# Testing various modes of installation for permanent broadband stations in open field environment

Jerôme Vergne, Olivier Charade, Benoît Arnold, Thierry Louis-Xavier

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## To cite this version:

Jerôme Vergne, Olivier Charade, Benoît Arnold, Thierry Louis-Xavier. Testing various modes of installation for permanent broadband stations in open field environment. European Geosciences Union General Assembly 2014, Apr 2014, Vienne, Austria., 2014. [hal-02271689](https://hal.science/hal-02271689)

HAL Id: hal-02271689

https://hal.science/hal-02271689

Submitted on 27 Aug 2019

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Distributed under a Creative Commons Attribution 4.0 International License
# Testing various modes of installation for permanent broadband stations in open field environment

Jérôme Vergne<fnref n="1" />, Olivier Charade<fnref n="2" />, Benoît Arnold<fnref n="2" />, Thierry Louis-Xavier<fnref n="2" />

## Abstract

In the framework of the RESIF (Réseau Sismologique et géodésique Français) project, we plan to install more than one hundred new permanent broadband stations in metropolitan France within the next 6 years. Whenever possible, the sensors will be installed in natural or artificial underground cavities that provide a stable thermal environment. However such places do not exist everywhere and we expect that about **half the future stations will have to be set up in open fields**. For such sites, we are thus looking for a standard model of hosting infrastructure for the sensors that would be easily replicated and would provide good noise level performances at long periods.

Since early 2013, we have been operating a **prototype station** at Clévilliers, a small location in the sedimentary Beauce plain, where we **test three kinds of buried seismic vaults and a downhole installation**. The cylindrical seismic vaults are 3m deep and 1m wide and only differ by the **type of coupling** between the casing and the concrete slab where we installed insulated Trillium T120PA seismometers. The downhole installation consists in a 3m deep well hosting a Trillium Posthole seismometer. For reference, another sensor has been installed in a ~50cm deep hole, similarly to the way we test every new potential site.

Here we **compare the noise level in each infrastructure** at different frequencies. We observe quite similar performances for the vertical component recorded in the different wells. Conversely, the **noise levels on the horizontal components** at periods greater than 10s vary by more than **20dB** depending on the installation condition. The best results are obtained in the completely decoupled vault and for the downhole setting, both showing performances comparable to some of our permanent stations installed in tunnels. The amplitude of the horizontal noise also appears to be **highly correlated to wind speed** recorded on site, even at long periods. The variable response of each vault to such external forcing can partly explain the variations of the seismic noise levels.

## 1. Noise power spectral densities differ at long period

<figure><img src="image_3.png" /><figcaption>**Figure 1:** a) Spectrogram of the North component data in each vault obtained by appending hour-long Power Spectral Density (PSD) functions. For each complete hour of data, the PSD is computed using the method of McNamara and Buland (2004) averaging the acceleration energy spectra over one octave bandwidth. White vertical strips indicate a lack of data. Right insets represent the relative variations of the PSDs with the hour of the day and over the frequency band. A clear diurnal variation is observed except in the microseismic band. Anthropogenic origin of noise at high frequencies explains the decrease of energy around noon. At low frequencies, the increase of noise during the day is probably due to temperature or wind. b) Median (bold line), 10th and 90th percentiles (dashed and dotted lines respectively) of the PSDs for the three components in each well (color code on central figure) computed for data during the purple periods on the bottom timeline in (a). Gray lines indicate the New Low Noise Model and New High Noise Model of Peterson (1993).</figcaption></figure>

## Setting

The prototype site is located in the Beauce plain, ~500m north of the small town of Clévilliers (Eure-et-Loir) and 14km from the major city of Chartres. The nearest highway is ~4km away. The rented 180m2 parcel is surrounded by farmed fields and protected by a 1m high fence. Near surface geology consists in arable soil and unconsolidated clastic sediments. Water table oscillates from 1 to 5m depth. The vaults and the borehole tube have been installed in 3m deep excavations, backfilled and covered with on-site soil and sand. Pressure, temperature and humidity sensors are installed within each vault and report stable conditions. All sensors are connected through buried cable paths to Q330 digitizers installed in a 1m tall outdoor box.

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**Reference 00**

- 50cm deep / 40cm wide

- HDPE casing

- Thin concrete slab coupled with casing

- T120 sensor with insulation cover

**Vault 01 coupled**

- 3m deep / 1m wide

- HDPE casing

- 20cm thick concrete slab coupled with casing

- T120 sensor with insulation cover

**Vault 02 decoupled**

- 3m deep / 1m wide

- HDPE casing

- 20cm thick concrete slab decoupled from casing

- T120 sensor with insulation cover

- Pump against floating

**Vault 03 sealed**

- 3m deep / 1m wide

- HDPE casing

- 20cm thick concrete slab separated from casing with EPDM seal

- T120 sensor with insulation cover

- Pump against floating

**Borehole 04**

- 3m deep / 21.5cm wide

- HDPE tube put in the excavation of vault 03

- Small concrete base coupled with tubing

- T120PH downhole sensor surrounded and covered with sand

## Summary

* Properly building the sensor's hosting infrastructure is crucial to record long period-low amplitude signals on the horizontal components

* Downhole installation and seismic vault with a decoupled concrete slab produce the best results

* Wavefield coherency between the wells strongly decreases for periods greater than 20s

* Site testing performed with a sensor buried at shallow depth provides a good estimate of the site's quality for the vertical component but overestimates the long period noise on the horizontal components

* Appart from the frequency bands of microseismic peaks and anthropic activities, background seismic noise amplitude is related to wind speed. Wind-induced tilting is observed at periods greater than 10s

McNamara, D., and R. Buland (2004), Ambient Noise Levels in the Continental United States, Bulletin of the Seismological Society of America 94, no. 4, p.1517-27
Peterson J. (1993), Observation and modeling of seismic background noise. U.S. Geological Survey Technical Report 93-322, p. 1-95

## 2. Waveform coherency decreases at short and long periods

<figure><img src="image_6.png" /><figcaption>**Figure 3.1:** Comparison between wind speed and temporal evolution of seismic-noise amplitude in the different wells. Wind data are recorded on site every 5 minutes with a ProVantage 2 meteorological station. a) Wind direction relative to north. b) Wind speed averaged every hour. c) Seismic noise amplitude on the north component at ~50s period. Colors refer to the different wells. Note the overall correlation with wind speed. Black arrows highlight M>6 teleseismic earthquakes.</figcaption></figure>

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## 3. Wind speed is highly correlated to seismic noise

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<footnote n="1">¹ Ecole et Observatoire des Sciences de la Terre, Institut de Physique du Globe, Strasbourg, France (jerome.vergne@unistra.fr)</footnote>

<footnote n="2">² Division Technique de l'INSU, CNRS, Meudon, France</footnote>