Geophysics is the study of the physical properties of the Earth. These include three which we can readily measure from a moving aircraft:
- magnetic field
- electrical conductivity
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 layer of glacial cover or peat obscures the solid geology.
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. The data are recorded digitally and removed to the processing centre at the end of each flight.
Previous surveys in Northern Ireland
The last major geophysical survey of Northern Ireland was a magnetic survey made in 1959 and the results show the broad structural elements at a regional scale. The new survey, flown at a lower altitude along more closely spaced lines and with significantly more sensitive equipment, has provided a wealth of new detail.
The Tellus airborne survey
The Tellus airborne geophysical survey of Northern Ireland is part of the HiRes geophysical mapping programme of BGS. The survey was flown by the Joint Airborne-geoscience Capability (JAC), a partnership of BGS and the Geological Survey of Finland (GTK). The aircraft was equipped with:
- two magnetometer sensors, each with a sensitivity 100 times better than that of the 1959 survey, which measure the magnetic field;
- an electromagnetic system, which measures the electrical conductivity of the ground;
- a gamma-ray spectrometer, which measures radioactivity.
The aircraft was a De Havilland Twin Otter, originally modified for this work by the GTK. The aircraft was manned by two pilots, a navigator and engineer.
Survey lines were spaced 200 m apart and orientated approximately north-north-west or south-south-east. The survey was flown at a nominal height of 56 m above the ground over rural areas and 250 m over villages and urban areas.
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. Prominent magnetic anomalies include those of the Antrim Lava Group, swarms of Palaeocene dykes, and the Palaeocene intrusions of the Mourne Mountains Complex.
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. The electrical conductivity map of Northern Ireland shows variations between the principal formations, areas of increased salinity, prominent expressions of major fault zones and certain industrial effects.
All rocks and soils are very slightly radioactive. Typically the radioactive content is only a few parts per million by volume but we can detect ground radiation with sensitive detectors in the aircraft. Most terrestrial radiation is from isotopes of uranium, thorium and potassium 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 provides a standard against which to measure any change in ground radioactivity in the future. Prominent anomalies include those of the intrusive rocks of the Mourne Mountain Complex and parts of the ancient metamorphic rocks in Counties Tyrone and Derry. The Antrim Lava Group and areas covered by peat have much reduced activity.