A Letter from Tony Beamish to Eric Robertson

This letter refers to the John Wren-Lewis correspondence

Low induction number, ground conductivity meters : a correction procedure in the absence of magnetic effects

Ground conductivity meters, comprising a variety of coil–coil configurations, are intended to operate within the limits provided by a low induction number (LIN), electromagnetic condition. They are now routinely used across a wide range of application areas and the measured apparent conductivity data may be spatially assembled and examined/correlated alongside information obtained from many other earth science, environmental, soil and land use assessments. The theoretical behaviour of the common systems is examined in relation to both the prevailing level of subsurface conductivity and the instrument elevation. It is demonstrated that, given the inherent high level of accuracy of modern instruments, the prevailing LIN condition may require operation in environments restricted to very low (< 12 mS/m) conductivities. Beyond this limit, non-linear departures from the apparent conductivity that would be associated with a LIN condition occur and are a function of the coil configuration, the instrument height and the prevailing conductivity. Using both theory and experimental data, it is demonstrated that this has the potential to provide biased and spatially distorted measurements. A simple correction procedure that can be applied to the measured data obtained from any of the LIN instruments is developed. The correction procedure would, in the limit of a uniform subsurface, return the same (correct) conductivity, irrespective of the ground conductivity meter used, the prevailing conductivity or the measurement height

A geological and hydrogeological assessment of the electrical conductivity information from the HiRES airborne geophysical survey of the Isle of Wight

A recent high resolution airborne geophysical survey across the Isle of Wight (IoW) and Lymington area has provided the first electromagnetic data across the relatively young geological formations characterising much of southern England. The multi-frequency data provide information on bulk electrical conductivity to depths of the order of 100 m. A GIS-based assessment of the electrical conductivity information in relation to bedrock geological classification has been conducted for the first time. The analysis uses over 104,000 measurements across onshore IoW and has established average and statistical properties as a function of bedrock geology. The average values are used to provide baseline maps of apparent electrical conductivity and the variation with depth (measured as a function of frequency). The average conductivity as a function of depth within the main aquifer units is summarised. The data indicate that the majority of the Palaeogene is characterised by values consistently in excess of 100 mS/m and with a surprisingly high degree of spatial heterogeneity. The youngest (Oligocene) Hamstead Member displays some strong edge effects and the largest localized values in conductivity. The central Upper Chalk is associated with the lowest observed conductivity values and mineral content and/or porosity appears to increase with increasing age. The large central outcrop of the Lower Greensand Group, Ferruginous Sand Formation provides persistently low (<30 mS/m) conductivity values which imply a relatively uniform distribution of clean sand content. Non-geological (e.g. environmental) responses are contained within the data set and examples of these in relation to a closed municipal landfill and an area of potential coastal saline intrusion are discussed. In the south, the Gault clay/mudstone of the Early Cretaceous appears as a distinctive conductive unit. Cross sectional modelling of the data has been undertaken across the aquifer units of the Southern Downs. The results indicate that the Gault Formation, acting as an aquitard, can be traced as a distinct unit under the more resistive Early Cretaceous Upper Greensand and Late Cretaceous Chalk formations. The conductivity modelling should therefore allow an estimation of the subsurface configuration of the aquifer and aquitard units

A radiometric airborne geophysical survey of the Isle of Wight

A high resolution airborne geophysical survey across the Isle of Wight and Lymington area conducted in 2008 provided the first modern radiometric survey across the geological formations that characterise much of southern England. The basic radiometric data are presented and it is evident that bedrock geology exerts a controlling influence on the broad response characteristics of the naturally occurring radioelements. A GIS-based geological classification of the data provides a quantitative assessment and reveals that a relatively high percentage of the variability of the data is explained by the Cretaceous bedrock geology but this is much reduced in the Palaeogene. The three traditional Chalk units (Lower, Middle and Upper Chalk depicted on the currently available Geological Map) provide the lowest and most distinct behaviour within the Cretaceous sequence. Mineral content within the Chalk appears to increase with increasing age. A new method of representing the baseline radiometric information from the survey in terms of the mean values of the geological classification is presented. The variation of radioelement geochemistry within individual formations is examined in two case studies from the Cretaceous Lower Greensand Group and the Palaeogene Hamstead Member (Bouldnor Formation). The Cretaceous sequences provide the higher levels of discrimination of localised variations in radioelement distributions. A more detailed case study examines the potential influences from the degree of water saturation in the soil and superficial deposits

Freezing and pressure-driven flow of solid helium in Vycor

The recent torsional oscillator results of Kim and Chan suggest a supersolid phase transition in solid He-4 confined in Vycor. We have used a capacitive technique to directly monitor density changes for helium confined in Vycor at low temperature and have used a piezoelectrically driven diaphragm to study the pressure-induced flow of solid helium into the Vycor pores. Our measurements showed no indication of a mass redistribution in the Vycor that could mimic supersolid decoupling and put an upper limit of about 0.003 um/s on any pressure-induced supersolid flow in the pores of Vycor.Comment: 5 pages, 4 figure

Anglesey (Ynys Môn) : 600 million years of Welsh history

Earth scientists from the British Geological Survey (BGS) are applying new techniques to advance our understanding of the geological evolution of Anglesey, North Wales, unravelling over 600 million years of Earth history. Now into their third year of study, they have already made some startling new discoveries about processes at opposite ends of the geological time scale. David Schofield, Emrys Phillips and David Beamish tell us more

Intrinsic and dislocation induced elastic behavior of solid helium

Recent experiments showed that the shear modulus of solid 4He stiffens in the same temperature range (below 200 mK) where mass decoupling and supersolidity have been inferred from torsional oscillator measurements. The two phenomena are clearly related and crystal defects, particularly dislocations, appear to be involved in both. We have studied the effects of annealing and the effects of applying large stresses on the elastic properties of solid 4He, using both acoustic resonances and direct low-frequency and low-amplitude measurements of the shear modulus. Both annealing and stressing affect the shear modulus, as expected if dislocations are responsible. However, it is the high temperature modulus which is affected; the low temperature behavior is unchanged and appears to reflect the intrinsic modulus of solid helium. We interpret this behavior in terms of dislocations which are pinned by isotopic 3He impurities at low temperatures and so have no effect on the shear modulus. At higher temperatures they become mobile and weaken the solid. Stressing the crystal at low temperatures appears to introduce new defects or additional pinning sites for the dislocation network but these effects can be reversed by heating the crystal above 500 mK. This is in contrast to dislocations produced during crystal growth, which are only annealed at temperatures close to melting