# Evaluation of the new infrastructure for French Permanent Broadband Stations in Auvergne

Jean-Michel Douchain, Edouard Regis

► To cite this version:

Jean-Michel Douchain, Edouard Regis. Evaluation of the new infrastructure for French Permanent Broadband Stations in Auvergne. 2017 AGU FALL MEETING, Dec 2017, Nouvelle Orléans, United States. hal-03116808

HAL Id: hal-03116808

https://uca.hal.science/hal-03116808

Submitted on 20 Jan 2021

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Distributed under a Creative Commons Attribution 4.0 International License
## National/Régional Network :

French seismologic and geodetic network (RESIF) is a national equipment for the observation and understanding of the solid Earth. It is an instrument aimed at acquiring new top-quality data for disciplines like seismology, geodesy and gravimetry to advance the understanding of the dynamics of our planet.

The seismology component of RESIF, with its **homogeneous coverage** of the territory, will allow **better localization** and characterization of seismic activity over a wide range of magnitudes as well as provide **high quality data** for research.

The Auvergne Seismic Network (ASN) manages seismic stations in the center of France since the beginning of the 80's and continuously upgrades them. Nowadays, 21 stations are deployed to monitor the Massif Central seismic activity. In the future, the ASN will include 15 broadband stations that will be part of RESIF network. In June 2017,

4 permanent seismic stations have been upgraded to the new standard installation type for open environment. The chosen infrastructure is a 5 meter drilling equipped with a posthole broadband sensor. Prior to these final installations, on each site, Trillium 120 PA have been installed for 2 temporary experiments.

<figure><img src="image_2.png" /></figure>

## Data processing :

We present comparative noise performances between the three types of installations calculated for vertical (Z) and horizontal (N) components. Experiments do not overlap.

We compare probability density functions to evaluate noise levels. Spectral Power Density (PSD) uses the MacNamara and Buland method (2004). Solid lines, in the figure below, represent the median value of PSDs and dotted lines, the 10th and 90th percentiles. The shaded lines describe Peterson's New High Noise Model and New Low Noise Model (1993). Number of triggers per hour are given as an indicator of the presence of HF perturbations. We also present spectrograms to illustrate noise spectral content during windy days.

## Installations :

### Direct Burial

<figure><img src="image_3.png" /></figure>

Prospecting sites. Duration : 3 weeks to 1 month

<figure><img src="image_4.png" /></figure>

### Seismic Vault

<figure><img src="image_5.png" /></figure>

Temporary Instrumentation. Duration : 6 months

<figure><img src="image_6.png" /></figure>

### Posthole

<figure><img src="image_7.png" /></figure>

Permanent Installation,
datas since 3 months

<figure><img src="image_8.png" /></figure>

**Electrical cabinet :**

- Power supply

- 2 AGM batteries

- Communications : ADSL

- Digitizer : Centaur 6

- Sensor : Trillium 120PH

<figure><img src="image_9.png" /></figure>

## Spectrograms :

A windy day (2017.11.22- Max speed : 72km/h at 8:00 pm) was selected to compare spectrograms between 3 RESIF stations and two reference stations such as SSB and CFF. SSB is part of Geoscope Global Network and located in the countryside, in a granite tunnel. It is a reference station for very low seismic noise. CFF signal is very noisy at HF because, it is located in the suburbs of Clermont-Ferrand (250 000 inhabitants). Both equipments are Q330 coupled with STS-2 sensors. Spectrograms show urban noise on posthole stations only at low amplitudes. Despite the presence of :

- a 30m high mast near PLDF,

- wind generators 3.5 km away from LBL,

- surface fences at all stations,

posthole stations offer a great signal to noise ratio to locate events.

PLDF is near a 30m high mast and its oscillations (~1Hz) are not observed on PSD. The drilling allows the installation of the sensor inside dense rock, improving signal quality :

- HF : ~5dB on Z and H sensors

- LP : 10dB on Z sensor, 20dB on H sensors.

Due to the presence of hard bedrock at shallow depth, the seismic vault could not be digged deep enough. This explains the high detection rate. With the 5m drilling, the detection number stays high. Seismic signals aren't affected by wind turbines 3.5 km away.

Drilling improves COLF station signal :

- Vertical : HF : 5dB, LP : 10dB

- Horizontal : HF : 5dB, LP : 15-17dB.

<figure><img src="image_10.png" /></figure>

ObsPy
A Python framework for Seismology

## Conclusion :

<figure><img src="image_11.png" /></figure>

RESIF installations greatly
improve data quality.

<figure><img src="image_12.png" /></figure>

Auvergne installations on
shallow bedrock are well
isolated from anthropic noise.