A multicomponent land sensor is proving its worth in trial surveys.

For many years, geophysicists have kept an eye on evolving technologies to see if they could offer an advantage for seismic acquisition. Moving coil geophones have been used successfully, almost universally, as seismic sensors, and they have enjoyed many performance enhancements during their 50-year history.
Despite this pedigree, interest in other technologies has been driven with an eye toward economics, logistics and added value. Reduction in size and weight, coupled with improvements in performance and reliability, are strong industry drivers. Integrated accelerometers have been available for more than a decade and have attracted interest from the geophysical community. Fiber-optic sensors have followed similar appraisals. Only recently have microelectromechanical system (MEMS) devices reached a level of common awareness. MEMS describes devices ranging from micro bio-pumps to mirrors to inkjet print heads. MEMS accelerometer designs are available for automotive applications and industrial vibration sensing. Now a micromachined digital accelerometer has been specifically targeted at the seismic acquisition industry.
The benefits of a MEMS accelerometer include reduced size and weight, integration of system electronics, calibrated response sensor-to-sensor, and flat transfer and phase response from DC through the seismic band coupled with ultralow distortion by the use of force-balanced feedback. These features make the sensor suitable for economic multicomponent seismic acquisition. The shear wave energy recorded in these surveys provides direct measurement of rock properties, lithology and fluid identification, fracture detection and orientation, and structural imaging through gas clouds. This data is of great value to seismic interpreters, providing essential information for detailed reservoir characterization and production monitoring.
The MEMS digital sensor has two principal components, a silicon micromachined accelerometer and a custom designed, mixed-signal, application-specific integrated circuit (ASIC).
The MEMS sensor is composed of a moving inertial mass, suspended by springs from a surrounding frame structure. The resonant frequency of this spring-mass sensor system has been moved above the seismic band into the kilohertz range. Designed to operate below its resonant frequency, the sensor behaves as an accelerometer. Moving coil geophones, which measure ground velocity, also are suspended mass systems but for mechanical reasons are designed to have their resonant frequency below the seismic band, typically at 10 Hz.
The upper and lower surfaces of the suspended mass have metal deposited on them to create conductive surfaces. Upper and lower wafer caps also have deposited metal to create a capacitor between the mass surfaces and the cap wafers. The MEMS assembly is formed from four individual silicon wafers, each wafer etched to form component structures and then collectively bonded to form the final die assembly. Figure 1 shows a schematic of the MEMS accelerometer cross-section.
Achieving the extremely low sensor noise performance required for seismic applications was a significant technical challenge. Two controllable parameters that have a major effect on inherent thermodynamic sensor noise are the size of the suspended mass (the smaller the mass, the greater the noise) and the damping of the resonant structure (greater damping results in greater noise). To fabricate the MEMS accelerometer, a technique called bulk micromachining was chosen over surface micromachining primarily to fabricate larger proof masses. To reduce the damping of the resonant structure, the sensor die is packaged and sealed in a high vacuum to produce an almost gas-free internal cavity containing the proof mass. Enclosed gas molecules strike the proof mass, increasing the device's noise level.
The MEMS accelerometer die is capable of being used as a stand-alone capacitive accelerometer, but achieving the performance necessary for use as a seismic sensor required the development of the custom mixed-signal ASIC.
The ASIC serves two important functions. First, the MEMS accelerometer is operated in a closed-loop, force feedback mode. As changes in capacitance are sensed by the ASIC, a restoring electrostatic force is applied to maintain the proof mass in a central (neutral) position. Second, the acceleration response, as measured by the feedback force, is digitized by an internal delta-sigma A/D converter. The output of the MEMS/ASIC accelerometer is an over-sampled digital bit-stream.
Considerable systems integration modeling and analysis were required to optimize the MEMS accelerometer and ASIC design parameters to achieve high performance and robust stability. The research and development efforts in advancing these components spanned 15 years and involved a sustained investment, including the establishment of the MEMS fabrication plant.
The MEMS accelerometer has undergone extensive laboratory and field testing to validate its performance during a 3-year period. Ambient sensor noise, dynamic range, harmonic distortion and cross-axis rejection are important performance characteristics for seismic applications. Results from an extensive 1999 field test program showed less than required noise performance and reduced dynamic range. These sensors, though otherwise excellent, would have had limited application in the demanding seismic environment. This serves to illustrate that there is a minimum entry-level performance for seismic acquisition. A concerted engineering effort through the end of 1999 resulted in a significant noise reduction and dynamic range improvement. The success of this effort was demonstrated in several field tests during the summer of 2000.
The first Input/Output (I/O) product using the MEMS digital accelerometer is the VectorSeis three-component (3-C) sensor module. Three of the digital sensors are mounted at right angles to each other in a tubular housing. This housing can be coupled to the ground for multicomponent land seismic recording. The performance of the digital sensor, with its broad dynamic range, high vector fidelity and low-frequency response, has the potential to markedly improve the subsurface imaging and interpretation of key reservoir attributes.
Following the successful developments and field trials conducted in August 2000, I/O and Veritas DGC Land equipped a crew with the latest precommercial version of the VectorSeis 3-component digital sensor module. The crew was formed to gain production-level experience with the new digital sensors. Veritas is providing the field personnel, survey design, operational experience and processing capabilities to make this venture a success. This precommercial system consists of 1,500 3-C VectorSeis stations coupled to the land recording system.
Most early activity has focused on surveys in the Western Canadian and Williston basins of Alberta and Saskatchewan, where temperatures have been as low as -32°F (-35°C) during deployment and recording. Six successful revenue-generating surveys have been completed for five exploration and production companies since the equipment first reached the field in February 2001. These deployments have ranged from small to medium-sized 3-D surveys to long 2-D lines, all recording shear wave energy as mode converted P-S or C waves from either dynamite or vibroseis sources. Based on the results of these surveys and additional field-testing, a busy schedule of work is leading up to the 2001-2002 winter season. Showing their confidence in the value this new technology brings to seismic acquisition, one of the early adopter exploration and production companies engaged the equipment for three additional surveys. Another client will use the equipment for a month in an extensive field trial in the United States.
Results from the surveys are still emerging as processing continues. A common trend has been improved P-wave data with higher-than-normal resolution and broader bandwidth, sometimes as much as a 25-Hz bandwidth improvement. In addition, the operational model has been verified and shows that the system can be used efficiently for improved P-wave data only. In this mode of operation the converted mode (P-S) shear wave data can be considered a bonus.
A commercial version of the sensor module is due for release later this year. The commercial system comprises a repackaged sensor module with a higher level of component and functional integration, reducing size and weight. This sensor is designed to offer more end-user value and meet the challenge of a new era of high-quality, high-resolution seismic surveys necessary for the detailed reservoir characterization and monitoring requirements of exploration and production companies.

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