To tow the streamer slantwise; divide into time-slices, depositional interrelationships of sedimentary rock; to deduce from an interpretation of seismic data; conspicuously stronger than adjacent portions; a noticeably flat reflection amongst arched reflections; to enter files into a gridding and contouring programs; to warrant wildcat drilling; to drain the reservoir with maximum efficiency; increases in computing capacity; survey vessels deploying multiple streamers; a swath of seismic data; paravanes steer the cables away from each other; the position of the streamers relative to the vessel; to reveal changes in pressure in different parts of the field.
48. Read the text and do the exercises.
Seismic sections provide 2-dimensional views of underground structure. By using special shooting techniques such as spaced air gun arrays or towing the streamer slantwise, or by shooting very closely spaced lines, it is possible to produce 3-dimensional (3D) seismic images. These images comprise vertical sections and horizontal sections ('time-slices').
Seismic stratigraphy is the study of the depositional interrelationships of sedimentary rock as deducedfrom an interpretation of seismic data; it can be used in finding subtle sedimentary traps involving changes in porosity. 'Bright-spots', short lengths of a reflection that are conspicuously stronger than adjacent portions may indicate gas: the velocity of sound is sharply reduced in gas-bearing rock, producing a strongly reflective contrast. A gas-water or gas-oil interface may stand out as a noticeably flat reflection amongst arched reflections.
The end-products of seismic surveys are interpreted sections showing geological structure down to fine sedimentary details. Maps are used to describe the topology of known rock units and 'isopach' maps are showing the thickness of these units. For the maps, reflections are 'picked' and their depths at points along parallel and intersecting survey lines plotted and contoured.
Seismic sections that have been picked by hand are digitised and the digital files entered into a gridding and contouring program. Contour maps can be plotted or 3D colour and shade enhanced images can be generated to illustrate the subsurface structure. Some rock layers produce wiggles with a distinctive character that can be followed right across a section; others may be identified by comparison with synthetic 'seismograms' made from logging and velocity surveys in existing wells in which the rock sequence is known.
The seismic maps are used to identify structures that would either repay more detailed seismic surveying or would warrantwildcat drilling. The interpreter studies the maps to identify areas that are shallower and form a dome shape (an anticline) or a shallow area surrounded by faults (a horst block) - within such structures it is possible that migrating oil or gas may have been trapped.
Initially 3D seismic surveys were used over the relatively small areas of the oil and gasfields where a more detailed subsurface picture was needed to help to improve the position of production wells, and so enable the fields to be drained with maximum efficiency. In the early 1990's, when exploration in the North Sea shifted to smaller and more subtle traps, 3D seismic surveys became more widely used for exploration work. The vast amount of data generated by even a small 3D survey meant that computer workstations were an essential tool for interpreting the data quickly. With a computer an interpreter can map a specific reflector by moving the cursor along it on the screen or, when a reflector is strong and continuous, the computer can 'auto-pick' that horizon through the whole 3D data set. Digital files of reflector picks can be transferred directly from the interpreter's workstation to mapping software. Visualisation software is an additional tool that allows the interpreter to view the whole 3D data set as a cube and rotate or cut it at any angle, allowing a picture of the subsurface geometry to be quickly seen
(Discovering the Underground Structure http://www.ukooa.org/education/storyofoil/index.cfm)
49. Replace the underlined words or word-combinations with synonyms from the vocabulary.
1. 3-dimensional (3D) seismic images can be produced by towing the streamer in inclined position.
2. The successful solution of the problem was made from the interpretation of seismic data.
3. Dykes traverse the surrounding layers.
4. The maps which show the thickness of the rocks are result of seismic surveys.
5. Several receivers can be connected in arrays to enhance the signal and reduce the noise.
6. Due to modern techniques the oil fields are commonly worked out with maximum efficiency.
7. This certificate gives you right to work as an interpreter.
8. As a result of echo-sounding the arched formation was discovered at the depth of several kilometers.
9. The interconnection between the weather and human health has been the subject of many discussions for a long time.
10. A new software was set up and the efficiency of the operations increased severalfold.
Дата добавления: 2015-09-10; просмотров: 7 | Нарушение авторских правWORDLIST | Terms and Vocabulary | Reflection Seismology | Form nouns from the following verbs. | Match the pictures with the extract for illustration of which they can serve. | Метод преломленных волн | Applications of Reflection Seismology | Terms and Vocabulary | Acquisition | Processing |