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The words seismic and geophysics are often associated with earthquakes, but seismic data are also a valuable technology used extensively by the oil and gas industry in its exploration, development and reservoir management operations. This interactive Geoscience Overview is a picture-based explanation of seismic data — how the data are gathered and how the data are used. There are 40 slides, each one should take only about 30 seconds to read and digest.
The main purpose of seismic exploration is to render the most accurate possible graphic representation of specific portions of the earth's subsurface geologic structure. The images produced allow exploration companies to accurately and cost-effectively evaluate a promising target (prospect) for its oil and gas yielding potential.
The theory behind seismic imaging is straightforward but it takes knowledge, experience and advanced technology to do it right. Acquisition of seismic data involves the transmission of controlled acoustic energy into the earth and recording the energy that is reflected back from geologic boundaries in the subsurface.
Information regarding the structure and nature of the reflecting strata can be derived from the two-way travel time and other attributes of the returning energy. Processing these reflections produces a synthetic image of the earth's subsurface geologic structure.
At sea, the procedure is essentially the same except that our instruments are continuously moving!
The seismic (energy) source is usually an array of airguns towed behind the survey vessel just below the sea surface. The airguns are fired at regular intervals as the vessel moves along pre-determined survey lines.
Energy reflected from beneath the seafloor is detected by numerous 'hydrophones' contained inside long, neutrally buoyant 'streamers' - often almost 5 miles long - also towed behind the vessel.
Two types of seismic surveys are available to the geophysicist: two dimensional (2D) surveys or three-dimensional (3D) surveys.
2D seismic data are displayed as a single vertical plane or cross-section sliced into the earth beneath the seismic line. 2D is generally used for regional reconnaissance or for detailed exploration work where economics may not support the greater cost of 3D...
3D seismic data are displayed as a three-dimensional cube that may be sliced into numerous planes or cross-sections.
More expensive than 2D, 3D data produces spatially continuous results which reduce uncertainty in areas of structurally complex geology and/or small stratigraphic targets.
Two or more 3D seismic surveys acquired at different times can be compared in order to search for changes in the fluids within the rock formations.
This type of survey is known as 4D, where elapsed time is the fourth dimension of information.
There are five key ingredients to acquiring useful seismic data:
Accurate positioning is fundamental and vital to acquiring seismic data. We must know precisely where all our instruments are on the earth's surface.
Postitioning and surveying is crucial because the data are worthless if we don't know where they came from.
In both marine and land environments, energy source and receiver layout patterns are pre-planned and their positions pre-determined, so that we can calculate precisely where our recorded seismic data originate.
Today we are in the 'space age' of GNSS - the Global Navigation Satellite System - which offers unprecedented positioning accuracy. Commonly known to the public as GPS, a modern COMPASS systems.
Combined, these systems will operate a constellation of around 100 (in 2012 around 60) satellites in orbit over 20,000 kms above the earth. The satellites act as precise reference points in space and transmit radio signals that allow a GNSS receiver on earth to triangulate its position to within around 2 meters.
While 2-meter accuracy is adequate for most purposes, for seismic we use Differential GNSS correction techniques to bring our levels of accuracy to within a few centimeters!
At sea, positioning is more difficult than on land because our vessel - and all its towed equipment - is continuously in motion.
Nevertheless, the precise locations of the energy source(s) and the streamer(s) must be known at all times.
In such a dynamic environment, real-time positioning is extremely complex and highly computer-intensive.
We use an integrated combination of multiple reference site DGNSS, relative DGNSS, laser measurements of ranges and angles, underwater acoustic ranging and digital compasses along the streamer(s).
Literally hundreds of complex mathematical position calculations are carried out every few seconds, enabling the precise positions of the vessel, the seismic source(s) and the individual hydrophone groups in the streamer(s) to be calculated in real-time as the vessel continuously moves along.
To gather seismic data, we must first generate and transmit controlled acoustic energy into the ground.
In the past, dynamite was the preferred seismic energy source both on land and at sea. Dynamite is still used on land, usually in areas of soft, unconsolidated or weathered surface layers.
When buried and detonated in safely plugged shotholes below the surface layer, dynamite produces a sharp, acoustically clean energy pulse. However...
...in urban and/or populous areas, dynamite is obviously not practical!
There are several other energy source technologies used for acquiring seismic data on land, but the main one is 'vibroseis'.
Large servo-hydraulic vibrators on vibroseis trucks are safer, faster, more adaptable and more environmentally friendly than dynamite.
These trucks can yield equal (or sometimes better) data quality to dynamite.
A vibroseis truck generates a controlled vibratory force of up to 70,000 lbs through a baseplate that is placed in contact with the ground.
In the marine environment (and sometimes in swamp or marsh) dynamite has been almost completely replaced by airguns.
High pressure air is stored in a firing chamber and explosively released through portholes by the action of a sliding shuttle with pistons at each end.
Seismic energy is generated by the rapid, explosive release of compressed air through the airgun's ports...
...into the surrounding water. This produces a primary energy pulse and an oscillating bubble.
Typically, multiple airguns are towed behind the vessel, several meters below the sea surface in a pre-determined combination or 'array' of different chamber columns designed to generate an optimally tuned energy output of desirable sound frequencies.
Some of the energy we send into the ground or water is reflected back from geologic boundaries in the sub-surface.
This reflected energy is detected by a connected network of geophones (left) planted in the ground, or by groups of hydrophones contained inside the neutrally buoyant seismic 'streamer(s)' towed behind the vessel at sea (main picture).
Similar to microphones, these devices convert the reflected energy into electrical energy which is transmitted to...
...a central recording system, usually housed in the instrument room (or 'doghouse') for recording as raw seismic data and for quality control checks.
Quality control is vital, not just during data recording but at every stage of a seismic project.
At sea, several lines of seismic data can be recorded simultaneously by towing multiple source arrays and streamers.
Here, two source arrays and four streamers allow eight lines of seismic data to be recorded at once.
Preservation of health, safety and environment (HSE) are of paramount importance in conducting any seismic operation.
By its nature, whether on land or at sea, seismic work is not without risk.
However, through effective HSE management, education, training and planning, and by following HSE rules that reduce these risks to a minimum, everyone can come home safely at the end of every day.
We must make sense of the recorded seismic 'squiggles' to produce the truest possible image of the earth's sub-surface geologic structure. Reflected seismic response is a mixture of our output pulse, the effect of the earth upon that pulse and background noise, all convolved together.
We must remove the input pulse and the noise to leave just the 'earth model'.
This is the role of seismic data processing, which requires accuracy, reliability, speed and...
...substantial computing power. The advanced mathematical algorithms and complex geophysical processes applied to 3D seismic data require enormous computing resources, not to mention the massive volumes of data involved.
For example, the amount of seismic data recorded by CGG during just one medium-sized marine 3D survey would fill more than 20,000 compact disks, forming a stack over 650 feet high!
An ideal seismic response would be a single sharp reflection for each sub-surface rock layer boundary. Actual seismic response is less than ideal because our input pulse is not perfectly sharp and changes its shape while passing through the earth. Deconvolution 'deconvolves' our input pulse from the seismic response and converts it into a cleaner, sharper, less confusing pulse.
Can you determine the number of rock layers here by examining the actual seismic response (before deconvolution)?
Seismic traces from the same reflecting point are gathered together (CRP gather) and summed, or 'stacked'.
The six seismic traces on the left are from the same reflecting point. As the traces are merged into one (right), background noise cancels itself out while the seismic signals add together, producing a stronger signal-to-noise ratio.
(The output trace on the right is shown here six times only to provide a better comparison.)
The more of these seismic traces we can stack together into one output trace, the clearer the seismic image.
Each seismic trace in this 'shot gather' represents what is recorded at each receiver for a single shot. As this image is from a 3D survey, we can see traces grouped in "banks" according to receiver lines that are laid out parallel to one another. Since the seismic wave takes less time to reach the receivers closest to the shot, their response shows up earliest on the shot record.
As we get further from the location of the shot in either the x- or y-direction, the wave takes longer to reach the geophone, so the time of the first arrival increases and it appears lower on the display (time increases as you move toward the bottom of the image). You can also see the various reflectors showing up deeper in the shot record.
The first attempt at combining the traces illustrates the advantage of 'stacking' the data.
At this point, the differences in the time it takes for a wave to travel due to the difference in elevation between the shot and the receiver have been accounted for.
A preliminary attempt at determining and accounting for the different velocities of the various layers of the subsurface has also been made.
As more precise corrections are applied to account for the effect of both the near-surface and sub-surface, and as an attempt is made to differentiate seismic 'noise' from 'signal' so that the noise can be removed, the image of what lies below the earth's surface becomes more focused.
Various faults and folds can be more easily identified.
This fourth image most closely resembles the subsurface geology.
A process called 'migration' moves the reflected energy to its true subsurface position of origin.
The interpreter can now not only take note of the various geologic structures under the earth's surface, but also the amplitude of the various events in an effort to unravel the mystery of what lies beneath the ground.
Comparing the preceding four steps, we can clearly see the gradual enhancement in seismic image achieved through data processing. The example showed only four steps. In data processing, there are many steps required to arrive at the final seismic image.
This particular project shown at left required approximately 25 separate steps!.
More advanced processing techniques, such as Prestack Depth Migration (PSDM), can significantly improve seismic imaging - especially in areas of complex geology. In this example from the Gulf of Mexico, PSDM has improved the imaging of a massive salt body, and sedimentary layers beneath the salt.
Processes such as PSDM take more time, expertise and resources to apply, but accurate 3D seismic images can mean the difference between success or an expensive dry hole.
Our customers usually need the data delivered as fast as possible!
In fact, today's industry demands for ever-faster turnaround of seismic projects necessitates that data now be processed, at least to a preliminary stage, in the field immediately after recording.
This requires equipment and personnel in the field to be almost as sophisticated as those onshore.
We must interpret the seismic data to understand the geology and assess the likelihood of finding oil and gas accumulations.
Geophysicists at CGG interpret the processed seismic data and integrate other geoscientific information to make assessments of where oil and gas reservoirs may be accumulated.
Powered by advanced supercomputer, rapid data loading, high-speed networking and high-resolution graphics, CGG visualization centers provide the ability to display and manipulate complex volumes of 3D data in a collaborative, team environment.
The result is better interpretation of more data, in less time.
CGG offers a broad range of advanced interpretation services including PSDM, seismic attribute analysis amplitude variation with offset (AVO) analysis and reservoir characterization.
Our visualization centers enhance our ability to integrate additional geophysical and geologic data such as well logs, and to visualize and rapidly mature prospects for testing.
The end product of all this work and technology is a graphic 3D representation of the earth's subsurface geologic structure.
Based largely on this information, exploration companies will decide where (or if!) to drill for oil and gas.
This example represents over 600 square kilometers of complex geology down to a depth of more than 6,000 meters!
We hope our online seismic 'guided tour' has given you a basic idea of what we do at CGG. If you'd like to learn more, please contact the CGG office nearest to you.