Geophysicists are responsible for the acquisition of gravity, magnetic and subbottom shallow seismic datasets. The equipment used during the Irish National Seabed Survey (INSS) for acquisition is a Lacoste and Romberg gravity meter, an IXSEA magnetometer towfish and a GeoAcoustics subbottom profiler.
Gravity Surveys
Gravity is the measure of the force of attraction at any point on the Earth's surface. This force will vary depending on the rock type present. Gravity surveys are used to denote the location of sedimentary basins. These basins are primary areas of interest in the search for oil and gas reserves.
Magnetic Surveys
Magnetism is the force of attraction or repulsion between substances. Many rocks have a magnetic component, caused by the presence of magnetic minerals within the rocks themselves, and this magnetic field can be measured by our magnetometer towfish. Igneous rocks containing the mineral magnetite tend to have the highest magnetic fields.
A graph of the magnetic field intensity is plotted in real time to show the geophysicist any anomalies during acquisition. Manmade objects containing iron and other metals will also produce a magnetic field, so the magnetometer can also be used to detect shipwrecks and debris on the ocean floor.
Subbottom Profiling
Subbottom image of sandwaves
The RV Celtic Explorer uses a 3.5kHz hull mounted pinger seismic source to acquire shallow seismic data. In principle, an acoustic signal is emitted by the pinger and travels downward through the water column into the underlying sediment, producing reflections at sediment boundaries. The energy that returns to the vessel is received in the form of a 'wiggle trace'. The onboard geologist interprets the reflections and denotes various sediment units, geological faults and folds, gas and buried objects up to depths of 50 metres beneath the seabed.
Subbottom image of bedrock