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Seismic

Today the main geophysical exploration method used in the oil and gas industry, and the state of the art, is 3D seismic. 

The use of advanced seismic techniques makes it possible to map the geological structure of the earth’s subsurface three-dimensionally, down to depths of 5,000–6,000 metres. Seismic exploration involves generating sound waves and picking up the echoes reflected by the subterranean layers of rock.

“Oil and natural gas are hydrocarbons formed from organic substances by biological, chemical  and physical processes in the course of the Earth’s history. They migrate from the source rock and, under the right conditions, accumulate in the tiny pores of subterranean reservoirs.”
Kurt Sonnleitner, Chief Technical Officer, RAG


The differing densities and acoustic properties of underground rock strata make it possible to locate potential oil- or gas-bearing formations. The signals (seismic waves) emitted by special vehicles, known as vibrators, are reflected by the layers of rock and detected by arrays of receivers (geophones), usually with diameters of about 15 centimetres, on the surface. Seismic campaigns normally last for between four and eight months. Powerful computers at RAG headquarters use the data acquired in this way to generate images similar to ultrasound scans. It takes several months to process the raw data accumulated from millions of seismic traces. The results are then subjected to structural and stratigraphic interpretation by RAG’s geologists and geophysicists, enabling our experts to draw conclusions about the probability that oil or gas accumulations have been identified. However, proof of the presence of hydrocarbons can only come from a well.


Data acquisition

The reflection seismic method has been and still is RAG’s main geophysical tool to develop economically viable hydrocarbon projects. This geophysical method is based on the principles where energy waves are emitted into the ground, which are reflected by the individual layers of the subsurface. These reflected travel times are registered at the surface by seismometer receivers, so called geophones. The relevant depths of the individual target layers in RAG’s concession areas range between 500 and 4000 meters.

The required energy is generated by vibrator vehicles which emit vibrations at a pre-selected frequency range (~10 to 100 Hz) into the ground with a controlled force of about 28000 lbs. With the 3D seismic method used almost exclusively nowadays, the reflected data are recorded on a rectangular patch (about 6 km2, its size depending on the target depths) of up to 1000 receiver stations. The digitized signals from the receiver stations are transmitted via cable to the recording instruments and written on magnetic tape. During recording operations, this patch is moved relative to the vibrator positions over the whole of the surface area to be surveyed.

This allows the three-dimensional registration of the subsurface. A seismic data acquisition campaign to record about 500 km2 will take about 8 months. Prior to such surveys RAG must obtain permission to carry out such projects from the relevant government agencies. Competent international service companies are selected by RAG on a tender – contract basis to carry out such seismic data acquisition projects. The contractor will contact the local communities and land owners in order to gain the necessary information about the installations and environmental conditions. Permission to access the property must be obtained from each land owner. Damages to land and crops are compensated for based on agreed upon established guidelines. Other damages, such as to roads and tracks, are reinstated to its original condition.


Data processing

The recorded raw data represents millions of individual subsurface measurements, which contains not only the desired reflection signal but is also contaminated by extraneous unwanted noise signals such as from unwanted sources, such as weather, agricultural activities, traffic as well as from source generated energy travelling in near surface geologic layers. One of the data processing steps is to remove or at least attenuate such unwanted signals.

The multifold method of data collection ensures that the subsurface is sampled at a predetermined rate. Statistical analyses of these multifold measurement provide the necessary information for the signal enhancement as well as for near surface and dynamic corrections, prior to the summing (stacking) of the individual measurements. High performance computers with extensive storage capacity (terabytes of disc capacity) and specialized seismic data processing software are used. The final product of the standard seismic data processing sequence is a 3D migrated data volume which approximates the geologic subsurface complexity.


Data interpretation

The 3D migration volume is the basis of the subsequent geological interpretation. The data volumes, either in depth or two-way seismic travel times, are evaluated in high-performance computers with using specialized interpretation and display software.

Based on knowledge and experience as well as existing information, individual reflections can be identified as geological markers. The objectives of the seismic data interpretation are to find and map subsurface traps where hydrocarbons may have accumulated. The steps include regional mapping to establish the geologic setting and hydrocarbon potential. Detailed project mapping must answer questions such as the hydrocarbon trap, reservoir potential and target depth.

All drilling projects are subsequently evaluated and reviewed for their technical and economic viability by a team of experts.


Seismic projects

To date RAG has conducted 3D seismic surveys of a total area of about 4,000 square kilometres in Austria, Bavaria and Hungary. RAG has been routinely using 3D seismic – by far its most important exploration method – to explore its concessions since 1992 (the first pilot project was carried out in 1984 in Voitsdorf). The first large area to be surveyed in this way was around Munderfing, in Upper Austria, and this was followed by acreage between the River Salzach and Strasswalchen, in Salzburg Province. We have now shot 3D seismic in a large part of our permits in Upper Austria and Salzburg. A major milestone was the discovery of the large Haidach gas field on the border of Upper Austria and Salzburg in 1997. Some years ago, after production at the field ceased, it was converted into the second-largest gas storage facility in Central Europe, and it now plays a key role in European supply security. Seismic campaigns have also brought us oil exploration successes in the shape of the Hiersdorf, Bad Hall and Sierning fields, discovered between 2006 and 2009. The most recent surveys have been in a 220 square kilometre area in Attergau in 2009/10, and a 380 square kilometre area straddling the Steyr, Steyr Land, Kirchdorf an der Krems and Amstetten districts in 2012. A seismic survey will be carried out in Bavaria in 2016.