Talking Sense About Sensors

INOVA Geophysical began exploring digital sensor technology with a solution-focused micro electrical mechanical systems (MEMS) seismic sensor chip in the late 1990s. Designed to overcome the limitations in sensor technology that are currently inherent in analog geophone sensors, the digital sensor helps to eliminate issues with broadband sensitivity and phase. Since the introduction of the MEMS technology, two products – including Vectorseis, a 3C all-digital sensor, and Accuseis, a single-component digital sensor – have seen commercial successes on the market. The digital sensor technology is currently in its 5th generation of commercial products, and is continuing to evolve.

INOVA’s Quantum® nodal system was introduced to the market as a fully practical, standalone seismic node and offers advantageous operational properties, compact design, and reliability.

Quantum with Accuseis is the fusion of INOVA’s 5th generation MEMS technology and the Quantum node, creating a product with the benefits of both.

A foundation of research and field knowledge

The 5th generation Accuseis technology is based on a foundation of research and field experiences. The Accuseis sensor is formed from a sandwich of silicon wafers with a tiny mass suspended by etched silicon springs, forming a unit that is about 1 cm.2 The output of the sensor is a ratio of charges on either side of the suspended mass. Updating approximately 156 000 times/sec., the charge ratio is the source of the data stream for digitisation. This technology outputs units of gravity (g) with a very high degree of fidelity.

Jason Criss, INOVA Geophysical, USA, discusses the use of a combination of digital sensor technologies to meet the demands of challenging projects in regions around the world.

A secondary but important benefit is that the unit self-calibrates each time it powers up, and at random intervals while recording. This ensures that the sensor response is uniform, repeatable, and does not age with continued use. The sensor will respond identically during thousands of deployments throughout the product life. Since the silicon chip is tiny and very rigid, the sensor has a resonant frequency of approximately 2000 Hz, which results in amplitude and phase response curves that are above the useful frequency range for seismic data. In effect, the resultant phase and amplitude response of the sensor is nearly flat. Functioning in gravity units from 0 to 400 Hz with a nearly flat phase and amplitude response results in a truly broadband sensor that can overcome the historical limitations of analog geophone technology.

Since the introduction of the digital sensor in the late 1990s, many projects have been implemented with this technology. Originally marketed in a 3C sensor called Vectorseis, the units have successfully recorded data in every environment, with both 3C sensors and 1C sensors. Projects with over 180 000 live channels have been successfully completed with Accuseis sensors.

A data comparison from North Africa demonstrates the advantages of the technology. In this example, a 2D line was recorded with coincident sensors, including single high-sensitivity geophones and the Accuseis sensor. The 2D line was shot and recorded into two systems simultaneously. This set-up eliminates environmental issues and potential source variances, and provides data sets that can be utilised to compare sensor responses (Figure 2). In this case, the Accuseis sensor produced

a stronger amplitude response when compared with the analog 5 Hz high-sensitivity geophone.

In a second data example (Figure 3), the Accuseis data is compared with the results of legacy seismic data. In this example showing fully-processed data, the reshoot using the digital sensor resulted in higher resolution and greater fidelity.

The Quantum node is under improvement through the development of new features, utilising high-sensitivity geophones as the initial sensor technology. The original Quantum node is a small 650 g full-featured recording node that records seismic data continuously for up to 50 days with a high-fidelity, industry-standard, dynamic range and sensitivity. A new development, HyperQ, utilises a low-power, long-range wireless technology to transmit data over long distances. The technology enhances the original product by allowing the status of deployed units to be monitored at ranges of as far as several kilometers, depending on terrain and the antenna height.

The technology helps makes seismic projects more efficient and operationally-flexible, which in turn makes them more environmentally-secure, profitable and safer to operate. The addition of the Accuseis digital sensor as a replacement for the geophone results in a recording system with strong operational properties and a true broadband sensor.

Figure 1. Accuseis sensor.

Figure 2. Stack sections from the 2D line show differences between the Accuseis sensor response (top) and the 5 Hz high-sensitivity analog geophone response (bottom).

Figure 3. Processed data comparing Accuseis sensor data against legacy data from the same location. Legacy data on the left and Accuseis data on the right geophone response.

Flexibility and feasibility

Areas with diverse topography, and zones ranging from urban environments and dense jungles, to shallow swamps and arid deserts, create tremendous operational challenges for seismic crews. The flexible technology can be configured and deployed to acquire data for many weeks, using duty cycling which turns nodes off during periods of no shooting activity and then begins recording at preconfigured times. This capability means that crews can work with a two-touch strategy, where the receiver station is visited once for deployment, and a second time for retrieval once the shooting has progressed past the area. This simplifies operations and the environmental impact, and is a key feature when analysing permitting issues.

Combined with HyperQ technology, the deployed Quantum nodes can be scanned for daily quality control through telemetry. Since the HyperQ technology is effective for use across long ranges, quality control can be accomplished with a variety of strategies including fixed masts, drive-by utilising work vehicles, and fly-by utilising drones. Drone QC has been proven effective in many regions, and is a growing technology that is ideal for this purpose, especially when ground access is restricted by wet areas and permit issues.

Furthermore, Quantum can be configured to work with a marsh geophone. This delivers a solution for areas where water is expected to cover zones of the project. When set up on purpose-built floats, marsh geophones are deployed in the soil at the water bottom, while the Quantum units remain active and record above water. This capability allows crews to maintain a consistent system type with a flexible and adaptable geometry that makes the Quantum solution a feasible and optimal one for challenging projects. The technology has made possible the acquisition of data in challenging environments around the world.

Technology fusion

The fusion of the Quantum technology with the Accuseis technology expands the capability of the system by providing a broader spectrum of user choices and solutions for deploying seismic equipment in any type of landscape. The technology is an effective solution in terrain types ranging from deserts with unrestricted access, to areas with dense jungles. The Accuseis technology is a seismic solution that helps overcome the technical disadvantages of the geophone, and is a truly broadband solution with a uniform phase and amplitude response. These technologies can help to expand and meet the needs of challenging projects in many regions.