In this article, we ask how the merging of the MEMS sensor with standard land seismic nodes combines the benefits of both technologies to form a unified solution for seismic acquisition.
Abstract
The all-digital MEMS (Micro-electromechanical system) based sensor designed for seismic acquisition was first introduced in the 1990s. Early implementations of the sensor showed great promise in ocean bottom cables where properties such as low-frequency sensitivity and tilt insensitivity were particularly advantageous.
Further implementation of the technology on land yielded three-component sensors (P and S wave) and finally single-component sensors (P wave) that are currently utilised on land production projects in several regions of the world. The seismic MEMS-based sensor has matured into a time-tested, highly refined tool for seismic exploration.
The next evolution is merging of the MEMS sensor with the standard land seismic nodes. Until recently, all seismic nodes have been implemented and deployed with analog sensors. However, nodes are currently getting deployed with MEMS-based sensors combining the benefits of both technologies to form a unified solution for seismic acquisition.
Fifth generation digital MEMS sensor
The MEMS sensor is etched from silicon wafers forming a tiny mass suspended with silicon springs that is approximately 1cm2.
Tolerances on the manufacture of the primary sensor component are so tight that Brownian motion of air particles becomes a concern requiring the manufacture to be completed in a vacuum.
Once the chip with the suspended mass is formed the MEMS sensor unit is completed with a tiny ASIC (Application specific integrated circuit) that controls the function of the MEMS chip.
In operation, the suspended mass is held stationary by the ASIC and the ratio of the hold charge is fed to the sigma-delta A-to-D converted at a fantastic rate of more than 150,000 updates per second. The result is an all-digital MEMS sensor that delivers acceleration data from 0-400Hz with a nearly flat amplitude and phase response and a dynamic range of 118dB at all frequencies.
The sensor is tilt insensitive, temperature tolerant, and due to a continual self-calibration function, it does not age or become worn with usage. These fundamental properties set the digital MEMS sensor apart from the traditional analog sensors.
The sensor has progressed through several generations.
The 5″ generation sensor has a lower noise floor and minimal power consumption making it ideal for all seismic experiments.
Self-calibration, which is an automated process that the MEMS sensor executes during power-up, re-establishes the precise function of the unit. This yields a sensor that responds to motion with a uniform result for every deployed sensor. This reduces sensor-to-sensor variation minimising data jitter observed with analog sensors resulting in the purest seismic response possible and eliminates sensor ageing which means the sensor responds with identical fidelity for each deployment.
Integration – Digital node
Along with the MEMS technology, the node technology has advanced along similar lines. Several generations of nodes have resulted in the single channel nodes that can be implemented with any analog sensor type with the correct form factor.
As an example, and not plugging it of course!. INOVA’s Quantum node technology is a single channel, all in one recording system that stores data continuously, has precision timing and self-locates, making pre-survey of projects increasingly obsolete. The form factor of the node optimises deployment and recovery due to its compact size and shape.
The node can be planted by field personnel the same way a traditional geophone is planted which assures the best possible coupling and alignment with the vertical orientation. In the case of Quantum, the node requires no additional electronics for deployment, once the node is placed vertically in the ground the node becomes active and begins recording seismic data. The node automatically performs self-check routines at predetermined intervals and can report status at the discretion of the operating crew.
The node technology is no longer a blind acquisition technology?. The Quantum node utilises low power long range radio technology (LORA), which can transmit status data to a central location continuously while recording at ranges of 5 kilometres depending on the terrain and vegetation. Previous versions of nodes were limited to mesh technologies or limited range communication technologies such as Bluetooth. The unique capability built into the Quantum node to monitor quality utilises a Low Power Wide Area Network (LPWAN) technology allowing tens of thousands of seismic nodes to broadcast status updates continuously with minimal additional power consumption.
The status updates can be captured, recorded, and displayed with various methods including handheld, vehicle-mounted and airborne devices. Since LPWAN enables long-range communications, large areas of deployed nodes can be managed with less field effort. In some areas drones have proven to be highly effective and further reduce the exposure of field personnel.
Monitoring large areas of MEMS-enabled digital nodes enhances the command andcontrol of the operational aspect of seismic projects, further benefiting the contractors and clients.
Combining the state-of-the-art Quantum node with the 5th generation MEMS sensor technology is yielding a next generation node product which will change the strategies currently used for seismic acquisition.
Benefits – All digital node
The benefits of the MEMS-based Quantum node are profound.
The node technology has matured to the point that most regions of the world are node-based operations. Crews in an open desert environment where exceptionally large projects are executed have realised cost benefits with nodes greater than a 13% reduction in the overall cost of operations, and up to 70% reduction in field resources such as personnel and vehicle requirements.
This is because nodes are smaller, lighter, and easier to deploy.
They require fewer personnel on the ground, as well as smaller and fewer vehicles. Nodes eliminate cable handling issues and eliminate the need for managing quantities of large batteries that further reduces QHSE concerns and operational complications.
The simplicity of node operations has motivated operators to begin using innovative geometry deployments that create operations that are source restricted rather than equipment restricted meaning that the project can be completed as fast as shooting can be accomplished. The latest generations of the MEMS sensor use minimal power and can be deployed in a node recording continuously for 50 days or more using duty cycles. Due to the combination of the tilt insensitivity of the MEMS sensor and deployment optimisation features built into the Quantum node, the vertical alignment of the node and sensor is improved, further enhancing the uniformity of large deployments and the response to seismic data.
Combining the benefits of the MEMS-based sensor with the successful node technology further enhances the benefits of the nodal operations. MEMS-based sensors have no spurious frequencies that are usual with analog sensors, and no amplitude distortions across the useful seismic bandwidth. The MEMS sensor is a true broadband sensor in every respect. These properties simplify the initial data processing steps by eliminating the need for geophone response corrections that are required with analog sensor data. The resultant seismic data derived from MEMS based sensors has acceleration units as its native format.
No special processing of acceleration data is required unless the end user requires the data to be converted to a different domain for data matching purposes.
The seismic data shown in Figure 3 illustrates the subtle advantages derived from MEMS sensors. Both sensor types were recorded simultaneously with a single source type. On the left side of the picture is the result from a single component high sensitivity geophone and on the right side of the illustration is the result from a MEMS-based sensor.
The processing steps for each section were identical with the exception of a 90-degree phase shift of the MEMS data so that the phase would match the analog result. It can be noted that the MEMS-based sensor produced slightly better amplitudes and frequency response when compared with the result of the analog geophone. The data benefits of the true broadband sensor are realised in processed data. Unprocessed MEMS data will commonly appear to have more noise than unprocessed analog data.
This is partly because the MEMS-based sensor does not discriminate any frequency response, particularly with the lowest frequencies. While analog sensors have a 12dB/octave amplitude response reduction below the resonant frequency, the MEMS sensor does not, and will record the lowest frequencies, below SHz with full fidelity.
Historically, nodes and MEMS sensors have been compared with analog sensors deployed in arrays. An array response creates a spatial filter that effectively limits higher frequencies greater than 100Hz regardless of noise or signal. With current trends in most projects implementing receiver group spacings as short as 25 m or less, any benefits realised by an array need to be carefully weighed against the benefits of high-fidelity broadband recording that does not limit the frequency response and that further results in the dramatic operational advantages inherent in node operations combined with the MEMS technology.
Conclusions:
Merging the latest seismic node with the 5-generation MEMS sensor delivers a node which is the result of two mature technologies that benefit from both in a product with proven operational and economic benefits. The MEMS sensor further enhances the all-digital node with true broadband recording which overcomes the drawbacks of traditional analog sensors.
References
Criss, C.J.[2022]. Digital MEMS and Seismic Nodes – Technology Fusion, Asia Petroleum Geoscience Conference and Exhibition (APGCE), Nov 2022, Volume 2022, p 1-5.
[2001]. MEMS for geophysicists, SEG Technical Program Expanded Abstracts 2001. January 2001, 21-24.
