Seismic Network Components

Introduction

The seismic network consists of a remote sensing station, equipment to transmit and receive data, a computer to log and display the data and an Internet connection to distribute the data. Signals from the seismometer are sent via a transmitter at Soldiers Delight to a receiver on the roof of the MGS building. From the rooftop receiver, the data signal is sent to a dedicated computer for processing, display and storage. The data is then made available on the Internet from our World Wide Web site.

Components

• remote station
  The remote station includes the seismometer (earthquake sensor), data processing equipment, transmitter, power supply and lightning protection. The earthquake sensor is a Guralp CMG-3T three-component, broadband seismometer, with a standard bandwidth of 0.01-50Hz. Three directions of motions can be measured by this instrument: north-south, east-west, and up-and-down (vertical). A built-in signal digitizer converts the analog signal from the seismometer sensors to a digital signal. This instrument is placed on a concrete pad which is is direct contact with bedrock. The sensor-digitizer equipment is contained in a below-ground vault which is covered with several feet of earth for thermal and mechanical isolation. The equipment is located in a quiet, tree-free area to minimize ground vibrations and wind noise, which would degrade the seismic signal. The data processing and transmission equipment is housed in the Soldiers Delight Visitors Center. Sufficient battery backup power is available to keep the remote station in operation for up to seven days during a power failure. Data is transmitted by a FreeWave Spread Spectrum wireless data transceiver. A time signal provided by a geographic positioning system (GPS) receiver is available at the station for data timing and synchronization. In the event of an extended power outage or transmitter failure a data backup unit provides up to one month of data storage. Finally, the station is protected from lightning strikes by a GPS lightning protection system.
• base station
  The base station receiver system consists of a FreeWave Spread Spectrum wireless data transceiver, a short-haul modem, and a computer for data processing, display, storage and Internet distribution of data. The receiver has a battery backup system similar to the remote station. A short-haul modem sends the incoming data from the rooftop transceiver to the MGS internal network and to the data processing computer. The computer uses Earthworm software to process seismic data. The resulting data can be viewed on the data processing computer, and is also available as near-real time data on our web pages. An entire day's worth of data is transmitted to the Lamont-Doherty Cooperative Seismic Network (LCSN) during the evening.
• high-speed Internet distribution
  Seismic information from the remote station is available over the Internet from MGS as uninterrupted data, or from LCSN as interpreted data. LCSN will integrate data from the Maryland station with data from the rest of the LCSN network to produce information about local and regional earthquakes. LCSN will determine when and where earthquakes occur based on the MGS seismic data and other LCSN network data. LCSN will then notify MGS of the event, and information on the event will be posted on our web sit

Data Collection and Processing

The Maryland Seismic Network uses a variety of software to collect, analyze and display data. Some of this software was developed by MGS, and some came from other sources.  Earth movements are detected by the seismometer and converted to a digital signal by the instrument's internal digitizer. The digitizer also compresses the data before it is transmitted to the Survey laboratories. The digitizer uses internal, proprietary software for these tasks. When the digital signal arrives at the Survey it is decompressed and made available to our network by SCREAM! software. This software is available from Guralp Systems

The raw data output by SCREAM! is processed by Earthworm software, available from USGS Earthworm Development Group. Earthworm provides several services for seismic data processing. One of the primary functions of Earthworm is to store seismic data from SCREAM!. Earthworm stores the data in a large file on a hard drive disk. Because the data is constantly being transmitted and stored, and there is a finite amount of disk space for storage, only a certain amount of data can be stored. Therefore, the most recent data received is stored and data older than some chosen date is discarded from the disk file. This kind of data storage file is called a circular buffer. The circular buffer for Earthworm data is refereed to as the wave ring. Thus the primary function of Earthworm is to create and maintain a wave ring.Earthworm also imports seismic data from other institutions into our wave ring. Any data in the wave ring is available for distribution, analysis and display.

MGS exports wave ring data to the Lamont-Doherty cooperative seismic network for analysis.