Fig 1. Using a large proofmass allows this HP silicon MEMS accelerometer to achieve ultra-low noise levels with a noise floor down to a few ngs/vHz.
Fig 2. HP’s silicon MEMS accelerometer’s design allows it to achieve a noise power density spectrum over a frequency range of l mHz to 100 Hz as compared with the USGS’s GS13 reference accelerometer.
Hewlett-Packard (HP) and Royal Dutch Shell are collaborating on developing a next-generation wireless sensing network for oil and gas exploration, using HP’s recent and notable ultra-high-sensitivity microelectromechanical systems (MEMS) accelerometer that’s 1000 times more sensitive than any commercially available MEMS accelerometers.
The sensor provides very low noise at frequencies below the bandwidth of traditional geophones and existing MEMS devices. The small size of the sensor and very low power consumption will significantly reduce the cost of large-scale deployments in a wireless sensor network, according to the companies, enabling data from more channels to be collected, increasing the channel density of any given survey.
The sensing technology has been demonstrated to have a noise floor of 10 ng/√Hz—a measure of the smallest detectable acceleration over a range of frequencies. “This approaches the quietest noise level detectable on earth” says Grant Pease, HP’s marketing manager for sensing systems platforms.
“This is similar to the ocean vibration noise impacting the quietest land on earth,” adds Rich Duncombe, senior strategist with HP’s Technology Development Organization in the Imaging and Printing Group.
“For seismic applications, low noise, low frequency, and low power dissipation are paramount,” explains Pease.
“Responding to the energy challenge, the oil and gas industry is tackling ever deeper and more complex reservoirs, as well as reservoirs in very tight rock systems,” says Dirk Smit, chief scientist for geophysics and vice president of exploration technology at Shell. “In particular, for on shore settings, this requires enhanced-quality seismic data as well as the cost-efficient, flexible deployment of seismic sensor networks.”
Put To The Test
The sensing technology’s results were demonstrated in tests performed at the U.S. Geological Survey’s (USGS) Seismological Laboratory in Albuquerque, N.M. The primary goal of the testing was to measure the self-noise of the sensor, with the testing performed in a vault on a block of granite. The vault is used for testing seismometers because of the extremely low ambient noise levels. The USGS provided a high-resolution seismometer (GS-13) to be used as a reference during the testing.
Key to the sensor’s performance is its surface electrode sensing technology, as well as an integrated custom ASIC that’s under development. When combined with HP’s MEMS accelerometer, the sensor is expected to have the 10-ng/√Hz performance down to the 1 Hz previously mentioned.
The sensor is fabricated from three separate single-crystal silicon wafers bonded together and singulated into a small vacuum encapsulated die (Fig. 1). The proof mass is suspended by silicon flexures etched through the center wafer. Electrodes are arrayed on one surface of the proof mass and on the stationary wafer opposite the proof mass. A small gap is maintained between the two wafers.
One advantage a MEMS accelerometer like HP’s has over geophones is a flat frequency response at low frequencies. While geophones roll-off below their resonance frequencies, the gain and phase of the HP unit is flat from 200 Hz down to dc and has a linear output with no hysteresis.
In the USGS testing, the sensor demonstrated a dynamic range of 120 dB and a noise power spectral density of less than 10 ng/√Hz (Fig. 2). The sensor’s performance was tested side by side with the USGS reference sensor under a real-time 6.7 Richter-scale earthquake in the Gulf of California that occurred October 21, 2010. The signal from the reference sensor was matched by HP’s sensor down to 25 mHz, verifying the sensor’s response at low frequencies.
The sensing system will be delivered by HP’s Enterprise Services and the company’s Imaging and Printing Group.
Royal Dutch Shell
www.shell.com