Seismic Stations

SCSN has more than 350 seismic stations in Southern California, creating one of the densest seismic networks in the world. Each station requires many hours of planning, during which time several factors are accounted for: site selection, environmental noise, security, power, telemetry, permit cost/attainability, ease of access, and geology. Site selection is decided regarding station density. If a large area of the Mojave Desert has very few stations, SCSN may choose to install a station in the least dense portion of this area. In addition to density, stations are separated into regional areas called subnets. Subnets are used to break up the network into many smaller networks, each of which is designed to trigger when a particular number of stations detect an event. This enables SCSN to regularly detect small events (<M 0.4).

Environmental variables, such as noise from wind or highway traffic, are taken into account when selecting a station site. The ideal site selection is far enough from roads to not detect the noise from passing vehicles, but close enough that the site is still accessible. Site security is very important, as sites that are in unsecured areas have been vandalized in the past. A power source is also an important factor, since a site in an area without power needs battery replacement regularly. Each station has an UPS system, in case the central power fails, the site will be able to rely on alternate power for several days to weeks if needed.

Telemetry is a very important factor when installing a seismic station. Most stations use frame relay via the major phone companies to telemeter data, but many other methods of telemetry are used. Microwave transmissions, radio signals, the internet, and earthworm systems are all used in addition to frame relay. This variety insures a healthy network, provided one particular method of telemetry fails. In addition to these methods, many utility companies share their networks with SCSN, which allow the data to be transmitted via private networks owned by gas, electric, water, and phone utility companies. , receive products generated by SCSN. ShakeMaps, CISN Display, and paging devices are provided to many first responders at utility companies by SCSN.

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 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.

SCSN Sensor Orientation Convention

1) Triaxial sensors will be oriented vertical, north-south, and
east-west with the greatest accuracy permitted by the circumstances and
in accordance with the manufacturer's recommendation. Orientations are
true and not magnetic. Field measurements of orientation generally have
an accuracy of +-2 degrees.

2) SEED orientation codes will be assigned as described in appendix A of
the SEED Manual; "Z" for vertical, "N" for N-S and "E" for E-W.

3) 1, 2, 3 will be substituted for Z, N, E for downhole sensors at sites
with surface mount sensors. This convention is kludge that should be
abandoned when location codes are fully implemented.

4) If a sensor's orientation is found to depart measurably from true
Z,N,E it will be corrected in the field. The departure will be measured
and entered into the orientation fields of the database as an epoch
ending at the time the sensor is reoriented.

5) There is no guarantee that imported data will conform to this
convention. However, the dbase entries, which are under our control,
should adhere to these rules.

Schema Convention for Sensor Orientation
1) Channel_Data.azimuth -- The azimuth of the instrument from north,
clockwise ranging from 0 to 360 degrees. When correctly oriented an N
component will have an azimuth of 0 and an E component will have an
azimuth of 90. The Z component when truly vertical has no azimuth and is
set to 0.

2) Channel_Data.dip -- The dip of the instrument, down from horizontal
ranging from -90 to 90 degrees. When correctly oriented a Z component
will have a dip of -90 and a horizontal will have a dip of 0. If the Z
component has a dip other than -90 or 90 the azimuth of the component
should also be recorded.

Seismic Station Configurations

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.