Malcolm B. Bertram

Field data comparisons of MEMS accelerometers and analog geophones

Geophones have been the motion sensor of choice in oil and gas exploration surveys for many years and for good reason: they require no electrical power to operate, are lightweight, robust, and able to detect extremely small ground displacements. However, the seismic industry recently has developed considerable interest in microelectro mechanical systems (MEMS) accelerometers, which are similar to those used to sense accelerations for airbag deployment and missile guidance (among many other uses). The MEMS element is tied to an application specific integrated circuit (ASIC) that includes sensor signal conditioning, feedback control, and digitization blocks. The result is a custom-built unit for seismic applications that requires a power supply to operate.

Seismic physical modeling at the University of Calgary

The seismic physical modeling facility at the University of Calgary has existed since 1985. Recently, we upgraded it by replacing obsolete components with modern alternatives. We constructed a 3D positioning system based on high-precision linear electric motors, and coupled it to arrays of multiple transmitting and receiving piezoelectric transducers. We wrote customized software executing on the latest generation of desktop computers for controlling transducer movements, selecting and activating individual transducers, and acquiring and recording files of digital seismograms. The modernized facility enables us to collect scale-model seismic gathers at rates of thousands of traces per hour. We present examples of data from modeled 2D marine and land seismic surveys.

Ground motion through geophones and MEMS accelerometers: sensor comparison in theory, modeling and field data

Summary Digital sensors based on micro electro mechanical systems (MEMS) accelerometers are one of the newest technologies being used in seismic acquisition. As such, some confusion remains surrounding similarities and differences relative to the coil-over-magnet geophone. An understanding of the functioning of these sensors and how to compare them can be facilitated by deriving transfer functions, which relate the data acquired through each sensor to actual ground motion. An equation is then derived to calculate acceleration comparable to unprocessed MEMS data from unprocessed geophone data. The inverse of this equation may be used to calculate geophone data from MEMS data. The effects of sensors on zero and minimum phase wavelets are modeled, demonstrating that the raw output from the sensors should be similar. The minimum phase wavelets are convolved with a random reflectivity series to test deconvolution of impulsive source data. Deconvolution produces geophone and MEMS processed traces that appear similar, and constant phase rotation of MEMS data after deconvolution cannot correct all remaining differences. The geophone-to-MEMS transfer equation will exactly transfer between sensors only in the absence of instrument noise. Comparisons between MEMS and geophones recording the same shots, and ground motion domains calculated from those records, show that the data is very similar in frequency content when the same domain is considered, and MEMS records will not necessarily have a larger magnitude contribution from low frequencies than

Azimuthal responses of some three‐component geophones

A significant and often overlooked aspect of seismic surveys is the importance of data fidelity. Geophone fidelity is particularly critical for multicomponent seismic surveys in which the full vector wavefield is sampled by geophones which house multiple elements. Traditionally, geophones are tested on “shake tables” in the laboratory (with field testing equipment) or evaluated on the basis of simulation or equivalent circuits. There appear, however, to be few studies which evaluate geophone performance in the field.

Accelerometer vs. geophone response: A field case history

There has been considerable interest in the geophysical community surrounding the use of MEMS accelerometers as a new sensor in the acquisition of seismic data (Dragoset and Gabitzsch, 2007; Laine and Mougenot, 2007). It has been suggested that accelerometers, with their flat response in acceleration, may have advantages over geophones at low frequencies as well as high frequencies - due to greater sensitivity (Maxwell et al., 2001; Mougenot and Thorburn, 2004). If both sensors’ frequency responses correspond to what the simple harmonic oscillator model would predict, then it should be straightforward to calculate an equivalent output in any domain desired. The output of the sensors could then be compared to show if differences are in evidence and whether either sensor more accurately represents ground motion.

Comparison of low‐frequency data from co‐located receivers

A low-frequency sensor comparison survey was acquired at the University of Calgary’s test site near Priddis Alberta in August 2009. Portable calibrated broadband seismometers, analog 3C 10 Hz geophones and two makes of digital 3C accelerometers were deployed at 1 m receiver line spacing, and used to record weight-drop and EnviroVibe (this report) sources at two source points located 50 m from the end of the receiver lines. This study shows that leastsquares-subtraction scalars (LSSS) depend on amplitude, frequency, phase, source-receiver offset, quality of sensor placement in or on the ground. LSSS show good promise for future use in quantitative sensor comparison studies.

Field data comparison: 3C‐2D data acquisition with geophones and accelerometers

We report on a field comparison of different seismic motion sensors. The CREWES Project at the University of Calgary acquired a 3C-2D seismic line in the Spring Coulee area of Southern Alberta in January 2008. This was a unique opportunity to compare two types of multicomponent sensors with acquisition occurring at the same time and with the same receiver parameters. This 6.52 km 2D acquisition was laid out with a digital MEMS accelerometer: the DSU3-428 and the accompanying Sercel 428XL recording system; as well as an analog 3C geophone: the SM-7 high resolution geophone element placed in a modified PE-6/S nail type case co-developed by Sensor Nederland (A Division of ION Geophysical) and ARAM Systems with the accompanying ARAM Aries MC recording system. There have been limited acquisition comparison tests performed and/or published with MEMS accelerometers and analog geophones in the past; the purpose of this study is to compare data acquired with single-point 3C receivers laid out side-side in a commercial recording environment.

Design of a Low Power, Wireless, Passive Seismic Monitoring Instrument

Rural and remote passive seismic monitoring sites are often challenged by the lack of power and communication utilities. Sites without power lines require costly on-site generation (from diesel, gasoline, propane) or self-sustaining power sources (solar, wind). In all cases, electrical power is a key component in the capital and operating cost of surface-based monitoring systems. An instrument was designed for environmental and oilfield passive seismic monitoring with very low power consumption. The device was designed to use either a wired or wireless network link for data communication. Designed for sample rates up to 4 KHz, the instrument is ideally suited for connection to seismic-band sensors such as geophones or accelerometers. A low-noise, high-gain, programmable gain preamplifier provides the necessary amplification for the A/D converter. Several of these instruments have been installed at two remote sites using wireless data transmission. Running on solar power alone, the instruments have operated very successfully over the past two years. Reliability and the availability of remote diagnostic facilities are found to be key components in a successful remote passive seismic monitoring program.