Passive Seismic Tomography
Active seismic tomography is routinely employed by oil and gas exploration firms to gain understanding of subsurface geology. This involves measuring the way in which acoustic waves travel through the Earth’s crust.
Much like the way light refracts as it passes through water; acoustic waves alter their speed and direction as they pass through a boundary between different materials. A network of sensors on the surface measure these vibrations and complex software analyses the data to create a map of subsurface geology. This method is ‘active’ because the acoustic waves are produced mechanically either with hydraulic rams on a ‘thumper truck’ to send vibrations through the ground, or for larger areas via controlled surface explosives. These acoustic waves are created on a grid pattern across the area to be surveyed, sometimes as close as 25 m apart. The surveying method is resource intensive and forms a considerably expensive component of oil and gas exploration.
Africa New Energies uses passive seismic tomography to survey the Namibian concession. Although it is more time consuming than the active seismic technique, the data collected is of the same quality, at a projected cost of $200 per km², in comparison with $20,000 per km² typical of active seismic tomography. Instead of producing acoustic waves on the surface and measuring a response, passive seismic tomography relies on natural micro – seismic activity to understand the subsurface geology. In this method, a grid array of geophones is placed on the surface and monitor naturally occurring micro seismic disturbances. When micro seismic activity occurs, each of the surface geophones records a disturbance. Comparing and analysing the disturbances recorded by each of the geophones for the same micro – seismic event, allows us to make interpretations of the subsurface geology. The concept is illustrated below.