Instrumentation












Analog versus Digital Stations--What's the Difference?

Seismic stations measure the ground motion at a specific location. The data are converted from the analog signal produced from the ground motion into digital information that computers are capable of understanding and manipulating. The difference between networks with analog stations and networks with digital stations is the point at which the analog signal is converted into digital information.

Analog stations are called "analog" because the analog signal is converted into digital information at the site of data processing. This means that the analog signal must be sent, in this case over phone lines, from each station to the central site. Each station's signal is then converted from analog to digital by hardware and processed by computers.

Signals from analog stations go off-scale quickly because the electronics and analog phone lines have limited dynamic range. However, each analog station is somewhat simpler, the time stamping of the data is done simultaneously, and the data conversion hardware is at the central site, so the analog stations are somewhat easier to maintain.

Digital stations, on the other hand, have high and low gain sensors and do their data conversion at the sensing site itself with 24 bit digitizers, thus allowing both small and large signals to stay on scale. The digital information is then sent via digital data link to the central site where it is able to be used immediately by the computers processing and storing the data.

Using digital stations instead of analog stations provides several important benefits:

The high and low gain sensors provide data on scale for both small and large earthquakes.
The digital data can be
tadalafil error checked so that line noise won't cause the data to be corrupted.

Although the data output by different dataloggers is often of different formats, the network can incorporate them through simple software changes.

Seismic Station Configurations

TERRAscope
K2
Squash
STS1
STS2
Episensor
etc.

Communication Equipment & Requirements

TriNet has the following data communication needs:

It requires moderate amounts of bandwidth
It is geographically dispersed
It involves multiple locations
It involves real-time need for information
It requires group addressing capability
It may require communications to more than one central site
It may use LAN interconnection in future applications

Frame Relay Service (FRS) meets all of these requirements.

Frame Relay Equipment

Motorola equipment is used for both the central site and for our remote locations. At the central site, we are using a Motorola 6525+, in a modulus 21 cabinet. At most of our remote sites, we are using the Motorola Vanguard 100 FRADS. At a couple of remote sites, where we have more than two data feeds over a single 56k line, we are using Motorola 6507 multi-port FRADS. The Vangaurd 300 Ether-FRAD is used with the seismic recording equipment which utilitizes TCP/IP telemetry.

Pictures of Equipment

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The K2 Strong Motion Accelerograph

The K2 datalogger with force balanced accelerometers, the seismometer, and communications equipment that are typically used:

The K2 as it would be set up at an actual digital site. The metal frame is used to keep things from bumping up against the seismometer while it is in operation. The K2 is crooked in its frame for a specific reason. It must be aligned in a northerly direction so that the North-South and East-West ground motions are indeed what are sensed by the device.

A closer view of the internals of the K2 accelerograph. The metal bar in the lower lefthand corner of the case is the battery holder. The three ground motion sensors (for vertical, N-S, and E-W motions) are in the top lefthand corner of the device, covered by a protective grille. The data conversion/storage/communications hardware is in the right half of the device. The orange rectangular object to the left is the rechargable battery which powers the K2 during power shortages.


The Central Site Data Acquisition System

This is a picture of the central site data acquisition system. The tower contains the actual data receiving hardware, as well as a computer and data storage system. The large terminal on the table allows system administrators to modify and maintain the system.


An Example of a TriNet Station

This is a photo of a digital seismic station. It uses a Quanterra digital datalogger. The two cylindrical objects on the floor are the broadband sensor and the strong motion accelerograph by which the datalogger records ground motions.


This page is www.trinet.org/instr.html. Last updated 03/30/00. Designed and maintained by Lisa Wald.
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