The exploration cycle between the planning of a seismic data acquisition project and the drilling of a well may encompass several years and includes several benchmarks of economic evaluations, field data recording, data processing, data interpretation and project evaluation. The following section describes the steps of seismic exploration activities in detail:
Data acquisition
BeiThe 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.