Data acquisition methods

Reflections and refractions of seismic waves at geologic interfaces within the Earth were first observed on recordings of earthquake-generated seismic waves. The basic model of the Earth’s deep interior is based on observations of earthquake-generated seismic waves transmitted through the Earth’s interior. The use of human-generated seismic waves to map, in detail, the geology of the upper few kilometres of the Earth’s crust followed shortly thereafter and has developed mainly due to commercial enterprise, particularly the petroleum industry.

Reflection seismology, or ‘seismic’ as it is more commonly referred to by the oil and gas industry, is used to map the sub-surface structure of rock formations and also to provide information about the physical properties of both the sub-surface rocks and the fluids within those rocks. These results allow oil and gas exploration companies to identify and de-risk hydrocarbon prospects prior to drilling.

Modern marine seismic data is collected by emitting acoustic energy below the water’s surface from energy sources towed behind a survey vessel. The energy source is typically formed by using high pressure air that is emitted through an array of source elements, typically called air guns. At rock layer boundaries beneath the seabed some of this acoustic energy is reflected back up to the seismic streamers, also towed behind the survey vessel. These streamers can be up to 12,000m long and have hydrophones within them that detect and convert this reflected energy into digital data, which in turn are recorded on-board the survey vessel. These data are processed both on-board and onshore and subsequently interpreted to provide an image of the earth beneath the survey vessel’s traverse. Geoscientists then analyse these images to identify potential hydrocarbon reservoirs.

Several seismic techniques are in use today to provide such information to the geoscientists. At the highest level these can be categorized into 2-, 3- and 4-dimensional seismic surveys.

A 2D marine seismic survey is typically carried out by a single survey vessel towing one streamer and one energy source. The data acquired represent a vertical cross-section of the subsurface layers along the line tracked by the vessel, referred to as a “seismic line”. A 2D survey is characterized by a loose grid of nonparallel seismic lines because the goal is to understand the coarse geology of the region. After analysis of the 2D data, candidate areas of interest are covered by the more dense type of seismic survey – the 3D seismic survey.

3D data is, in effect, compiled by combining a dense set of parallel 2D seismic lines that can be processed to produce a three-dimensional image of surface strata. 3D seismic surveys involve the acquisition of a dense grid of seismic data using multiple streamers over a precisely defined area. Such data acquisition requires the use of sophisticated navigation equipment that permits the constant and precise determination of the positions of streamers and energy sources during the acquisition process. This is required to produce accurate subsurface images.

When 3D marine seismic data is acquired, multiple vessels with multiple streamers and energy sources may be used to acquire the large number of seismic lines needed to produce the 3D data volume. Each streamer towed by a survey vessel gathers information from each energy source such that the total amount of seismic information acquired is represented by the number of seismic lines, combined with the number of streamers and energy sources used. Using this method, large surveys can be performed more rapidly and cost-effectively with, for example, a 12-streamer vessel than a 6-streamer vessel. Surveys performed with 12 or more streamers are characterised as “high-end 3D”- surveys.

4D seismic surveys involve exactly repeating a series of 3D surveys over the same survey area at different times to understand the draining dynamics of a reservoir, as witnessed by the changed geophysical response with time. Useful 4D information requires a higher density of 3D data and increased computing capacity to reflect the relatively subtle changes in geophysical conditions that may occur over time.

Oil and gas companies find high density data to be particularly useful from 4D surveys for animating reservoir behaviour. This helps with the development of more efficient drainage strategies that increase recovery from producing petroleum reservoirs.

A 4C seismic survey is a special type acquisition technique that requires the receivers to be in direct contact with the seabed, which allows for extra information to be detected from the reflected subsurface waves. In towed streamers, the hydrophones can only measure the small variations in water pressure that are caused by the reflected subsurface waves but a seabed streamer (simply called a cable) is able to measure the particle velocity (shear wave) of the soil too. Particle velocities are measured in the three orthogonal planes (XYZ) and when the original hydrophone signal is added, the cable is able to deliver four sets of data – or 4C.

There are several advantages to acquiring both pressure and shear wave data:

 The ability to image geology under gas-invaded strata (a “mirror” to towed seismic)  Improved lithology prediction and fluid discrimination

 The ability to characterise rock fracture orientation to help understand drainage

Compared to towed streamer acquisition, this technique is a much more time and cost intensive operation. For that reason, 4C tends to be focused on problematic producing reservoirs. However, as has been demonstrated from the towed streamer sector, acquisition costs are expected to fall as expertise and acquisition spread multiplicity improves.

Permanently trenching a 4C cable into the seabed over a reservoir represents the ultimate marine seismic implementation because it delivers both 4C and 4D: illustrating subtle variations of reservoir characteristics over time. There is a greatly improved signal-to-noise over conventional towed seismic because the sensors are in stationary the soil and not close to the sea surface where they are affected by swell and movements.

The seabed array of 4C sensors is buried over the area of the producing reservoir and is tied back to the recording equipment on the central platform via standard umbilical-through-J tube. Repeat source-boat surveys are conducted typically every three months to build up the time-lapse movie. Extra surveys may be called upon before a drilling campaign. Such a survey is much simpler, and faster, than repeat towed-streamer surveys and the latest reservoir data is therefore more current.

Wide and multi-azimuth

By synthesizing 3D data from different angles, the seismic companies are able to produce better images of the sediment structures. There are two main techniques for gathering such data:

 Wide-azimuth  Multi-azimuth

Wide-azimuth is acquired using a high end 3D vessel, called the receiver vessel, together with two dedicated shooting vessels, named source vessels. Alternatively, datasets may be obtained using a multi-azimuth technique, in which a

The main reasons for applying these high end acquisition configurations are to improve the illumination of the sub- surface i.e. to avoid shadow zones, and to provide images with less noise. These techniques are therefore typically applied in areas with complex geology like salt, basalts and rotated fault blocks and have a word wide application. Depending on streamer requirements and acreage, the time taken to acquire wide-azimuth and multi-azimuth surveys are usually two to four times the time it takes to acquire one traditional survey. The azimuth technique is still fairly new, so the actual acquisition technique applied may differ between projects and companies.