Site Station Subsystem Sensor Sampling Non-seismic channels Comments

LIGO Data Channels

The LIGO e-Lab features data from a set of seismic sensors that the LIGO experiment uses to monitor ground vibration, which is a significant background to its gravitational wave searches. The data from a particular sensor is called a channel. LIGO's seismic sensors operate around-the-clock, and each channel represents a continuous stream of seismic data over the life of that sensor.

Summary: There are nine total seismic sensors (SEIS) between the two sites. From each, we get "raw" data as well as processed data divided into 6 frequency bands. That's 7 streams per sensor, or 63 seismic streams; 14 of these are currently available in the e-Lab. In addition, each sensor measures vibration in the x, y, and z-directions independently and delivers this data as three separate signals, giving 189 total seismic stream files (42 available in the e-Lab).

Thus, data channels are identified by the site, subsystem, station, sensor, and sampling of the data.

The e-Lab data covers the period from 2003 to the present. From June 2011 to April 2013, however, LIGO was shut down for major upgrades, and there is no data from this period. The upgrades included substantial changes to the data streams and file formats, so these two periods (2003-June 2011, April 2013-present) are treated separately by the e-Lab and its data structures.

Site

The two LIGO observatories are located in (H)anford, WA and (L)ivingston, LA.

Station

"Station" refers to the physical location at the observatory site that houses the seismic sensor. They are

Both Livingston and Hanford (six total):

• LVEA - Laser and Vacuum Equipment Area, also called the "corner station" at the vertex of the two interferometer arms
• EX - End X, the end of the X arm of the interferometer
• EY - End Y, the end of the Y arm of the interferometer

Hanford only (three more):

• MX - Mid X, the midpoint of the X arm of the interferometer
• MY - Mid Y, the midpoint of the Y arm of the interferometer
• VAULT - Underground concrete vault outside of the primary LIGO buildings

Each of the arm detectors (EX, EY, MX, MY) is additionally labeled with VEA in the stream filename, but this is distinct from the LVEA station. Presumably there are additional systems along the pipes to help maintain vacuum, and seismic sensors are placed with this equipment in the respective Vacuum Equipment Areas. There's no L because there's no laser equipment along the arms, only at the vertex.

Physically, there's no real qualitative difference among the data originating at various stations. The exception is the Vault sensor, which has historically been problematic to keep functioning correctly (possibly because of a bad water chip), and should be interpreted with skepticism. For example, in May 2016, we discovered that someone at Hanford had "borrowed" the detector from the Vault for some other purpose, and that the Vault data from this time period is therefore nonsense. As of Q1 2017, we're still not 100% certain it was ever returned.

Subsystem

PEM refers to the Physics Environment Monitoring system, software that processes the data from all sensors. The seismometers take take data at a rate of 256Hz; the PEM system takes 1-second averages of the signals and delivers this 1Hz data to the e-Lab otherwise as-is. After the restart (April 2013 - present), these files are labeled with a "DQ" (for Data Quality, I think) at the end. PEM-DQ streams are as close as the e-Lab gets to "raw" LIGO seismic data.

DMT refers to Data Monitoring Tool, which takes data from the PEM and calibrates and filters it. This processing includes an FFT of the data into six frequency bands that are of particular interest in seismology. This processed data is then delivered to i2u2-data divided into one stream per frequency band. These streams are labeled with the frequency band at the end of the filename.

The e-Lab divides the data into "PEM" and "DMT" subsystems to indicate whether or not the DMT processing has been applied. Nonetheless, all data originates in the PEM system, so, confusingly, DMT stream filenames still have the label "PEM". Filenames for the two subsystems are instead distinguished by whether they end in a "DQ" (raw-data PEM) or a frequency (DMT-processed PEM).

GDS refers to the Global Diagnostic System. This data is not (yet) available in the e-Lab.

Sensor

Each sensor comprises three accelerometers arranged to measure motion in the X, Y and Z directions. In addition, the choice of frequency band of DMT-output data is lumped in with this option, though a single sensor generates all bands associated with a channel (there aren't different sensors for different frequencies - that's all done later through Fourier analysis).

Sampling

Default for most streams is rms data sampling. PEM (raw data) also offers mean data sampling.

Non-seismic channels

In addition to the seismic sensors there are also tiltmeter (TILT) and magnetometer (MAG) sensors at each site. Each delivers separate x, y, z data (but without frequency-banding). These are stored by the ELabs data system but are not used for the e-Lab (as of June 2016). There are eight TILT and eight MAG sensors, one of each located with each of the nine SEIS sensors except for the one in Hanford's Vault.

-- Main.JoelG - 2016-12-09

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