簡介：最近WesternGeco公司推出新的陸上點接收地震勘探設(shè)備UniQ，它將是地球科學(xué)的一項重大突破。它延伸了點接收數(shù)據(jù)收集的范圍、靈活性、可靠性，并提高了其效率。
The recently launched integrated point-receiver land system might be the next breakthrough geoscientists are seeking. The system can record up to 150,000 live channels at a 2-millisecond sample interval. The new system is designed to combine all of the capabilities described above and also extend the capacity, flexibility, reliability, and efficiency of point-receiver acquisition.
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Desert Explorer MD Sweep vibrators probe shifting dunes of indeterminate velocity and thickness to image the rock formations beneath.(Images courtesy of WesternGeco)
It could be said that acquiring land seismic data is one of the greatest challenges in the oil and gas industry today.
Explorationists and reservoir managers are trying to find and produce hydrocarbons in increasingly demanding conditions. On the Earth’s surface there are requirements to acquire exploration data in challenging frontier areas but also to acquire high-quality data in noisy, producing oil fields for reservoir development and production. At the same time, prospects are smaller, deeper, and often concealed within complex geological settings that test the assumptions upon which conventional seismic surveying has been based.
A key part of the difficulty of land seismic acquisition is the continuing expansion of prospects into challenging terrain, from the freezing Arctic to the hottest desert areas. These areas present a set of unique challenges that may seem obvious, but go far beyond the fact that the acquisition environments are typically very cold, very hot, or sometimes both, depending on the time of year or even just the time of day.
To be sure, surface environments have affected the reliability of equipment and have hampered field crews. However, the presence of near-surface heterogeneity plays havoc with seismic velocities and makes data processing extremely complex. In arctic regions it is critical to understand and determine how permafrost, frozen lakes, and sea ice affect seismic data quality. Likewise, in desert areas, shifting sand dunes of indeterminate thickness can conceal subsurface features in much the same way as a rippled glass window.
Exploration and reservoir development has progressed into carbonates and unconventional shale gas plays as well. In the former, porosity ranges from low in impermeable oolites to extremely high in naturally fractured zones with permeability measured in Darcies. In the shales, thick zones of low contrast conceal so-called “sweet spots” of hydrocarbon accumulation in micropores with permeabilities measured in nanodarcies. Characterizing these extreme formations requires a level of detail and repeatability never before required.
The application of seismology to reservoir characterization has resulted in many new challenges for the technique.
No longer focused solely on structure, seismic technologists have been asked to provide answers regarding fluid migration, permeability barriers, and reservoir compartmentalization. These answers require an unprecedented level of detail and fidelity.
Point and counterpoint
In 2002, WesternGeco introduced the Q-Land point-receiver seismic acquisition and processing system. The system was built to simultaneously record traces from all of its 30,000 sensors. The geophone accelerometer (GAC) was selected as a robust and elegant way to reduce signal distortion while increasing bandwidth. The ability to record data from this high channel capacity enabled breakthroughs in subsurface imaging through improved signal and noise sampling and the associated data processing.
Since that time, several source technology developments have further improved and refined the ability to acquire increasingly detailed seismic surveys. Maximum Displacement sweep (MD Sweep) allows seismic vibrators to optimally deliver the required source output, adding half an octave of low-frequency bandwidth when compared to conventional sweeps. When coupled with the latest high-power, high-fidelity vibrator unit, the MD Sweep technique provides an even more powerful, low-distortion, broad-bandwidth signal.
There has also been increasing interest in simultaneous source techniques. Such techniques potentially bring a step-change in acquisition efficiency. The idea of deliberately firing multiple seismic sources to interfere with one another seems counter-intuitive. However, the ability to identify and separate the information provided by each source using innovative separation algorithms and powerful digital processing techniques has enabled simultaneous source acquisition to gain acceptance.
The next step-change
The recently launched UniQ integrated point-receiver land system might be the next breakthrough geoscientists are seeking. The system can record up to 150,000 live channels at a 2-millisecond sample interval. The new system is designed to combine all of the capabilities described above and also extend the capacity, flexibility, reliability, and efficiency of point-receiver acquisition.
The system was field-tested extensively in both arctic and desert environments. An initial test conducted in the United Arab Emirates (UAE) focused on assessing the system’s robustness. This was followed by a second field test in Norway where fidelity of measurement and robustness were evaluated in freezing conditions. For the third and final test, the crew returned to the UAE deserts to conduct a thorough 2-D survey.
Suitable for fast-moving, fit-for-purpose exploration surveys and wide-azimuth, broad-bandwidth appraisal and development surveys, the same system can be used to deliver optimal solutions across a full range of seismic applications. The GAC sensor (Figure 3) now features “plug-and-play” capabilities, automatically powering up and self-testing upon connection. Cable connections have been minimized, ruggedized, and designed to permit arctic field crews to make connections while wearing heavy mittens — they also resist the formation of ice on connecting parts. For desert operations, the connections resist sand intrusion, and the GACs can tolerate intense heat environments without fidelity loss.
Network advances also add reliability and flexibility. Sensor strings are double-ended to ensure uninterrupted power and data paths. The system features alternate fiber-optic pathways for seismic data to reach the recording system, enabling it to sustain multiple line breaks or cable cuts and still continue acquisition. Even so, any cable break triggers an alarm that pinpoints the break location so repairs can be made quickly.
The system is capable of operating essentially unlimited numbers of vibroseis units. Combine this with a continuous recording architecture, GPS time-stamped records, and zero dead-time acquisition, and the result is a system built to support all simultaneous vibroseis techniques in use today and well into the future.
Why all those channels?
The challenge of imaging through complex geology (such as the folding and faulting in overthrust geology) or analyzing the azimuthal variations in seismic response to accurately determine reservoir subtleties requires seismic data pathway from all directions. This requires a high channel count. Combined with a requirement for imaging deep targets or sampling finely to characterize and remove noise from the data, an extreme channel capacity becomes essential. The high channel counts of the system enable fielding of the right number of point-receivers to optimize the job, including full-offset full-azimuth reservoir surveys. The extreme channel capacity also can be used together with simultaneous source techniques to improve efficiency of exploration surveys, where the objective is a fast structural image.
Combining forces
Point-receiver acquisition and processing techniques have recently been combined with other geophysical measurements, including gravimetric and magnetotelluric techniques. These complementary measurements provide additional constraints during processing and interpretation of seismic data to deliver more reliable subsurface images.
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