Abstract Resistivity methods are widely used as an aid to siting water-supply boreholes in basement terrain. Adverse conditions due to shallow bedrock can be identified readily but specific targets such as narrow conductive zones may be missed; quantitative interpretation may be less reliable than is often assumed. Electromagnetic (EM) profiling will detect localized zones of deeper weathering and the effects of fracturing at shallow depths, and it is used for the precise location of anomalous ground indicated by lineations on aerial photography. Routine EM surveys give better lateral resolution than resistivity for the conductive targets of interest. The 2-D and 3-D EM modelling techniques needed for interpreting the conductor geometry realistically are still under development. Geophysical methods must be applied with due regard for the targets being sought and the local geological setting: they will not always be appropriate and resources should be targeted on districts where problems can be solved. Qualitative interpretations are adequate in some situations but more rigorous field procedures and analytical techniques are needed to ensure that useful hydrogeological information is obtained.
Geophysical studies were an integral part of a multidisciplinary investigation
of the basement aquifer in Zimbabwe, carried out by British Geological Survey
(BGS) and the Ministry of Energy and Water Resources and Development (MEWRD)
of Zimbabwe, and funded by the Overseas Development Administration (ODA) of
the British Government. The study was centred on Masvingo Province, in
conjunction with the Provincial Water Engineer based in Masvingo as well as
the Hydrogeology Department in Harare.
A geophysical study had been made in 1986 (Smith and Raines, 1987) of a number
of key areas, which represented the range of major rock types, in a variety of
different hydrological and hydrogeological settings. It was intended as an
orientation study, to obtain additional data on a number of previous drilling
exercises which had fully reported, but which had a proportion of unsuccessful
boreholes. On the basis of this experience, it was hoped to apply methods
which might enhance the success rate.
This second season's work represents the application of this experience to the
identification of suitable drilling targets in conjunction with parallel
hydrogeological, structural and geomorphological assessments. It was hoped
that the drilling would improve our understanding of the structure and
hydrogeological behavior of major linear features identifiable from aerial
photography, as well as provide suitable sites for the installation of pumps
for water supply.
British Geological Survey is collaborating with Satellite Observing Systems Ltd. to produce high quality gravity coverageof the world's oceans using satellite altimeter data. As part of this collaboration BGS has compared the free-air gravity anomaly derived from marine gravity surveys with data derived from the ERS-1 satellite.
Marine gravity surveys provide good coverage of most of the UK continental shelf and recently released, closely-spaced, satellite-derived gravity data extends the coverage to the whole of the continental margin to the northwest of the British Isles. This has, for the first time, enabled regional gravity maps to be produced of the entire Shetland-Faeroes-Hebrides-Rockall area with sufficient resolution to image upper-crustal structures and putative sedimentary basins. The use of gravity data for exploration in continent margin areas, however, poses some particularly challenging problems in data processing -and presentation.
Two geothermal wells have been drilled in or near Southampton since 1979. One is at Marchwood, just outside Southampton, and the other is in the centre of the city; the wells are 1.85 km apart. They proved a geothermal reservoir in the upper 25 to 40 m of the Triassic Sherwood Sandstone at a depth of about 1700 m. The reasons for siting the wells in Southampton are discussed and the nature of the reservoir described. The wells have been extensively tested. The Marchwood well yielded 30 ls −1 for a pressure reduction of 3.7 MNm −2 after a test of 33 days, while the Western Esplanade well gave 20 ls −1 for about 3.0 MNm −2 . Both yielded brine with a salinity of over 100 g l −1 at a well-head temperature of between 70 and 74°C. The transmissivity of the reservoir is 6 m 2 /d (3.5 D.m) and the storage coefficient 4 × 10 −5 . Computer modelling of changes in reservoir pressure suggests that near the wells there is a region of relatively high permeability but the permeability declines at distances of more than a few kilometres from the wells. This could take one of several forms including a bounded reservoir or a narrow wedge-shaped reservoir. The thermal yield from either well at an abstraction rate of 20 ls −1 would be about 3 MW.
The British Geological Survey, in collaboration with other data holders, has recently completed major new compilations of regional gravity and aeromagnetic data covering most of Britain, Ireland and the surrounding continental shelf. The gravity compilation includes over 300,000 line-km of marine data and over 150,000 land gravity observations. The magnetic compilation incorporates 400,000 line-km of aeromagnetic data which have been reprocessed to a consistent reference field. Colour shaded-relief maps at a scale of 1:1,500,000 have been produced for publication in 1995. These, together with other filtered image maps, provide a new insight into the tectonic framework of the British Isles and Ireland, and in particular into the relationship between the development of the major hydrocarbon-hearing sedimentary basins and the underlying tectonic fabric.
A closed Bouguer gravity low is revealed by land and marine gravity survey data centred on Holyhead Bay, northwest Anglesey. The anomaly extends across areas on land and offshore mainly occupied by Precambrian rocks. A concealed granite offers the best explanation for the anomaly and a late Caledonian age seems most probable for the intrusion, although Precambrian–early Cambrian ages are also possible. The existence of a granite in this area could have influenced the development of mineralization and the low density rock mass also formed a relatively buoyant core to the basement block. It could therefore have been instrumental in the preservation of this Precambrian structural high.
Published maps showing the regional coverage of both gravity and aeromagnetic data for the land and parts of the sea areas of the UK have been available for a number of years. These data are now available in digital form, which allow the geophysicist to take advantage of advanced analytical and imaging techniques. Horizontal and vertical derivatives, inversions, grey-scale and colour shaded relief plots are startlingly effective in defining structural trends and characteristic anomaly pattems.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
