J. Morales , G. Alguacil, J.B. Martin, A. Martos

Instituto Andaluz de Geofisica, Universidad de Granada, Campus Universitario de Cartuja s/n. 18071-Granada, Spain

Introduction

Since 1902, when several mechanical seismometers (pendula Stiattessi and Vicentini) were installed in Cartuja Observatory (now Instituto Andaluz de Geofísica-Universidad de Granada, IAG-UGR), the seismic instrumentation has been the focus of a permanent development. In 1983, the IAG-UGR started to operate a high-gain local short period network. In fact this was the first local radio-telemetered permanent seismic network installed in the Iberian Peninsula (Alguacil, 1986). The principal goals were the monitoring and recording of the microseismicity activity in the Granada depression (Southern Spain) and surrounding areas, and to overcome the lack of microseismic information in southern Spain due to scarcity and low-gain of seismic instrumentation. Until 1989, when the network was changed over to digital recording, it consisted of seven vertical short-period stations with central visual recording on paper. The network was increasing until 1995, up to a configuration with 13 short period seismometers, deployed around Granada depression and Almería province –these latter with central recording at Almería, with analogue transmission and digital recording in a central data acquisition system based on a PC-on board card system (Alguacil et al., 1990).

By the end of the 90’s, 4 broad band (BB) stations (STS-2) were installed in southern Spain by the IAG-UGR: SELV, VELZ, ANER and ORGV (last one no longer operating) in order to improve the dynamic range and bandwidth of the data. At a first stage, the acquisition system was DOS based, with a digitiser of effective resolution 18 bits, which required a manual interrogation via modem with proprietary software. The exception was station SELV (the first BB station installed) which was equipped with a Quanterra 360 datalogger with a dial up interrogation mode under TCP/IP. After 2000 a new project of mixed seismic network including short-period high-gain and broad band seismometers was planned and progressively deployed in Southern Spain. The main goal of the project was to extend the coverage and resolution of the network in order to be able to record on-scale small and moderate earthquakes generated in the Iberian-Magreb plate contact, with homogeneous instrumentation. We want with this article give an overview of this instrumentation.

IAG regional network. Current state, configuration and communication.

The present spatial configuration of the regional IAG broad band and short period network is shown in figure 1. The broad band network consists of a total of twelve seismic stations based on triaxial seismometers STS-2. An additional seismic station (ACBG) with a triaxial (To=5 s) seismometer (Lennartz LE-3D/5s ) is included in this net because it is using the same soft and hardware configuration except in the type of seismometer. The high resolution digitizer Earth Data 24 bits (PS2400) is time-synchronised with a Garmin GPS receiver. The digitiser is connected through a serial port to the datalogger, a SeisComP industrial PC from Alpha 2000 with a 30 Gbytes hard disk, where the sampled signal at 50 s.p.s is stored in half-hour ringbuffers, under the control of Seislog, a software package (Utheim and Havskov, 2001) installed in the stations and running under QNX operating system. Seislog writes continuous data files and by means of a simple STA/LTA detection algorithm also produces event files.

Figure 1. - Broad band and short period stations in southern Spain managed by the Instituto Andaluz de Geofisica. Red triangles are broad band stations except ACBG (see text) installed and operating. Yellow are planned in near future. Blue triangles represent short period seismic stations installed and operating.

The data communication between the central recording site (Instituto Andaluz de Geofisica) and the seismic stations is implemented in several ways, depending of the infrastructure available in each place. Digital telephone line is used for communication and interrogation in SELV and ANER at practical speed of 56 kbaud. GSM mobile communication for data transmission is used in VELZ, ESTP, ACLR, ASCB, JAND and ACBG (speed 9.6 kbaud). DSL line (presently 256 kbaud) is installed at SESP, ARAC, GORA and HORN and for CEUT we use the high speed intranet of the Granada University since this station is located at the Granada University Campus at Ceuta. The policy of the Institute for the future is to have all seismic stations linked by ADSL lines terrestrial or GPRS and to transfer continuous data in near real-time. Some of the main characteristics of the broad band stations are summarized in the table 1.

Table 1. - Broad band seismic station characteristics managed by the Instituto Andaluz de Geofisica - Universidad de Granada

click for larger table

* ED24= Earth Data 24 bits

** SCP= SeisComP with 30 Gbye H.D.

*** T= trigger segments. C= continuous data storage since January 2004; From installation date to January 2004 the data were storage by trigger.

Additionally, nine short period stations are still operating at present, after several stations of the original 1990’s network have been upgraded to broad-band (especially those in the Almería province). The remaining stations are situated at distances of up to 60 km from the central recording site at Cartuja Observatory in Granada. The sensors are vertical 1 s, except for station CRT, with triaxial, extended period up to 10 s. Data transfer uses real-time analogue radio telemetry. The signals are recorded continuously on drum paper and the declared events in digital form. The data is acquired by a PC based data logger that was home-designed. The upgrade plan includes field digitisers and digital telemetry, as well as continuous digital recording at the central recording site.

Network data collection

The Seisnet (Ottemöller and Havskov, 1999) package to combine seismic stations of various types with communication capabilities into a network is installed on a Sun work station under Solaris 8.0 platform. The main routines carried out by Seisnet are transfer of parametric data, network event detection, transfer of waveform data and automatic determination of epicentre location and magnitude. The data are stored in a central Seisan database.

Currently this system collects data automatically from the BB and SP networks and the PDE (USGS preliminary determination of recent epicentres) sources

The Seisnet process is started up every thirty minutes. It will then collect data from short period and BB’s with TCP/IP protocol (intranet or DSL facilities). Data from BB with modem (phone or GSM) are only collected regularly at night time in order to save communication costs, except in the case of an important local or regional event, in which case the data are gathered by the operator as soon as possible. For short period and broad band stations under dial-up protocol, only data segments (event data) are transferred. Broad band stations under TCP/IP protocol provide a continuous time series of half hour data blocks. To obtain and store continuous data for those broad band stations without TCP/IP protocol, every 3 to 4 months the stations are visited, checked and all the continuous data are recovered.

The Seisan package (Havskov and Ottemöller, 1999) is used for most of the routine data analysis at the IAG. This system organizes data from all kind of seismic stations into a simple database. Seisan comprises all the tools needed for routine processing, and also conversion to other standard seismic data formats to be used in external software. The Seisan database facilitates research tasks, currently focused on regional studies of earth structure and seismic sources.

Portable BB network

More recently, a broad band “portable” set with fifteen BB sensors, was designed to be operative in the IAG for large scale experiments involving temporal seismic networks. This portable network is based on the same specifications that the permanent network: STS-2 triaxial seismometers, 24-bits Earth Data PS2400 digitisers with GPS receiver and a PC based data logger, except the remote communication facilities.

Conclusions

At present, waveforms collected by the broad band, both permanent and portable networks, together the short period net are used in several projects studying seismic sources (IAG-regional moment tensor for the Ibero-Maghrebian region; Stich et al. 2003a,b Stich 2006) and the velocity structure below southern Spain.

Acknowledgments

We received financial support by the Spanish DGI project CGL2005-04541-C03-01-BTE, FEDER founds and within the Research Group RNM#104 of Junta de Andalucía.

References

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