Technical Details on the GSC's Internet Accelerograph

Tests

Prototypes of the Internet Accelerograph were extensively tested on shake-tables at Zentrum für Angewandte Raumfahrt und Mikrogravitation (ZARM) in Germany, the Department of Civil Engineering at the University of British Columbia (UBC) in Vancouver, BC, and the Institut de Recherche d'Hydro-Quebec (IREQ) in Varennes, Quebec ¹.

Noise tests were conducted at the Pacific Geoscience Centre in Sidney, BC. For this purpose, instruments were installed in the seismic vault, which is build on bedrock, and data recorded during a seismically quiet period were analyzed.

Time synchronization is maintained by means of the Internet Network Time

Protocol (NTP). The internal clock is thus continuously synchronized to highly accurate time sources, the residual error is typically smaller than 10 ms.

Deployment and Networking

All instruments installed so far are on "continuous feed" high speed Digital Subscriber Line (DSL), TV-cable or direct links to the Internet in schools, offices and warehouses of private companies, churches, hospitals, and in private residences. Due to the extremely low network traffic and uncomplicated integration into a wide variety of Local Area Network configurations, our co-operating partners were, without exception, willing to donate the use of their network resources. No computer network expertise is required from our partners, the instrument adapts to their networks rather than requiring changes in the local network configuration.

The instrument can operate from a private IP-address range and a Network Address Translating (NAT) router as well as from behind a traffic controlling Firewall. It supports static and dynamic IP address assignments. The only requirements are access to at least one Internet time-server and very limited access from the instrument out to the Internet and into one particular host computer at the Pacific Geoscience Centre. Connections from a LAN out to the Internet are in general tolerated in even the most security sensitive environment and the instruments use an authenticated and encrypted channel for secure communications.

Maintenance

The procedures and configurations used to communicate with a network of instruments have so far proven to be robust and reliable. Our strong motion network communications survived the "MS-SQL worm" infection of late January 2003 with a quick recovery from Internet outages. The instruments can be controlled entirely over the Internet. Acquisition parameters can be adjusted, trigger-parameters can be changed and even part of the network environment can be re-configured remotely.

Unless some physical damage occurs, the instruments are expected to be virtually maintenance free. The backup battery will be replaced every 2 to 3 years. The instrument prototypes have now operated for over two years without interruption and interference in the seismic vault at the Pacific Geoscience Centre. The first production instrument was installed on November 20, 2002 in the City of Richmond. It has operated continuously since.

The system interfaces with a database containing site locations, contacts and instrument-related data. Status reports for each instrument are automatically generated daily and forwarded by e-mail to staff members. We also automatically generate a map displaying instrument locations and basic status information every hour for internal use.

Day-to-day Operations

During normal operation, each instrument acquires and stores 3-component DC to 42 Hz acceleration data in a ring-buffer with a size of about two and a half days worth of data in MiniSEED format. Data are assembled in 5-minute files, which can be retrieved from the instrument at any time. Data from all or a group of selected instruments can be retrieved almost instantaneously by a simple request to the network management system at Pacific Geoscience Centre in Sidney, British Columbia.

Data streams of continuous 0.1 Hz to 42 Hz acceleration, velocity, displacement and spectral intensity ² are computed by the instrument in real time.

When an event is detected, peak values from the data streams are determined for a certain configurable time interval.

These values are immediately reported as peak ground acceleration (PGA), velocity (PGV) displacement (PGD) and spectral intensity (SI) values in a brief message to a computer at the data-centre . They can then be forwarded by e-mail to a group of users or as a short message to a group of cell phones.

Work is progressing to use ground motion parameters reported by the instruments to generate a shake-map with reports from all stations in near real time ².

Instrument Specifications

• Sensor: Each axis has four ADXL105 iMeMS accelerometer chips connected in parallel (to reduce noise and enhance linearity)
• Full-scale: +/-3g (g = 980.6 cm/s2)
• Noise: 0.5mg RMS, DC to 42 Hz
• Bandwidth: DC to 42 Hz (set by 151-coefficient FIR filter)
• Sample Rate: 100 samples per second, optional 50, 60, 75, 150, 300.
• Total instrument delay: 685 ms due to a two-stage FIR filter implementation.
• Nonlinearity: 0.2% of full scale.
• Operating Temperature: -20 to +50 degrees C
• Temperature Sensitivity: less than +/-0.5% change over the operating temperature range.
• Cross-Axis Sensitivity: +/-1%
• Alignment Error: +/-1 degree
• Zero-Adjust: None needed, provided the instrument is installed level.
• Power Consumption: 8.1 W max at +9 V to +18 V DC, 5.7 W typical when the internal battery is fully charged.
• Built-in UPS: More than 6 hours run time with internal 6 V lead-acid battery.
• Computer: National Semiconductor Geode, 266 MHz, 128 Mb RAM. 32 Mb Flash disk, optional up to 512 Mb Flash disk.
• Operating System: Linux, Kernel 2.2.16
• Data Format: Standard: 5-minute miniSEED files, optional CNSN 5 minute CA data or ASCII data files.
• Timing Accuracy: +/-10 ms, assuming reasonable access to NTP-servers.
• Total Data Time Lag: 690+/-10 ms, due to FIR filter length.
• Data Retrieval: Via SCP, FTP or TFTP, the 80-MB RAM disk stores about 2.5 days of data.
• Internet security: password protection for FTP access, secure shell access for maintenance.
• Connectors: 10-pin bayonet connector, cable pig-tail with break-outs for RJ-45 and DC power supply, DB-9 serial provided on installation cable.
• Indicators: Heartbeat / diagnostic LED, Ethernet activity LED.
• Physical dimensions: ABS enclosure with aluminum lid, 22 cm x 18 cm x 11 cm high (8.5'' x 7'' x 4.25'').
• Mounting: Case is bolted to the basement floor through the central recess in the bottom of the enclosure.

References