In this article we explain how an application based on the SensorTile.box has been able to detect an earthquake of magnitude 4.3 on Mount Etna in December 23, 2021.
Article created by Rosario CataniaTable of Contents
In this work we compare different sensors, built with different technologies but located in the same site, which acquire data 24 hours a day of some physical parameters, to test the accuracy, reliability and quality of MEMS sensors. These are compared because they are cheap, need very little energy, can be activated when needed, and offer the possibility of being used in the field with extreme simplicity thanks to smartphone applications. The MEMS capsules, integrated into the SensorTile.box, manufactured by STMicroelectronics, are used on the Etna volcano, which features an active geodynamic environment, perfect for testing the dynamic components of the sensor in response to geophysical stresses. During the test period, numerous events occurred on Etna between lava fountains (over 50 in the year 2021), which produced a drastic increase in volcanic tremor and a continuous vibration of the ground, and seismic swarms. An earthquake of magnitude 4.3 occurred on the southern outskirts of the volcano on December 23, 2021, and was detected by SensorTile.box for the first time.
Etna ash column (photo by the author)
Graph of volcanic tremor (source imagine INGV Osservatorio Etneo)
SensorTile.box (STEVAL-MKSBOX1V1), is a development board that integrates several sensors, including some MEMS, for the detection of rotations, accelerations, inclination variations, magnetic field vector and for the measurement of environmental physical parameters, such as temperature, humidity and pressure. But it also contains sensors for the acquisition of audio signals, thanks to miniaturized MEMS microphones. SensorTile.box is powered by a lithium polymer battery and is equipped with Bluetooth connection, micro USB port and slot for recording data to microSD card. The programming of SensorTile.box is very simple, thanks to the intuitive graphical interface of the "STBLESensors" application, downloadable from the Android and iOS app-stores. The firmware used in the test versions on Etna, has been programmed according to specifications suitable for use in the geophysical field. For the transfer of a signature on the SensorTile.box, please refer to the STSW-MKSBOX1_BL guide.
Etna has interesting geological elements to experiment with sensors, such as active seismic faults or the rim of craters. These are ground movements that can be detected by an accelerometer if it is sensitive enough. Even a volcanic eruption if preceded by a tremor that increases in intensity, causes a general movement of the soil, as well as an earthquake at the very moment in which it manifests itself. This is the scenario to which to apply the technology, taking the acceleration of the soil, as a useful parameter to intercept a movement; but in some cases, it is useful to measure the change in inclination of a surface or the rotation of a plane due to an angular force. In other cases, it may be useful to measure environmental parameters. Some sites used for preliminary tests were chosen in remote areas, where electricity is lacking, to test the possibility of mounting an inexpensive sensor that works for a long time on a battery. The biggest challenge for a remote recording unit is to operate for a long time on battery power alone, especially during the winter period with low temperatures and snow. Adopting the best strategy is what we are learning with testing. Knowing the SensorTile.box hardware thoroughly, it was possible to customize a firmware suitable for each scenario. For this purpose, it was decided to use a setup in which the gyroscope of the LSM6DSOX chip acts as a sentinel, keeping both the accelerometer and the microcontroller dormant. At the occurrence of an event of acceleration g of the ground, but exceeded a set threshold, the microcontroller is awakened that starts the data collection from the 3-axes of both the accelerometer and the gyroscope, and records them on the SD card for a sufficient time to the analysis,which is done a posteriori.
The underground geophone and the SensorTile.box resting on rigid plates (photo by the author)
In geophysics, the geophone is a device capable of measuring one or three axial components of a seismic event (earthquake) and the vertical one is among the most widespread. The one we use is of the ZF-4.5HZ capsule type which, although economical, is very sensitive and able to detect weak ground oscillations. Thanks to the high knowledge of the response of this capsule, the ZF-4.5HZ was taken as a reference for comparison with the MEMS sensor and was placed at its side. The two sensors, due to their different construction, have different sensitivities, but the presence of a vertical geophone can provide a further indication of an event, then crossing the data with those of the permanent stations of the National Institute of Geophysics and Volcanology, trying to understand if the same is of natural or artificial origin, often linked to human activities. It is also used to estimate the sensitivity level of the accelerometer, with a certain setup configuration. The big advantage of SensorTile.box is to have on board a data processing unit and a digital output with a ready-to-use file, while a geophone must be equipped with a low-noise amplifier and a signal acquisition and processing system for the production of the final data; in our case a sound card and a windows PC. During the earthquake of December 23, 2021, the geophone recorded the main event of magnitude 4.3 and the image attached here (obtained with the freeware software Spectrum Lab), shows the spectrum visible inside the yellow box below. There are also other footprints but of lesser intensity, being a seismic swarm that involved the area. The strongest event (Motta S. Anastasia) was analyzed.
Spectrogram developed with Spectrum Lab (source author's archive)
The earthquake under analysis occurred at 22:33 Italian time and was also felt by the population around the epicenter area. In the following days the SD card was analyzed, containing the recording files and among these, the one produced on the date / time of the seismic event was searched. Before starting the recording, SensorTile.box was synchronized, via Bluetooth, with the SMARTPHONE APP. The setup was the same as tested in the remote units, so we proceeded to process the data in graphic form. From this it emerged that the axes of the accelerometer and gyroscope contained in the LSM6DSOX chip, showed an evident imprint of the beginning of vibration, which detaches from the bottom, more clearly on the Z axis, which represents the vertical axis of acceleration, therefore comparable with the vertical capsule geophone. SensorTile.box during installation was placed parallel to the ground, with orientation of the X>EST, Y>NORD axes, to have the correct information on the position of the sensor in space.
After a first phase of remote testing, we are developing a new application of SensorTile.box as a high-frequency vibrometer, where the system is always in acquisition with the accelerometer at 6.6KHz, but powered by solar panel, storage system, and recording on the hard-disk of a windows PC. The higher frequency and the continuous acquisition will allow us to be faster and more precise when an event occurs, although less performing in costs and consumption. The goal is to know the behavior of the hardware exposed to all the possible cases defined by the scenario and find the best setup for programming the remote unit.
(source author's archive)
The LSM6DSOX sensor included in the SensorTile.box that is being tested experiment ally on Etna, recorded a vibration during the main event of a seismic swarm, applying the sampling specifications that the research team has designed for application in the field of geophysics and that it has programmed in the custom firmware. It is a concrete case of automatic recording, of a seismic event on Etna, obtained with a development kit, designed for other uses, but which offers, thanks to its wide flexibility, interesting technological developments. Geophones are confirmed as valid and sensitive working tools for the capture of even weak seismic events, but new silicon technologies, such as MEMS mechanisms, represent a new frontier, and given their low cost and low energy consumption, they can be used in remote places that are difficult to access, such as on Etna or other volcanoes. They can be used to monitor the movement of active faults or to collect environmental data inside caves or collect vibration data on the edge of active craters. SensorTile.box offers interesting development margins, thanks to the presence of other accelerometers inside it such as tri-axis (LIS2DW12 and LIS3DHH). It is an example of how technology can stimulate ideas and projects, and how these, in turn, can develop new technology.