Description of a GSN Station

The Science Plan for New Global Seismographic Network (IRIS, 1984) documented the need for a broadband station with certain characteristics. This issue was followed up in greater detail in the Design Goals for GSN (IRIS, 1985). Both documents had a distribution of about 500 copies, roughly half sent to scientists outside the United States. In the spring of 1986 these documents were translated into design specifications and submitted to IRIS's administration.

In April 1987, following appropriate procurement procedures, IRIS entered into a contractural agreement with Gould, Inc., for construction of a prototype station processor. The choice of the architecture of the system was guided by several considerations:

1. Meeting of the scientific objectives

2. Reliability and ease of maintenance

3. Modularity and minimal dependence on unique products

4. Feasibility of future upgrades and modifications of the chosen elements of the system

5. Scientific and operational benefits for the host organization

Figure 6 is a block diagram of the system. The two principal components are the data-acquisition unit (DA) and the data-processing unit (DP). They are connected by a serial link or any form of telemetry, from telephone lines to satellites. The following is a brief description of the major features of a GSN station.

Data-Acquisition (DA) Module

The DA is the heart of the system. With only minor software modifications, relative to the agreed-upon specifications, and the addition of a tape recorded, this unit would be, in principle, capable of fulfilling the principal objective of the entire system, namely, the acquisition and recording of data.

The major components of the DA system are:

1. A very broadband seismic sensor system. The STS-IV(H)/VBB, built by Streckeisen & Co., is a set of sensors that satisfy the IRIS design goals for installation in a vault.

2. A high-resolution digitizer/calibrator unit (HRDCU). This is a three-channel 24-bit formatted digitizer/calibrator for use with the VBB sensors. Its data rate is twenty samples per second (sps) per channel.

Figure 6
Block diagram of the prototype GSN station. From report U87-260 by Gould Inc.

3. Kinemetrics OMEGA UTC time receiver.

4. Auxiliary digitizer unit (AUXDU). This is for monitoring various state-of-health parameters of the VBB system and, optionally, other parameters such as atmospheric pressure or geomagnetic field. Data rates are programmable.

5. Optional low-resolution digitizer-calibrator unit (LRDCU). This is for digitizing the output of additional sensors.

6. Optional very-short-period (VSP) sensor system. "Design Goals" suggested a sampling rate of 100 sps. Consideration is being given to increasing it up to 200 sps.

7. Low-gain (LG) sensor subsystem. This was originally thought of as a means to extend the dynamic range of the VBB system and could serve as a strong-motion monitor. Its sampling rate is the same as that of the VSP system. There could be a limitation on the aggregate data rate of the VSP and LG channels (about 600 sps total—all sensors and all components).

8. Data-acquisition unit (DA). This is based on a 68000 or 68020 CPU with a VME bus, operating under an OS-9 operating system and special application software. The DA processor interfaces with the three GSN digitizers (HRDCU, LRDCU, and AUXDU) and with the data-processor unit (DP). All interfaces are bi-directional. The major data-manipulation functions of the DA processor are to collect data from the three digitizers, time tag data blocks, compress VBB data, perform VSP and LG event detection, and transmit data to the DP.

Data-Processor (DP) Module

The DP module provides the means for on-site and remote monitoring of system performance, communications with remote users, digital and analog recording of data, maintenance of data buffers, display of data for a selected time window on a graphic terminal, and many other functions. Its principal role is the enhancement of the functions of the DA unit, on-site quality control, and providing station personnel and remote users with information on seismic events.

The station DP consists of commercially available hardware modules and is based on a standard VME bus. Figure 7 gives some details of the DP configuration. The computing power of the 68020 chip is such that only a small fraction of its capacity will be used for the planned tasks, which leaves much room for expansion. Even now it is a fairly complex, sophisticated system.

Figure 8 is a block diagram of the DP unit's data-manipulation functions. The VBB data are decompressed so that they can be processed through a series of digital filters that produce short-period (SP), long-period (LP; 1 sps), and very-long-period (VLP; 0.1 sps) data. The LP and VLP data

Figure 7
Configuration of the GSN station data processor (DP). From report U87-260 by Gould Inc.

Figure 8
Data processing functions carried out by the DP unit. U87-260 by Gould Inc.

streams are obtained using finite-impulse-response (FIR) filters with a very sharp corner near the edge of the passband. This assures retention of the maximum information in the passband and may modify the way in which seismologists evaluate the usefulness of various data streams.

Figure 9 is taken from Steim (1986). It shows the original VBB channel and five channels derived from it. There is a twenty-second delay caused by a FIR filter. The trace labeled LP is obtained from the VBB stream using a 201–points FIR filter. It is clear that is contains most of the information present in the VBB stream, but with twenty times fewer samples. It very well may be that this will be the most frequently used data stream, particularly because of its relatively light archival burden. The other data streams shown contain less high-frequency energy, with the simulated SRO-LP channel representing an extreme. The VBB response is particularly convenient for a stable recovery of the ground displacement function (bottom trace).

The command, control, and algorithmic functions executed by DP are shown in figure 10. Rather than discuss them in detail, let us list functions that the operator can execute from the system console without interrupting acquisition of data:

1. Adjust the scale and select active analog monitor channels, including the selection of simulated WWSSN and Seismic Research Observatory (SRO) response functions.

2. View a continuously updated full-screen status display that shows a snapshot of all data channels, the internal and received UTC time, and several other system parameters.

3. View the system event log.

4. Change the tape cartridge without loss of data.

5. Examine the status of active processes.

6. View selected data waveforms from buffers or in real time.

7. Set, change, or display event-detection parameters.

8. Exchange message text over the real-time and dial-up ports.

Figure 9
The information content of the "broadband" 1-Hz LP data stream, obtained by processing VBB data
with a FIR filter with a very sharp roll-off near the edge of the passband. The four lower traces,
showing alternative responses, are derived from the LP data stream. The 20-s delay is due to the
finite length of the FIR filter. From Steim (1986).

Figure 10
Command, control, and diagnostic function structure within the
data-processor (DP) unit. From Gould Inc. report U87-260.

9. Control and set up a calibration cycle or program the onset time of a calibration sequence to be recorded with the station data.

10. Run a calibration analysis as a low-priority background job.

11. Log messages and the results of calibration to the system's mass-storage device.

Many of the functions described above can be performed through a dial-up port, which allows for frequent checks of station performance from the network maintenance center.

It is expected that a prototype of the GSN system will be delivered in the fall of 1988 and, following tests, production units will be ordered. The first ten, budget permitting, will be deployed in 1989.