Hi-RES - What is measured by an airborne geophysical survey?
Surveys are carried out along closely-spaced lines (usually 200 m apart) at low altitude (50 – 90 m in rural areas and 250 m over urban areas). Instruments can be mounted on fixed-wing aircraft or helicopters, but for regional surveys a fixed-wing aircraft is the best option for cost-effective coverage of large areas.
The optimum instrument package measures three physical parameters:
- Radiometric sensors measure natural and man-made radioactivity.
- Electromagnetic (EM) sensors measure the electrical conductivity of the shallow sub-surface.
- Magnetic sensors measure perturbations in the Earth’s magnetic field created by sub-surface structure.
The radiometric and magnetic measurements are passive (i.e. they measure .natural physical fields). The electrical conductivity of the ground is usually .measured by an active ‘transmitter-receiver’ setup; in the system used by .BGS low-power electromagnetic fields are generated in wing-tip coils and .the ground response is measured by similar coils on the other wing.
The electromagnetic system used by BGS uses four different frequencies, each providing information at a different depth. Together the four frequencies give a 3D picture of electrical conductivity in the shallow subsurface (down to around 100 m).
