Time to Investigate Deep Horizons. Geology Without Limits' Unique Approach
01 October 2015
For example, in the Gulf of Mexico for many years only the upper part of the geological section was considered promising and only relatively recently by studying the deeper part, a number of major discoveries have been made (e.g. Great White, Tobago and Silvertip discoveries in Alaminos Canyon). Similarly large hydrocarbon accumulations have been discovered in the deep strata of the Brazilian offshore (e.g. Carioca-Sugar Loaf field, the third largest accumulation of oil in the world). These examples clearly show us the direction in which modern seismic surveys should be carried out.
The most important objectives of seismic exploration techniques are the study of the structure of the sedimentary cover, the mapping of the surface of the basement, the allocation of tectonic faults, structures and lithological unconformities; and the estimation of oil and gas potential. One of the effective methods of their solution is Refraction Seismology as an essential adjunct to the more widely used Reflection Seismic techniques.
One of the main advantages of the Refraction Seismic method is the ability to determine the marker velocity which can define the physical properties of the seismic horizons and indicate lithology. The use of the refracted method provides a deep-velocity model, the definition of which, in practice, is quite challenging. Mistakes in the determined velocity values can significantly distort the resulting images of the geological subsurface - deforming the shape of the boundaries and their position in the deep section. Therefore, the definition of velocity properties of the subsurface is one of the most important tasks in seismic data processing. In addition, an accurate depth-velocity model is in itself a significant geological and geophysical result, as it provides additional information on the geological structure of a region. Improving the efficiency of seismic surveys can only be achieved by integrating the various methods, primarily reflection and refraction.
Complex methods used by the company "Geology Without Limits"(GWL) in the planning and implementation of large regional marine seismic surveys, including gravity and magnetic surveys which are outside the scope of the present article , provides the opportunity to build a high-precision deep geological model of the region under study.
In conventional reflection surveys , processing geophysicists are limited by streamer length while building deep-depth velocity models in order to investigate the whole sedimentary cover and reveal basement structure, especially in the deep horizons areas, that are the principal points of interest nowadays. Even a 12 km seismic streamer length can be not enough if we are targeting to study geological aspects of deep horizons. Seismic interpretation geophysicists and geologists can be restricted by the datasets they have to deal with. As a result, basin models may reflect only minor aspects of the oil and gas fields, leading into the incorrect prediction of oil and gas occurrence and inaccurate estimated reserves. But what if we want to go beyond and see the bigger picture?
One of the cutting-edge geophysical technologies designed to address the issues described above is a unique refraction seismic data acquisition technique conducted with help of GWL’s self-developed seismic recording equipment – GWL Seismobuoy.
GWL Seismobuoy helps to triumph over streamer length constraints and extend seismic data offsets up to 150 km, enhancing depth of the survey up to crustal boundaries and the Moho discontinuity. Adjustable sensor depth makes GWL Seismobuoy flexible to be utilized in various scenarios and hydrographic conditions: from shallow waters and transition zones to deep ocean waters areas and regional basin-wide surveys. Online satellite based tracking position gives an opportunity to follow GWL Seismobuoy and its recording status anywhere anytime, you always know where your data is and have total control over it. GWL Seismobuoy’s rechargeable battery life is up to 15 days of continuous recording, enabling the acquisition of seismic data on ultra-long (up to 2000 km) 2D regional seismic lines. GWL Seismobuoy’s major technical characteristics are represented on figure 1.
-191-/+ dBV re 1 µPa @ 20°C, 27.22 V/bar
Operating Life (100% charge)
8; 4; 2; 1; 0.5; 0.25 ms
Up to 15 days continuous record
GWL Seismobuoy appearance and major technical characteristics
A range of tests performed to compare GWL Seismobuoy and industrial Ocean Bottom Seismometers (OBS) (used as a reference point to the obtained seismic data) has proved the concept of the unique refraction seismic data acquisition technique and has shown the excellence of the recorded data by GWL Seismobuoy in comparison to the data recorded by industrial reference OBS (GWL Seismobuoy has higher sensitivity of the sensor and a broader recorded frequency range under equal environmental recording conditions – figure 2). Moreover, geological interpretation of the seismic gathers recorded during one of the tests perfectly matches with a detailed geological survey performed earlier in the project site. Thus, GWL Seismobuoy has no limits in its utilization whether it is in its initially designed implementation for 2D regional seismic surveys or in the field of 2D/3D engineering seismology studies with applications to geotechnical engineering.
Sample of common receiver gather after preprocessing designed to refracted waves event picking. Left, OBS with overlaid Average amplitude spectrum. Right, GWL Seismobuoy with overlaid Average amplitude spectrum
Additionally GWL has developed a unique concept where the simultaneous acquisition of broad-band reflection data and ultra-long offset refraction data along the same seismic lines will provide superior velocity information for the data processing and the geological model building. This concept lends itself particularly to basin-wide investigations and is about to be applied in key complex basins across the world such as the Caribbean.
Flexible seismic design of the refracted wave survey (receivers distance, receivers density, maximal offsets, etc.) can be optimized for an individual geological environment providing the best production conditions and high quality data Thus, refracted wave seismic data obtained with the same source (specially tuned up to a low frequency spectrum) is easily integrated with reflected wave seismic data on the processing and interpretation stages. Ray tracing modeling and combined reflected and refracted waves seismic tomography are used for advanced velocity imaging (an example of seismic tomography velocity imaging is shown in figure 3). Derived reliable velocity models can be used for migration procedures in the time and depth domains – providing us with a cleaner image of the seismic streamer dataset itself and true position of the deep-depth horizons. Geological horizon mapping including crustal boundaries and the Moho and forward problem solution modeling gives us an opportunity to drive forward renewed regional geologic and geodynamic basin models to the next level.
An example of sedimentary cover and Moho discontinuity where the velocity model’s ray coverage is derived from joint refracted/reflected waves and seismic tomography velocity imaging
Developed by GWL new technology for the recording and processing of extra-long offset data, and based on the application of GWL Seismobuoy, the methodology described above allows us to create integrated, high-precision models of the geological structure of marine basins, which will be the foundation for further scientific and practical studies, significantly reducing the time and cost for their preparation and realization.