It takes seismic force to make the ground give up its secrets. Through the years, those searching for oil and gas have used varied methods to send sound energy into the ground and to record the waves reflected by the geological features beneath the surface. Modern methods include large vibrator trucks and many thousands of surface sensors called geophones, all precisely located to obtain the most useful information with which to explore for hydrocarbons. Today, seismic surveys planned with satellites are yielding clearer, deeper subterranean views at reduced cost.
Often carried out in the remotest parts of the planet, these surveys are almost military in scale and expense; a seismic crew exploring a 500-square-kilometre area can require 400 people with up to 50 small and 15 large vehicles working with up to 600,000 geophones, and carrying out 600 seismic ‘shots’ daily.
Seismic surveyor WesternGeco, has been working with ESA for the last three years to integrate satellite data into its working practices. What Earth Observation can provide is a detailed preview of a region’s topography and geology, valuable for assessing areas that will produce the best and worst seismic quality – meaning the sending and receiving of vibration signals – far in advance of commencing the survey. To achieve high-fidelity reservoir characterisation, the surveyors aim to exclude as many variables as possible. Around 80% of acoustic signal distortion comes from propagating through the top 100 metres of ground, with the most problems encountered nearest the surface.
Space-derived topographic information is important because rises or falls in the landscape delay signal arrival time, and if they are not compensated for, they cause blurring of imagery. No single space-borne instrument can supply all the data required. Instead, data from a variety of different satellites are collected and combined within a geographic information system to yield information on accessibility, data quality, and source and receiver coupling. The process begins with a digital elevation model, available from many sources including space shuttle mapping and ESA’s ERS-tandem mission. This provides topographic and gradient information for logistics and safety planning. Next comes radar imagery – from spacecraft such as Envisat and ERS – to measure surface roughness, forming the basis of a map of coupling potential.
Visible light images provide infrastructure and land use information that help determine accessibility for vehicles and people. Also, surface vegetation detected in this imagery may indicate sediment-buried water channels that weaken signal propagation.
The ground reflects short-wave infrared (SWIR) light immediately, revealing the spectral characteristics of minerals at the surface. SWIR imagery, obtained from hyperspectral satellite sensors, is particularly sensitive to carbonates such as limestone and basalt and occurrences of softer materials such as gypsum or quartzite gravel. Advancing further into the infrared spectra permits surveyors to peer deeper beneath the surface. Thermal infrared (TIR), or heat radiation from the surface, is a delayed response to incoming solar radiation, coming from the top half-metre of subsurface.
UK-based Infoterra and WesternGeco have jointly developed a seismic-quality mapping service as part of an ESA Earth Observation Market Development (EOMD) project. EOMD is a programme aimed at strengthening the European and Canadian capacity to provide geoinformation services based mainly on Earth Observation data, with a particular emphasis on addressing the needs of small value-adding companies.