Successful sensor installation is important, as it directly affects how the sensor will perform. If conducted incorrectly, it could seriously degrade the data received and collective system performance. This is especially pertinent for three-component (3C) seismic sensors that have the additional parameters of orientation and leveling in addition to the need to be well-coupled to the surrounding media.
Installations at several meters’ depth within a borehole are more challenging, since achieving coupling and orientation underground cannot be easily confirmed. Protecting the sensor cable and connection on the surface while working at several meters’ separation from the sensor in a borehole is an added challenge. Three-component seismic sensors are of interest, as they have additional advantages over their one-component counterparts because they sense motion in XYZ directions commonly used as vertical, east-west, and north-south directions; whereas one-component geophones sense only a single component, typically vertical motion.
Sensor installations need to be adapted to their specific geologic or environmental setting to ensure proper installation and optimal sensor performance. Installation procedures should be tested and evaluated for that specific environment prior to sensor deployment, especially when those sensors have a potential for long-term, several-year usage.
There is no commercially available installation tool for the specific 3C seismic sensor used in this test. An installation process and hardware were developed by Raytheon BBN Technologies for a previous government experiment to ensure that the sensor was oriented correctly and coupled to the surrounding rock at several meters’ depth. This research details the process used to install the sensors in a cohesive soil environment, which had never been tried before.
The target scenario for these sensors is that the source and receivers are in the near-surface (e.g. 100m or less) and the signals of interest are high-frequency (e.g. 10-500Hz). Near-surface maximum depth extent is not an absolute value definition that applies to any or all near-surface applications and the bottom depth of near-surface will vary depending on the focus of the target. The sensors are buried in the near-surface with the purpose of being closer to near-surface sources as well as minimizing surface noise; both aspects improve the overall signal-to-noise ratio.
High-frequency signals attenuate quickly with distance; therefore, sensor placement needs to be close to the target source in order to capture relevant signals. As the propagation path is in the near-surface, this installation method has an objective to not change or have minimum impact on the native conditions of the subsurface. This extends to, for soil settings, the sensor installation using native materials for coupling and backfilling and typically not using a permanent cased borehole or drill mud for installation. This style of installation has a tradeoff as once the sensor is buried (installed) there is a minimal surface footprint; however, it is not possible to recalibrate, easily recover, or repair a buried sensor.
The seismological community are the most common users of borehole installations for their instrumentation; it is for different reasons, however. For example, their objective sensor condition is a stable thermal and pressure environment that their sensors need for optimal operation and to escape surface noise. They utilize boreholes tens to hundreds of meters deep in bedrock to provide these conditions. In addition, the sources they are trying to locate (e.g. earthquakes) are at kilometers to global distances where they use low frequency signals (0.008 to 50Hz) with their sensor responses focused on low frequencies. As their end goals are different, their guidance is not used in this installation method as it is not immediately applicable to the objective sensor use and near-surface target scenario discussed in this report.
This work was done by Alanna P. Lester, Erin P. Simpson, Gabrielle J. Rigaud, and Jennifer R. Picucci for the Army Engineer Research and Development Center. ERDC-0004
This Brief includes a Technical Support Package (TSP).
Deep Installation Method for Three-Component Seismic Sensors
(reference ERDC-0004) is currently available for download from the TSP library.
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