Exploration surveying
In the first stage of the search for hydrocarbon-bearing rock formations, geological maps are reviewed in desk studies to identify major sedimentary basins. Aerial photography and satellite imagery may then be used to identify promising landscape formations such as faults or anticlines. More detailed information is assembled using a field geological assessment, followed by one of three main survey methods: magnetic, gravimetric and seismic.
The Magnetic Method depends upon measuring the variations in intensity of the magnetic field which reflects the magnetic character of the various rocks present, while the Gravimetric Method involves the measurements of small variations in the gravitational field at the surface of the earth. Measurements are made, on land and at sea, using an aircraft or a survey ship respectively. Seismic survey is the most common assessment method and is often the first field activity undertaken. The Seismic Method is used for identifying geological structures and relies on the differing reflective properties of sound waves to various rock strata, beneath terrestrial or oceanic surfaces. An energy source transmits a pulse of acoustic energy into the ground which travels as a wave into the earth. At each point where different geological strata exist, a part of the energy is transmitted down to deeper layers within the earth, while the remainder is reflected back to the surface. Here it is picked up by a series of sensitive receivers called geophones or seismometers on land, or hydrophones submerged in water Special cables transmit the electrical signals received to a mobile laboratory, where they are amplified and filtered and then digitized and recorded on magnetic tapes for interpretation. Dynamite was once widely used as the energy source, but environmental considerations now generally favour lower-energy sources such as vibroseis on land (composed of a generator that hydraulically transmits vibrations into the earth) and the air gun (which releases compressed air) in offshore exploration. In areas where preservation of vegetation cover is important, the shot hole (dynamite) method is preferable to vibroseis.
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In the first stage of the search for hydrocarbon-bearing rock formations, geological maps are reviewed in desk studies to identify major sedimentary basins. Aerial photography and satellite imagery may then be used to identify promising landscape formations such as faults or anticlines. More detailed information is assembled using a field geological assessment, followed by one of three main survey methods: magnetic, gravimetric and seismic.
The Magnetic Method depends upon measuring the variations in intensity of the magnetic field which reflects the magnetic character of the various rocks present, while the Gravimetric Method involves the measurements of small variations in the gravitational field at the surface of the earth. Measurements are made, on land and at sea, using an aircraft or a survey ship respectively. Seismic survey is the most common assessment method and is often the first field activity undertaken. The Seismic Method is used for identifying geological structures and relies on the differing reflective properties of sound waves to various rock strata, beneath terrestrial or oceanic surfaces. An energy source transmits a pulse of acoustic energy into the ground which travels as a wave into the earth. At each point where different geological strata exist, a part of the energy is transmitted down to deeper layers within the earth, while the remainder is reflected back to the surface. Here it is picked up by a series of sensitive receivers called geophones or seismometers on land, or hydrophones submerged in water Special cables transmit the electrical signals received to a mobile laboratory, where they are amplified and filtered and then digitized and recorded on magnetic tapes for interpretation. Dynamite was once widely used as the energy source, but environmental considerations now generally favour lower-energy sources such as vibroseis on land (composed of a generator that hydraulically transmits vibrations into the earth) and the air gun (which releases compressed air) in offshore exploration. In areas where preservation of vegetation cover is important, the shot hole (dynamite) method is preferable to vibroseis.
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