Bright spots

In a seismic survey, sound waves from a sound source partially reflect off boundaries between different underground strata to form echoes that are detected at the surface by receivers called geophones. Those echoes provide information about the subsurface geology, including the locations of potential oil and gas traps.

Oil and gas occupying rock pores in an oilfield reservoir affect the physical properties of the rock in a way that could alter those sound echoes and thus provide direct evidence of subsurface oil and gas pools. For example, a quite modest amount of natural gas inside a rock will significantly reduce the velocity of sound passing through the rock. That velocity reduction can increase the acoustic contrast between the gas-bearing rock and the adjacent rock formations. And the increased contrast can, in turn, cause an abnormally high amplitude seismic reflection, giving rise to what geophysicists refer to as a ‘bright spot’ in a seismic section.

However, there can be more than one possible explanation for a seismic phenomenon such as a bright spot, and so this type of indicator suggests, but does not prove, the existence of subsurface hydrocarbons. And this is especially true when trying to use seismic data to detect oil. Oil has a much lower acoustic contrast with rock than gas. And, to make things even more tricky, there’s quite a low acoustic contrast between oil and water, thus making these two liquids difficult to distinguish.

Task 2. Answer the questions to the text:

Задание 2. Ответьте на вопросы к тексту:

1. How is natural gas detected in a seismic survey?

2. Why is it more difficult to detect oil?

Task 3. Read the extracts and answer the questions:

Задание 3. Прочитайте и ответьте на вопросы:

1. How does AVO improve the performance of a survey?

2. What is the difference between 3D and 4D seismic?

Developments in seismic technology

Developments in seismic technology have enabled geophysicists to further reduce drilling risk. Here are two examples:

The first technique is known as amplitude variation with offset. AVO is a bi-product of the way in which a seismic survey involves recording underground sound reflections using sound sources and geophone sound detectors in a series of increasing offsets from a single survey point (an offset is the distance between the seismic source and the receiver). The seismic crew record the data from different offsets so that they can add the data together. This addition tends to remove random noise while enhancing coherent signals from underground sound reflections. AVO analysis is more successful in young, poorly consolidated rocks, such as those in the Gulf of Mexico, than in older, well-cemented sediments, such as those from the mid-continent region of the USA.

Another technique is known as 4D seismic. This technique involves shooting several 3D seismic surveys over the same area over a time period of perhaps several years (a 3D survey is a type of survey that results in a three-dimensional image of the subsurface geology). Changes in seismic signals from one survey to the next can provide insights into the movement of fluids, such as oil and gas within the field reservoir.

Task 4. In seismic acquisition, a source of sound energy and a receiver are required. Read the description of the use of a vibrator truck in seismic surveying and decide whether each sentence refers to the source (S) or the receiver (R):

Задание 4. Для сбора сейсмических данных необходим источник звука и приемник. Прочитайте описание работы вибрационной сейсмической установке и определите, относится ли высказывание к источнику (S) или приемнику (R):