The all three-component broadband seismic array ARCES/PS28 M. Roth, J. Fyen, T. Kværna, S. J. Gibbons

NORSAR P.O. Box 53, N-2027 Norway. Contact: michael@norsar.no

ARCES/PS28 • Constructed 1987

• Certified 2001

• Recapitalized 2014/2015

• 3 km diameter

• 25 sites

• 25 broadband seismometers

with hybrid response

1 (360s – 50Hz) 3C at central site

24 (120s – 50Hz) 3C sensors

• 150 channels (40Hz and 80Hz)

• 4 experimental infrasound sites

• Central timing

• FO intra-array communication

• VSAT, BB over satellite and

GSM communication

3 km

NORSAR operates 10 permanent stations and arrays: 4 non-IMS installations

(TROLL (Antarctica), AKN, JETT, NORES seismic and infrasound array) and the

IMS-stations IS37, RN49, JMIC/AS73, SPITS/AS72, NOA/PS27 and ARCES/PS28.

ARCES had been build in 1987 and it has been providing continuous seismic data to

NORSAR since then. ARCES had a major upgrade in 1999 and was certified

November 8, 2001. After 15 years of operation ARCES was due for a

recapitalization in 2014/2015.. The recapitalization comprises the replacement of

sensors, digitizers, central acquisition system (done in 2014), a new power backup

system and the refurbishment of central recording facility (to be completed in 2015).

We received the first 2 ARCES prototype sensors in April 2013. Through long-term testing in our facility we confirmed that the

instruments were build according to the specifications and that the sensors performed reliable. Only then the remaining 28

seismometers went into production and were delivered in May-July 2014. For all sensors we checked the mass position drift

and performed electronic calibrations to verify the system response. We compared power spectral densities and waveforms

collected in huddle tests in order to determine sensor noise, sensor homogeneity and signal coherency.

The seismometer pits at ARCES are quite small and shallow. To accommodate the size we designed our own pit box

integrating the fiber optic converters and power distribution and mounted the box to the digitizer. Spare units are prepared in

the same way, so that we easily can exchange the entire unit in case of a failure.

The central timing system was newly developed by Guralp. It consists of a voltage controlled crystal oscillator trained to GPS

and a chip scale atomic clock for free running periods. We tested the first prototype for a time period of about half a year and

after several iterations of firmware and hardware modifications the system was ready for deployment.

At the end of the testing period digitizer and sensors were paired for deployment in the individual pits. Channel naming, data

transmission protocols and individual system responses were in place before the field installation.

Old and current system responses at ARCES. Until 2014 ARCES had 25 short-

period (SP) sites equipped with GS13 seismometers. Four of the SP sites were

three-component. From 1987 to 1999 the central site had additionally a

KS36000 3C seismometer with long-period (LP) and intermediate-period (IP)

output. In 1999 all Sandia digitizers were replaced with Nanometrics HRD24

and the sensor at the central site was replaced with a Guralp 3T seismometer.

In 2008 the digitizer at the central site was replaced by a Guralp DM24. In

2014 we removed all pit equipment and installed Guralp 3T with hybrid

response and Guralp EAM digitizers. A broadband sensor with hybrid response

provides the possibility to monitor both high-frequency seismic events as well

as low-frequency. Compared to a velocity-proportional sensor the hybrid

response allows to monitor high frequency-signal (e.g. explosions) with high

sensitivity and more low-frequency events (e.g. regional and global

earthquakes) without the risk of clipping.

All sites at ARCES are shallow pits standing directly on the bedrock. They are covered with moss and have additional insulation of the lid. Each pit has its own

power and fiber optic cable connected to the central recording facility (CRF). The power cable has a coarse lightning protection outside the pit and an additional

fine lightning protection inside the power pitbox. Since all sites got 3C instruments, it was necessary to orient 21 of the 25 pits. For this purpose we had the

opportunity use a TOGS (Tiny Optical Gyro System) compass, that was provided to us by the IMS Engineering and Development section. The TOGS compass

was extremely useful – it has a low weight (we walked a total of about 35 km to reach all sites) and we could determine the accurate (<1deg) geographical

North direction within 10-15 minutes.

In the 2014 campaign we installed two new communication systems, i.e. cellular network and broadband over satellite, in addition to the existing VSAT. The

main task in the CRF was to replace the complete central acquisition system, i.e. central timing, media converter and acquisition computers. All the components

are currently on a mobile rack in order to facilitate the structural refurbishment of the CRF (floors, walls, roof, etc.). In order to minimize downtime of the array we

had set up and tested beforehand data acquisition and transmission in the individual new digitizers and the acquisition computers. We operated old and new

system in parallel as much as possible. The installation of the 25 sites took 3 days and during this time period we always had more than 15 sites operational.

Except for a one-hour time window, when we rerouted the power cables from the UPS system, ARCES was operational.

Even though ARCES has shallow

pits, the array has low ambient

noise over the entire frequency

range. Due to the remote place

there is hardly any man-made

noise in the high-frequency range.

At the low frequency range, the

vertical as well as the horizontal

components are well below of the

upper limit of the Peterson New

Low Noise Model.

744 hourly power density spectra (blue) and the monthly average (red) of

central site ARA0 for January 2015.