Geophysical Surveys
Geophysical properties include:
- magnetisation
- thermal and electrical conductivity
- radioactivity
- speed of sound
Geophysicists observe how these properties and their effects vary and interpret the results in terms of underlying geological structures. Geophysical methods are particularly useful for mapping regions, such as Northern Ireland, where a thin layer of glacial cover or peat obscures the solid geology.
Some geophysical measurements can be made from the air, using a small aircraft or helicopter equipped with a range of sensors, navigational equipment and recording systems. Commonly, we measure the magnetic field, electrical conductivity and natural radioactivity with airborne platforms.
The aircraft flies along a network of parallel lines taking regular readings at intervals of between one tenth of a second and one second. The aircraft's position is recorded simultaneously with a Global Positioning System and the height measured accurately with a radar altimeter. All the data is recorded digitally and removed to the processing centre at the end of each flight.
The Tellus survey of Northern Ireland is part of the HiRes geophysical mapping programme of the British Geological Survey (BGS) that ultimately will cover all of the UK.
Previous surveys in Northern Ireland
The BGS made the last regional geophysical survey of Northern Ireland in 1959. This was an airborne survey, measuring only the magnetic field and the results show the broad structural elements at a regional scale. The new survey will add considerable detail.
The Tellus airborne survey
GSNI has flown the Tellus airborne geophysical survey with the most up-to-date integrated airborne geophysical system available in Europe. The aircraft was equipped with:
- two magnetometer sensors, each with a sensitivity 100 times better than that of the 1959 survey, measure the magnetic field
- a two-coil electromagnetic system, which measures the electrical conductivity of the ground
- a gamma-ray spectrometer, which measures the natural radioactivity of the ground
The aircraft is a De Havilland Twin Otter, originally modified for this work by the Geological Survey of Finland (GTK), which used this aircraft to survey much of Finland. The aircraft is now owned by the Natural Environment Research Council and managed by a joint venture between the British Geological Survey (BGS) and GTK. The aircraft, registration OH-KOG, is manned by two pilots, a navigator and engineer.
The operation in Northern Ireland was authorised by the Civil Aviation Authority and the Irish Aviation Authority and daily operations were controlled by Air Traffic Control at Belfast International Airport. The CAA and IAA permit our aircraft to fly at a distance of 56 m (185 feet) from any structure in rural areas. Survey lines were spaced 200m apart and orientated approximately north-northwest or south-southeast. Tie-lines were flown at 2,000 m interval in the orthogonal directions.
Progress and schedule
Between July and October 2005 Tellus completed the survey of Northern Ireland west of a line joining Coleraine and Carlingford Lough. The survey recommenced in March 2006 and by June 2006 the remaining areas had been completed. There was a substantial programme of public outreach prior to and during the survey to ensure the reasons for the work were well understood and to minimise disturbance.
Magnetic field
We can detect and map the magnetic field of the earth with a sensitivity of about one part in five million. Most rocks are slightly magnetic and differences in the measured magnetic field indicate variations in the type of rock and soil beneath the aircraft. The pattern of the magnetic map shows both major geological structures deep within the earth and the shallower effects of magnetic rocks nearer the surface. In Northern Ireland, the flat-lying lavas of the Antrim Plateau are particularly magnetic, as are the sets of in the north and west.
Electrical conductivity
The electrical conductivity of rocks and soils varies largely according to porosity, salinity, saturation and clay content. We use the variation in conductivity to help map rock and soil types and conducting structures such as faults.
We may also be able to detect contaminants (for example drainage from an industrial site) that typically raise ground conductivity. In Northern Ireland, shaly sediments typically show high conductivities than the harder igneous or metamorphic rocks.
Natural radioactivity
All rocks and soils are very slightly radioactive. Typically the radioactive content is only a few parts per million by volume so the effect is slight. Although it is small, we can detect ground radiation with very sensitive detectors in the aircraft.
Most radiation from rocks is from uranium, thorium and potassium isotopes and the proportions of these vary among different rock types. Mapping natural radioactivity is therefore another useful means of differentiating rock and soil types.
The radioactivity map will also provide a baseline or standard against which to measure any change in ground radiation in the future. The intrusive rocks of the Mourne and parts of the ancient metamorphic Dalradian rocks in Tyrone and Derry show relatively high radioactivity.