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New methodologies reduce cost, improve data quality.
Periodically technological breakthroughs in a particular segment of the industry seem to come fast and furious despite the industry’s reputation for being slow to try new things. Land seismic acquisition could be said to have experienced this type of renaissance in the past few years.
While efficiencies in marine seismic acquisition can be as simple as adding more streamers to the back of a boat, land is a different animal. The conventional procedure for a 3-D survey is to lay out a grid of receivers according to a survey design, shake the ground with vibrators or dynamite sources to activate the source, record the resulting seismic waves, then pick up the whole mess and move it to the next location. Noise issues have prevented any type of simultaneous sweeping, and adding to the inefficiency has been the sheer bulk of the cables and geophones, which often need to be deployed in very inhospitable terrain.
But new methods of simultaneous sweeping, new types of vibrator sources, and cableless and passive seismic systems all have played a part in dramatically improving the efficiency of land seismic surveys, even in some very difficult situations. Several of these were discussed at the recent Society of Exploration Geophysicists meeting in Houston.
There’s something about the desert that makes geophysicists think hard about their sweep configuration. Three papers discussed new acquisition methodologies taking advantage of recent developments in recorder systems that allow for a recording spread to be continually active. This ability to record continuously significantly speeds up the acquisition process.
One case study examined a new concept called independent simultaneous sweeping (ISS), first introduced by BP in 2008 and deployed in a full-scale implementation in Libya in 2009. This method allowed the company to acquire a very large 3-D survey of some 5,000 sq miles (13,000 sq km).
In this method all vibrators work independently without any attempt to synchronize their activity. Any interference can be treated as noise, which is randomized as much as possible. A standard suite of noise removal tools is used during processing.
The writers note that in their “simple and robust approach, there is no apparent limit to the number of sources that can be operated simultaneously, there is no waiting time for any vibrator, and very little central control or communication is required.” They add that it is well-suited for difficult terrain.
Greater efficiency does not mean cutting corners in data quality. The survey delivered very high-fold data and indicates that 3-D land exploration surveys can be acquired at costs comparable to marine surveys. Currently the company is studying the feasibility of combining this technique with cableless systems.
Another BP invention is distance-separated simultaneous sweeping, or DS3 for short. BP acquired a large 3-D survey in Oman using an 8,000-channel recording patch. The physical dimensions of this deployment (77 sq miles or 200 sq km) allowed the vibrator units to be spread apart over a significant separation distance.
The DS3 method makes use of the large physical separation to eliminate interference between the sources. Units are spaced more than 7.3 miles (12 km) apart so that two or more seismic recordings can be obtained in the same time that only a single record used to be acquired.
The application of this technology has facilitated achievement of world-record onshore 3-D production rates. The Oman 3-D has achieved more than 12,200 vibrator points (VPs) per day, which equates to more than 15.4 sq miles (40 sq km) in a single day, a figure which rivals marine seismic rates. In total, there were 830,000 VPs acquired in less than five months.
CGGVeritas is looking at using longer sweeps in vibrator acquisition. This provides increased spatial sampling and wide-azimuth coverage of seismic acquisition designs. More than 10,000 active channel counts can be achieved with small receiver arrays, allowing for high productivity in deploying receiver equipment. But the source productivity needs to be able to match this pace.
The use of long sweeps along with slip-sweep and simultaneous acquisition methods can reduce acquisition time, but again the data quality shouldn’t
suffer as a result. In an example, the authors show that a single 48-second sweep requires a total of 53 seconds when “listen time” is included, whereas six 8-second sweeps take 88 seconds because of the additional listen and reset times between sweeps. This increase in source productivity offers the opportunity to increase source density, which results in improved imaging.
Concerns about using long sweeps have centered on the supply of hydraulic oil or oil flow required to produce the reaction mass-to-baseplate displacements at low frequencies. This issue has been partly addressed by new vibrator designs. Another concern relates to ground roll, but when using fundamental amplitude control, these effects are not observed.
These types of advances have led to what authors at Petroleum Development Oman LLC have dubbed “the super crew.” In a well-documented case study, the combination of slip-sweep technology, DS3, and 24-hour operations resulted in an average daily production of 5,500 VPs and 8,300 recorded channels. “A step change in both productivity and data quality is achieved,” according to the authors.
In addition to new land acquisition methodologies, improvement to the equipment has been ongoing. ION Geophysical Corp. has focused some of its attention on improving hydraulic vibrators for increased productivity and data quality. Recent work has led to physical modifications to enhance performance at both the low and high ends of the frequency spectrum and also provide a more accurate measure of the ground force from the weighted sum.
After modeling to investigate how the physical properties of a hydraulic vibrator could be modified to improve this relationship, modifications to the hydraulic system were made to enable better low-frequency performance. Modifications were also made to the reaction mass and baseplate assembly to improve the high-frequency performance. This is also expected to yield a greater similarity between the weighted sum and the ground force.
“This development in turn opens up the scope for improving the quality of the seismic data acquired using Vibroseis and is particularly applicable to simultaneous Vibroseis techniques.,” the authors note.
Cableless acquisition technology
Most purveyors of cableless technology tout it as an efficiency and productivity enhancer, not a replacement for cabled systems. But the use of these systems can be particularly helpful in areas where multiple objectives are present.
AscendGeo geophysicists presented a paper on this topic and pointed out that multiple objectives present multiple challenges in an acquisition survey design. Shallower targets require denser sampling for higher frequencies but can be sampled with shorter offsets, whereas deep or steeply dipping targets need larger apertures, longer offsets, and different sampling intervals. In the past, the authors note, surveys were designed for the target needing the densest sampling, resulting in the over-sampling of other targets.
Deploying independent nodes on top of a cable design or by deploying all independent nodes allows for more complex and multiparameter designs. And passive recording stations eliminate the need to manage complex spread control as sources move through the survey. Designs can be more geologically driven without the limitations of channel count or cultural and environmental issues that constrain cable deployment.
Efficiency gains will always drive the industry to develop new technologies. Right now companies that acquire land seismic data are benefiting from these developments.