The HiRES airborne geophysical survey of the Isle of Wight: Logistics Report

This report provides a summary of the logistics of the HiRES airborne geophysical survey conducted in September and October 2008 across the Isle of Wight and part of the Lymington area. The survey was carried out by the Joint Airborne-Geoscience Capability (JAC) established between the Geological Survey of Finland (GTK) and British Geological Survey (BGS). The project is a HiRES survey carried out by the Geophysical Baselines Team under the Environmental Geoscience Baselines Programme. The survey was conducted at high resolution (a flight line spacing of 200 m) and at low altitude (56m) rising to >200 m in the vicinity of conurbations. The three main data sets acquired are magnetic, radiometric (gamma ray spectrometry) and active frequency domain electromagnetic (AEM). The aim of the present report is to provide descriptions of the logistical and in-field processing elements of the survey operations

Local and regional induction in the British Isles

Electric currents induced in the shallow seas and deep ocean around the British Isles have a profound effect upon the electromagnetic fields observed at stations on the land. The configuration of anomalous currents changes with frequency, and causes corresponding changes in the geomagnetic transfer functions. Magnetic variations have been recorded at a dense network of sites in southern Scotland and northern England. Single­ station transfer functions have been used to generate hypothetical event maps of the anomalous vertical field, and hence to infer the configuration of the anomalous internal currents. At periods exceeding 2000 s, the vertical field is dominated by the effects of electric currents to the west, presumably in the Atlantic Ocean . In the period range 400-2000 s, anomalous currents are concentrated in a thin sheet comprising the shallow seas, the t hick sequences of post-Caledonian sedimentary rock which underlie them, and the extensions of the sedimentary basins into the land. The response of the individual basin is determined not only by its local conductivity structure, but also by the extent of its connection to the shallow seas,i.e. its regional importance within the conductive sheet. At periods less than 200 s on the other hand, the anomalous fields at inland sites are principally determined by the local geological structure . These results confirm conclusions reached from theoretical studies of electromagnetic induction in a heterogeneous surface layer (Park, Orange & Madden); that electromagnetic response data measured in a 3-D environment such as the Northumberland Basin must be interpreted using 3-D models . If one or 2-D models are used, the data must be corrected on the basis of regional measurements

Melting in the crust and upper mantle beneath the Kenya Rift: evidence from Geomagnetic Deep Sounding experiments

A Geomagnetic Deep Sounding experiment in and around the Kenya Rift Valley has shown that telluric currents are concentrated by 3 regions with high electrical conductivity. Two of the anomalies are related to the rift structure, and the combination of different lines of geophysical evidence strongly suggests that the high conductivity is due to the presence of molten material in the rocks of the lower crust and upper mantle. The shallower zone where melt is present is located directly beneath the floor of the rift valley. Its upper surface is no deeper than 20 km; it may be as shallow as 5 km. The electrical conductivity is compatible with an average melt concentration of 5-10%, but could also be explained by discrete magma chambers where the concentration was much higher. The deeper conductor is located to the E of the rift valley beneath the Aberdare Mountains, and possibly extends beneath Mount Kenya. The Geomagnetic Deep Sounding data suggest a depth of 100 km to the high conductivity body, which appears to correspond to the core (the region with the highest melt concentration) of a zone of more diffuse melting in the mantle, that is responsible for seismic and regional gravity anomalies, and which supports a part of the topographic elevation of the Kenya dome

Geomagnetic variation anomalies in northern England: processing and presentation of data from a non-simultaneous array

The block and basin structure of northern England has been investigated by an array of 35 magnetometer stations over a grid with a spacing of 10km. Recordings at these sites were made simultaneously with those at a reference site. Using the common reference data, transfer function analysis enables the three anomalous field components to be estimated at each site, for the period range 10-10**4 s. Using the transfer functions obtained, maps of the anomalous field components may be generated across the array, and used for interpretation of electrical conductivity structure. Maps thus obtained from the non-simultaneous array data in northern England are used to illustrate the advantages of the method, and provide evidence of an east-west current flow concentrated along the southern margin of the Northumberland Trough. The limitations of the procedure, although slight, are considered. The errors which arise in the method are deduced and tech­niques for their removal are suggested

The High Resolution Airborne Resource and Environmental Survey- (Phase 1) (HiRES-1): background, data processing and dissemination and future prospects

This report provides an overview of the HiRES-1 airborne geophysical survey of Central England. The BGS and World Geoscience (UK) Ltd. carried out the survey jointly in 1998. The three main survey data sets acquired were magnetic, radiometric (gamma ray spectrometry) and Very Low Frequency (VLF) electromagnetic. The main aim of the report is to provide information on the acquisition, processing and storage of the final data and map products produced by the HiRES-1 project. Additional descriptions of ground truthing activities, data licensing and dissemination are also provided. A significant aspect of the project was the assessment of the potential of, and issues raised by, modern, multi-parameter, regional-scale airborne geophysical surveys in the UK context. Some of the main issues outlined in the report are: • The practical difficulties of conducting extensive low level, fixed-wing geophysical surveying in the UK. Issues discussed include CAA regulatory permissions, flight height adjustments above conurbations and surveying near areas with dense air traffic. • The processing challenges introduced by cultural (nongeological) influences on high resolution airborne magnetic data sets in the UK. • The significant amounts of detailed geological and environmental information contained within the radiometric data. • The weak application potential provided by passive (i.e. VLF) measurements, in contrast to active airborne electromagnetic techniques. The future prospects for regional-scale, airborne geophysical surveys in the UK are considered. Such prospects are also influenced by a further set of trial airborne data, obtained in 1999, but not described here. Further detail of the HiRES-1 survey, productivity, technical specifications and data pre-processing are contained in the survey logistics report prepared by WGL: ‘British Geological Survey “Hi-Res Phase One” Airborne Geophysical Survey (Survey Details, Technical Specifications & Processing Summary)’ (WGL 2000)

Trial airborne environmental and geological survey: an initial appraisal of relevance to land-use

A series of four trial airborne environmental and geological surveys was flown by the Geological Survey of Finland (GTK) in collaboration with the British Geological Survey (BGS) in June 1999; the trials were co-sponsored by the Department of the Environment, Transport and the Regions and the Environment Agency. The main objective of these surveys was to test the efficiency of the GTK airborne electromagnetic system in the mapping of potential pollution problems in the UK environment. Gamma spectrometric and magnetometric measurements were also collected to see to what extent these techniques provide complementary information. This report is an initial consideration of the multi-parameter airborne geophysical data in relation to land-use issues. A variety of earth science information for planning and development already exists. The baseline information is reviewed according to scale and resolution. It is noted that the airborne data is of a new type with regard to both information content and spatial continuity. The specific characteristics and advantages of the airborne geophysical data are emphasised. The trial survey data were obtained at high resolution and a large number of anomalous responses have been observed. Since the information is provided at a local-scale a number of issues concerning the nature of the responses observed and the underlying causes have been raised. It will be necessary to improve our understanding of the data before an unambiguous discussion of their specific relevance to land-use issues can proceed. In advance of detailed interpretations of the trial data, the data are presented and discussed in relation to their potential relevance to five land-use issues: (i) Waste planning/Planning and pollution control, (ii) Minerals Planning, (iii) Water supply and water resource protection/Coastal zone management, (iv) Urban regeneration/Peripheral development and new communities and (v) Agriculture and Forestry. It is acknowledged that there are differences between the information needed for planning the use of land, and information which is required for monitoring environmental strategies. The geophysical data appear to have greatest relevance to the latter requirement. Recommendations for ground and subsurface calibration of the data are made

Trial airborne environmental and geological survey of target areas in the English Midlands

A series of four trial airborne environmental and geological surveys was flown by the Geological Survey of Finland (GTK) in collaboration with the British Geological Survey (BGS) in June 1999; the trials were co-sponsored by the Department of the Environment, Transport and the Regions and the Environment Agency. The main objective of these surveys was to test the efficiency of the GTK airborne electromagnetic (EM) system in the mapping of potential pollution problems in the UK environment. Gamma spectrometric and magnetometric measurements were also collected to see to what extent these techniques provide complementary information. The EM data are particularly encouraging and have identified anomalies which may relate to subsurface pollution. The data have identified conductive zones both on the ‘local’ scale (i.e. possibly emanating from domestic landfills) through to regional scale features up to many kilometres in length. The information obtained in relation to spoil tip drainage is of particular note. The data have provided a wealth of information on ‘point source’ anomalies (requiring ground truth information) and diffuse sources that may be connected with groundwater extraction. At two sites (Trent Valley and Langar) the EM technique appears to have met the challenge of detecting conductive targets in a conductive host. Many other features of the data will take time to assess. The radiometric data indicate features of land fill and colliery spoil sites and allow comparison to be made rapidly with the EM data to define the lateral spread of conductive pore fluids beyond the site itself. They can also be used to map site-specific contamination where radioactive elements are present. The radiometric data also give direct indications of solid and drift geology and soil type, providing additional detail to that shown on geological maps (soil maps are not available for much of the UK). The magnetic data have suggested the presence of significant volumes of metallic debris in the Langar landfill sites. This may be significant in terms of the leachates derived and the compaction (through time) of the contrasting fill materials. Compared with the EM and radiometric techniques, the applications of magnetics in environmental studies are rather limited, but such data is worthwhile collecting in view of the marginal additional costs involved

Airborne geophysical surveys applied to urban/brownfield investigations

A number of airborne geophysical surveys have now been conducted in the UK using a fixed- wing system operated jointly by the British and Finnish Geological Surveys. The system provides magnetic, radiometric and frequency domain electromagnetic survey measurements. Regulatory survey permissions force a variety of flight elevations typically between 54 to 244 m. On the fringes of major conurbations, we encounter urban greenspace and brownfield areas. These areas may contain mixtures of infrastructure and built structures together with derelict and green areas. In order to provide valid interpretations of the airborne data, detailed studies of the behaviour of the data to a variety of influences found in the urban fringe of a town have been made. Non-geophysical spatial data sets are increasingly being applied to aid both the processing and interpretation of the survey data. This is particularly true in non-rural areas. Examples from a high-resolution survey acquired across a 5 x 5 km coastal area to the west of Irvine, Scotland are considered

Hydrogeological classification of superficial clays: apparent resistivity measurements from the Garboldisham, Norfolk pilot study area

Three types of apparent resistivity measurements have been undertaken at the Garboldisham pilot study area in Norfolk in support of the superficial clays programme. Resistivity soundings have established the intrinsic resistivities for the strata encountered. Cover sand (possibly mixed with glacial sand) ofresistivities 100 - 200 ohm.m overlies till ofresistivities 22 - 32 ohm.m which lies directly on chalk of resistivities 65 - 85 ohm.m. At the sounding sites, depth to chalk is in the range 11 - 15 m. Azimuthal apparent resistivity measurements were made in an attempt to identify fracture sets within the till and to establish the degree of fracturing between measurement sites. These fracture sets will have an important influence on the hydrogeological regime. At only two of the five sites investigated was a fracture trend recognised and this had an orientation of 210-225 degrees. There was an indication of a change in the intensity of fracturing between the two sites. The results indicate that there is either a lack of fracturing with a consistent trend within the till or that there is an insufficient resistivity contrast between the conductive till and the fractures. Detailed dipole-dipole apparent resistivity measurements undertaken with the BGS RESCAN system were able to map the thickness ofcover sand over a suspected sand channel. The sand body has a north - south orientation with a variable depth to the till surface. Maximum depths are up to two metres. The effectiveness of the resistivity technique for mapping sand lying on till has been demonstrated in this study

Applications and challenges for high resolution airborne surveys in populated areas

The role of high resolution airborne surveys is extending well beyond the traditional areas of mineral and hydrocarbons exploration and regional mapping, which were often focused on the relatively unpopulated and remote regions of the world. As an increasing proportion of the Earth’s surface is affected by human activity, airborne survey techniques are being adapted and applied to development-related issues in more populated areas. The value of radiometric and electromagnetic data (in particular) has been recognised for addressing a range of environmental issues and, more generally, airborne surveys are seen as an effective, non-invasive means of providing multiple development-related geoscience data sets in a single operation. These new applications present many operational, processing and interpretation challenges. This paper provides an overview of some of the issues addressed in this Special Session on ‘The role of high resolution airborne surveys in environmental monitoring and sustainable development of resources’