtering methods to attenuate undesired artefacts are available and are demonstrated

Assessing soil wetness with airborne radiometric data

A valid interpretation model for UK radiometric data requires a joint assessment of both soil and bedrock variations. Although the geological bedrock (the parent material) provides a specific radiogenic level with associated radiochemical attributes, attenuation of the signal level is controlled by soil wetness in conjunction with the density and porosity of the soil cover. Peat soils, in particular, produce readily identifiable attenuation zones. Other soil types are also predicted to attenuate radiometric signal levels but at lower wetness sensitivities. The amount of water stored in the soil is of fundamental importance to agriculture Studies of soil attenuation zones have now been undertaken using a number of modern UK radiometric data sets. Case studies are presented which highlight the procedures and results obtained. A variety of implied increases in wetness, at the scale of the airborne measurements, have been identified across a range of soil types, including those categorized as freely draining. The level of attenuation (and hence wetness scale) remains uncalibrated since ground information on wetness is not generally available at the appropriate scale

Measurement of model aeroelastic deformations in the wind tunnel at transonic speeds using stereophotogrammetry

A stereophotographic method of determining the aeroelastic deformations of an airplane model under aerodynamic load in the wind tunnel was evaluated. Wind tunnel tests were conducted in the Langley 8 foot transonic pressure tunnel on the wing of a 0.0625 scale model of the TF-8A supercritical wing research airplane to obtain simultaneously the aerodynamic forces and moments, pressure distributions, and stereophotographs. Tests were conducted at Mach numbers of 0.80, 0.95, and 1.20, and at free stream dynamic pressures of 20,349 Pa and 40,698 Pa. Accuracy of the stereophotographic technique in determining wing deflections was within 0.013 cm under static conditions. This value translates to an error in wing twist of 0.10 deg inboard and increases to 0.20 deg outboard. When the model was under aerodynamic load in the wind tunnel, the accuracy of the stereophotographic technique of determining wind deflections increased to 0.052 cm when compared with static wing loadings because of the dynamic motion of the model in the tunnel

An AEM survey of a leaking landfill

This study presents results obtained from a remarkably small-scale helicopter airborne electromagnetic (AEM) survey of a closed landfill. The landfill, occupying a former quarry, is situated among shallow, worked-out coal seams (pillar and stall workings) and was located over at least two mineshafts that occupied the quarry floor. The landfill was known to be leaking from an extensive borehole investigation that took place in the 1970’s, when the landfill was active. Redevelopment issues and associated, proposed surface extraction of coals have renewed interest in the possible pollution threat of waste products. Of particular concern is the extent to which pollution has, potentially, transgressed a fault to the north and entered the area of proposed regeneration. The airborne survey data (EM and magnetic) were obtained using the six-frequency RESOLVE bird using a flight line spacing of 40 m. The data acquired have been subjected to 1D regularized inversion to enable a 3D conductivity model to be assessed. The model clearly identifies the landfill as a source term of highly conductive materials. At conductivities that are a factor of 3 above background, evidence for a variety of vertically compact, lateral migration pathways, largely below the base of the quarry, is obtained

Optimal mapping of terrestrial gamma dose rates using geological parent material and aerogeophysical survey data

Regulatory authorities need ways to estimate natural terrestrial gamma radiation dose rates (nGy h−1) across the landscape accurately, to assess its potential deleterious health effects. The primary method for estimating outdoor dose rate is to use an in situ detector supported 1 m above the ground, but such measurements are costly and cannot capture the landscape-scale variation in dose rates which are associated with changes in soil and parent material mineralogy. We investigate the potential for improving estimates of terrestrial gamma dose rates across Northern Ireland (13542 km2) using measurements from 168 sites and two sources of ancillary data: (i) a map based on a simplified classification of soil parent material, and (ii) dose estimates from a national-scale, airborne radiometric survey. We used the linear mixed modelling framework in which the two ancillary variables were included in separate models as fixed effects, plus a correlation structure which captures the spatially correlated variance component. We used a cross-validation procedure to determine the magnitude of the prediction errors for the different models. We removed a random subset of 10 terrestrial measurements and formed the model from the remainder (n = 158), and then used the model to predict values at the other 10 sites. We repeated this procedure 50 times. The measurements of terrestrial dose vary between 1 and 103 (nGy h−1). The median absolute model prediction errors (nGy h−1) for the three models declined in the following order: no ancillary data (10.8) > simple geological classification (8.3) > airborne radiometric dose (5.4) as a single fixed effect. Estimates of airborne radiometric gamma dose rate can significantly improve the spatial prediction of terrestrial dose rate

UNICEF IWASH Project, Northern Region, Ghana : an adapted training manual for groundwater development

This report is an adapted training manual, with specific best practice recommendations for groundwater development practitioners working in the Northern Region, Ghana. It is designed to be used in conjunction with the existing comprehensive training manual ‘Developing Groundwater: a guide to rural water supply’ by MacDonald, Davies, Calow and Chilton (2005). The additional guidelines provided in this supplementary report are specific to the Northern Region of Ghana, and have been informed by a review of groundwater development in the region which BGS carried out on behalf of UNICEF in 2010-2011. The Northern Region is a difficult area in which to find and develop groundwater resources. For this reason, more resources – time and money – need to be focussed on careful borehole siting and development in order to maximise success. This includes detailed desk and field reconnaissance surveys; the effective use and interpretation of geophysical siting methods; collection of good quality data during drilling and test pumping; rigorous recording and management of data; and effective interpretation, sharing and use of hydrogeological information by all groundwater development practitioners. This report, and the associated manual ‘Developing Groundwater’, provide practical help for carrying out these activities effectively. The authors gratefully acknowledge those persons who contributed to the formation of these guidelines, who include: UNICEF Ghana – Othniel Habila, Kabuka Banda and David Ede Community Water and Sanitation Agency (CWSA), Ghana – John Aduakye Canadian International Development Agency (CIDA) – Hydrogeological Assessment Project (HAP) – James Racicot All participants at the UNICEF/BGS workshop and training programme held in Tamale, Northern Region, from 7 to 18 February 2011

Low frequency elastic measurements on solid 4^{4}He in Vycor using a torsional oscillator

Torsional oscillator experiments involving solid $^{4}$He confined in the nanoscale pores of Vycor glass showed anomalous frequency changes at temperatures below 200 mK. These were initially attributed to decoupling of some of the helium's mass from the oscillator, the expected signature of a supersolid. However, these and similar anomalous effects seen with bulk $^{4}$He now appear to be artifacts arising from large shear modulus changes when mobile dislocations are pinned by $^{3}$He impurities. We have used a torsional oscillator (TO) technique to directly measure the shear modulus of the solid $^{4}$He/Vycor system at a frequency (1.2 kHz) comparable to that used in previous TO experiments. The shear modulus increases gradually as the TO is cooled from 1 K to 20 mK. We attribute the gradual modulus change to the freezing out of thermally activated relaxation processes in the solid helium. The absence of rapid changes below 200 mK is expected since mobile dislocations could not exist in pores as small as those of Vycor. Our results support the interpretation of a recent torsional oscillator experiment that showed no anomaly when elastic effects in bulk helium were eliminated by ensuring that there were no gaps around the Vycor sample.Comment: Accepted by Journal of Low Temperature Physic