CGS seismometer array
2016-09-03 | Dr. John Hopkins
The CGS recently installed the first of five new seismic recording stations that will collect information on seismic events around the state and the region. The CGS seismic network acts in conjunction with those maintained by the University of Colorado and Colorado State University, the Incorporated Research Institutions for Seismology (IRIS), and the US Geological Survey‘s National Earthquake Information Center (NEIC)—to provide near real-time earthquake detection. The addition of our monitoring capacity, the wider network allows the geoscience research community to better understand background seismicity in Colorado and better discriminate between natural and induced seismic events that may occur in the region.
The CGS already operates four other stations with Streckeisen STS-2 Broadband Sensors (capable of sensing ground motions over the frequency band 0.01 Hz (100 sec) to 15 Hz). They were part of a national consortium—USARRAY—that was a portable seismic network migrating around to different locations in the US several years ago. State-level organizations were allowed to ‘adopt’ some of the stations that were deployed within each state. The CGS purchased those four stations in 2010—their approximate locations are included on our ON-001 Colorado Earthquake and Fault Map.
The set-up for a typical recording station includes the seismometer and its associated data recorder, a power system, and a communications system. The install site is carefully chosen for its relative acoustic silence—such that human-caused (road and air-traffic) and natural (wind, animal) noise levels are minimal at the relevant frequencies. The CGS cooperates with the Colorado State Land Board and the Colorado State Parks system in locating optimal sites for the stations in the CGS network. The particular station illustrated here is our Briggsdale Seismic Station #T25A-1 near Greeley, Colorado.
The physical installation of an isolated off-grid seismometer station includes the excavation of a pit for the seismometer ‘vault’ to sit in, a trench for cabling from the seismometer to the recording and power equipment, and a photo-voltaic (solar) power and data transmission tower. Adequate fencing to isolate the installation from noise and physical disturbance—in this particular case, grazing cattle—is important.
A seismometer is a device that can sense a wide range of ground motions or vibrations. Environmental considerations require that its underground installation be both level and thermally insulated. A sub-surface concrete pad is prepared with a glass plate embedded on the top to provide a perfectly flat platform for the seismometer to sit on. After precise leveling, the seismometer is then connected to a data recording system that is installed some distance away in a weather-proof console—again to keep possible vibrations from the tower at a minimum. The data recording box includes an A-to-D (Analog-to-Digital) converter that digitizes the signal and prepares it for transmission via the communications system.
The communications system consists of a modem and a GPS transceiver. Once recorded by the seismometer, a seismic trace is converted to a digital signal, processed and sent via the modem to a local cell tower where it is relayed first to IRIS and then on to the NEIC for correlation and display. The GPS provides a standard clock signal for data synchronization, an important factor in coordinating each individual seismic station with the wider network of stations. The IRIS website provides current near-real-time data for the Briggsdale station as well as all other stations in the network.
The power system includes a deep-cycle marine battery, and a photovoltaic panel for recharging along with a voltage inverter/charge controller to ensure a stable power supply for the data recording and communications system.