INSTALLATION OF A DIGITAL, WIRELESS, STRONG-MOTION 

 NETWORK FOR MONITORING SEISMIC ACTIVITY IN A 

WESTERN COLORADO COAL MINING REGION

Peter Swanson, NIOSH/Spokane Research Laboratory, Spokane, WA

Collin Stewart, Bowie Resources LLC, Paonia, CO

Wendell Koontz, Mountain Coal Company, Somerset, CO

Abstract

A seismic monitoring network has recently been installed in the North Fork Valley coal mining region 

of western Colorado as part of a NIOSH mine safety technology transfer project with two longwall coal mine 

operators. Data recorded with this network will be used to characterize mining related and natural seismic 

activity in the vicinity of the mines and examine potential hazards due to ground shaking near critical structures 

such as impoundment dams, reservoirs, and steep slopes. Ten triaxial strong-motion accelerometers have been 

installed on the surface to form the core of a network that covers approximately 250 square kilometers (100 sq. 

miles) of rugged canyon-mesa terrain. Spread-spectrum radio networks are used to telemeter continuous 

streams of seismic waveform data to a central location where they are converted to IP data streams and ported 

to the Internet for processing, archiving, and analysis. 

Background

During underground mining operations strata respond to excavation-related stress with deformation 

mechanisms that sometimes include dynamic rock mass failures. Experience in western coal mines shows that 

such dynamic failures can produce microtremors or seismic events with magnitudes in the range of M = 2-3 

with infrequent occurrences up to M = 4 (Arabasz et al., 2005). Factors affecting the susceptibility to mininginduced seismicity include mechanical properties of the local strata, amount of overburden, and size, geometry, 

and mining sequence of excavated areas. 

In anticipation of mining near structures such as earthen dams and reservoirs, Bowie Resources LLC 

and Mountain Coal Company have joined together with NIOSH to develop a digital seismic monitoring 

network to collect background data on mining-related seismic activity and to implement a real-time hazard 

monitoring tool. In addition to the mine-safety technology transfer activity, NIOSH is participating in this 

project to gather data for use in its mine safety research program. Data collected with this network will be used 

to (i) identify and characterize dynamic failure mechanisms of potentially hazardous rock mass failures (e.g. 

coal bumps), and (ii) determine empirical relations between levels of ground shaking, seismic event magnitude, 

and distance to the seismic sources. The latter can then be used to help evaluate the potential responses of 

surface and underground structures to shaking and identify minimum distances between the seismic activity and 

these structures to keep levels of shaking below key damage thresholds. 

Setting

The longwall coal mines are located in the rugged canyon-mesa terrain of western Colorado (Figures 1 

and 2) with vertical relief up to ~ 1 km (3300 ft). The major geologic units present are representative of the 

Upper Cretaceous Mesa Verde Formation and include six coal seams with thickness from 2 to 6 m (6 ft to 20 

ft). Overburden above longwall panels in the district extends up to ~ 0.7 km (2400 ft). Yearly production from 

all of the coal mines in the North Fork Valley exceeds 16 million tons per year. 
Preliminary Seismic Study 

Prior to the network design and installation a pilot study was carried out using a single triaxial seismic 

station deployed at the King Mine portal area adjacent to the Bowie Mine. Differences in arrival times of the 

compressional (P) and slower shear (S) waves constrain the distance between the station and the recorded 

events. For example, Figure 3 compares traces recorded on this instrument for a mining-related event at Bowie 

(ML ~ 2.8, upper traces) with an S-P time of ~ 0.7 s against those for a regional earthquake (ML ~ 3.3) with S-P 

time of ~12 s. The distances to these events are approximately 2 and 100 km, respectively. 

Of the 350 events that were recorded over a two-month period and analyzed, most fell into three groups 

with similar S-P times (Figure 4). As there are three different coal mines operating in the North Fork Valley 

with workings that clustered at three different distances, the different S-P time groupings were consistent with 

events originating at all three of these mines. 

North Fork Seismic Network

Seismic stations

Ten triaxial strong-motion seismometers (Altus K2 instruments with internal EpiSensors) and one shortperiod seismometer (WE1) were installed in two groups centered over the two participating mines (Figure 5) 

providing coverage over an area of approximately 250 square kilometers (100 sq. miles). Strong-motion 

seismographs, or accelerographs, are specifically designed to measure ground motions near seismic sources that 

are strong enough to potentially impact the integrity of engineered structures. With the nominal 24-bit 

resolution of their internal data recorders, they can also resolve background vibrations in the micro-g (10-6) 

range. At each strong-motion station a fourth channel records an additional co-located 1-Hz moving-coil (L4-

C) seismometer, providing higher sensitivity to smaller and/or more distant seismic events. The data recorder is 

configured to (i) continuously stream data at 100 samples per second, and (ii) locally trigger and save event data 

to compact flash cards providing backup in case of telemetry failure. Time synchronization is achieved at each 

digitizing/recorder unit using GPS receivers. 

Seismic station sites were selected to provide suitable azimuthal coverage of the mining areas, a 

workable telemetry path, concealment from view, and, wherever possible, access to bedrock. Separate 

instrument and power vaults were installed by digging shallow holes and installing vertically-oriented 

corrugated steel culverts with locking lids. In the instrument vault, a flat concrete floor was poured onto 

exposed bedrock, onto which the strong-motion seismometer is bolted. Remote power sources consist of two 

75-watt photovoltaic panels and four 115 Amp-hr batteries. 

Installation of the accelerographs at the Bowie Mine was complete by October 2005. The West Elk 

instruments were added to the network and made operational starting July 1, 2006. 

Data communications

Communication with, and data transfer from, the strong-motion seismometers is achieved with licensefree 900-MHz frequency-hopping spread-spectrum radios equipped with 10dB directional Yagi antennas. With 

the use of three repeaters, the ten remote instrument sites are connected in four separate point-to-multipoint 

radio subnets operating in a bandwidth-sharing time-domain multiple-access (TDMA) mode. Radio 

interference issues have been experienced with other nearby 900-MHz data-comm networks associated with 

ventilation and methane drainage drillhole monitoring. The interference has been mitigated by appropriate 

antenna selection, positioning and RF power level adjustments. 

Each seismic station outputs continuous digital seismic waveform data at 9600 bits per second (RS-232) 

and is received at a site in town with a DSL Internet connection (Figure 7). A dedicated server demultiplexes 

the serial data from each subnet and converts it to individual station-based IP data streams. The serial/IP data 

network allows complete control over the instruments from distant Internet-connected user locations. Currently 

this data is received, processed, and re-distributed at NIOSH’s Spokane Research Laboratory. 
Data Processing

A large portion of the software for data collection, processing, analysis and display is based on the U.S. 

Geological Survey’s (USGS) Earthworm system (Earle et al., 2003). At its core are real-time TCP/IP data 

distribution and data processing modules interconnected by a message-passing system. The software can be 

configured to automatically detect, process, archive and analyze seismic events from numerous types of 

instruments and make the raw data and processed results available over a computer network/Internet. 

At this stage of implementation, software modules have been configured to coordinate data collection 

from the remote seismic stations, merge and time align all of the data streams in a common buffer containing 

several weeks worth of data, identify both individual station triggers and network (event) triggers, save 

triggered event data to disk, filter traces and create helicorder/webicorder plots from individual stations. Other 

module functions in the process of being automated include arrival-time picking, magnitude estimation, event 

location, and detection and email notification of user-specified threshold acceleration levels. 

Example Results

As with any new seismic network there is a period of time where the parameters that control the data 

acquisition and processing functions are adjusted as experience is gained with the different types of seismic 

sources of interest. In the case of mine seismicity, the occurrence rate and magnitude of the activity can vary 

tremendously over relatively short periods of time as working areas encounter variable geologic and mining 

conditions. Both of these mines have gone through long periods of time without experiencing any seismic 

activity that is noted on regional seismic networks, only to have, for example, 5 or more seismic events with 

magnitudes between 2.5 and 3.0 occur within a single one-month period of time. One goal of this joint effort is 

to document these mining and geologic conditions so that we are better able to identify, and anticipate, 

circumstances that may lead to dynamic rock mass failures. 

Data Visualization

Webicorder plots provide the digital equivalent of the old pen and ink and/or smoked-paper seismograph drum (helicorder) records and provide a convenient way of evaluating overall levels of seismic 

activity on a daily basis. Raw and/or filtered seismic records are plotted in continuously updated GIF image 

files (Figure 8) that are accessible via a web interface. Waveform data satisfying a user-controlled triggering 

condition (i.e. events, Figure 9) are saved in separate files and can be viewed with a number of different 

software utilities. 

Comparison with USGS/NEIC Locations

During the time period between July 1 and December 1, 2006 the USGS’s National Earthquake 

Information Center (NEIC) in Golden, CO reported 7 seismic events in the North Fork Valley area with local 

magnitudes (ML) between 2.5 and 2.9. Figure 10 compares NEIC locations, calculated with data from 

available regional network stations and using a generic global earth model, with those determined using data 

strictly from the local network. Event locations determined using local network data were calculated using 

Hypoinverse (Klein, 2002) with a layered velocity model (Swanson and Koontz, 2006). The average difference 

between the two calculated locations is 8.5 km (5.3 mi). Event locations based on the local network data 

coincide closely with areas of mining activity underway at those times. Six events clustered in one group over 

a one-month period of time. The event locations correspond to an area of multi-seam mining (mining below a 

previously mined seam separated vertically by 90 m (300 ft)) where an unusually thick sandstone channel was 

encountered in the roof. 

Conclusions
NIOSH, Bowie Resources LLC and Mountain Coal Company have cooperated on a joint project to 

develop a digital wireless seismic monitoring network to collect background data on mining-related seismic 

activity in western Colorado and to implement a hazard monitoring tool. The ten station strong-motion array 

provides real-time monitoring capability using wireless serial and IP communications networksData collected 

with this network will also be used to (i) identify and characterize dynamic failure mechanisms of potentially 

hazardous rock mass failures, and (ii) determine empirical relations between levels of ground shaking, seismic 

event magnitude, and distance to the seismic sources. 

Figure 1: Location of North Fork Valley Mining Region. 

Figure 2: View to the SE toward West Elk property from Bowie Mine. 

References

Arabasz, W., Nava, S., McCarter, M., Pankow, K., Pechmann, J., Ake, J., and McGarr, A., 2005, Coal-mining 

seismicity and ground-shaking hazard: A case study in the Trail Mountain area, Emery County, Utah, 

Bull. Seism. Soc. Am., v. 95, no. 2, pp. 18-30. 

Earle, P., Bittenbinder, A., Bogaert, B., and Johnson, C., 2003, Tune to the worm: Seismic network operation 

using the USGS Earthworm system, in Observations and Research Facilities for European Seismology, 

Orfeus Newsletter, vol. 5, no.1, March. 

Klein, F., 2002, User’s guide to HYPOINVERSE-2000, a Fortran program to solve for earthquake locations 

and magnitudes, USGS Open-File Report 02-171, vers. 1, 123 pp. 

Swanson, P., and Koontz, W., 2006, Measurement and Analysis of Mining-Induced Seismic Ground Motion in 

the Vicinity of the West Elk Coal Mine, Somerset, CO (abstract), Seism. Res. Lett, v. 77, no.2, p. 318. 
Figure 3: S-P arrival-time intervals observed on triaxial sensor for events at 2 and 100 km distance. 

Figure 4: Three distinct S-P arrival-time intervals observed with single-station monitoring at the King Mine portal area. 
Figure 5: Stations of the seismic montoring network. Stations north and south of the North Fork river are centered on 

Bowie and West Elk workings respectively. 

Figure 6: Remote seismic station installation 
Figure 7: Serial/IP data communications network. Example of radio telemetry link shown for one radio subnet serving 

three remote seismic stations. 

Figure 8: 12-hour long webicorder style plot for vertical component at station BWK. 
Figure 9: Waveform record from mine-related event (M ~ 2.8). Each station shows accelerometer components Z, N, E, 

and short-period vertical Z. 

Figure 10: Comparison of event locations reported by NEIC (squares) with those calculated using local network data (circles).