Technology News

No. 466 November 1997

U.S. Department of Health and Human Services

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Use of Seismic Tomography To Identify Geologic

Hazards in Underground Mines

Objective

Develop geophysical techniques suitable for enhancing miner

safety. Specifically, to use geophysical methods to detect

hazardous conditions in a rock mass in deep underground mines as

well as to assess the mechanical integrity and monitor changes of

the rock.

Background

Currently, no adequate method exists to determine the amount of

relative stress in large rock pillars in deep underground mines.

The greater the stress, the greater the chance of violent ground

failure, which is a safety hazard to miners. Existing methods to

determine stress are time consuming and expensive and can only

be used in localized areas. Also, the measurements collected are

often of limited quality. Seismic tomography, as used to determine

stress, is based on the principle that P-wave velocities in highly

stressed rock will be relatively higher than P-wave velocities in

rock under less stress. In this technique, seismic waves are

generated that penetrate a rock mass and, based on measurements

of apparent velocities, used to develop stress gradient contours

(tomograms). The benefits of using seismic tomography to identify

high stress in deep underground pillars include the method’s

reliability, cost effectiveness, and efficiency. Engineers can

evaluate stresses large rock masses, as well as monitor pillars

during the mining cycle.

How It Works

This method requires striking a source location with a 4.5-kg

sledgehammer fitted with a trigger connected to a seismograph and

then recording the first arrival of the seismic wave at stations

where receivers (geophones) have been installed. Geophones are

mounted to a rib using rock bolt plates that are drilled, tapped, and

surveyed using the mine coordinate system. Each geophone is

hooked to a seismic cable connected to the seismograph, and a twopair shielded cable is used both for communication and as a trigger

for the seismograph. A signal-stacking seismograph is used to

record P-wave arrivals, and all seismic data are transferred from

the seismograph to a personal computer (PC). The travel times

(first arrivals) are “picked” from the seismic records. All

receiver and source location coordinates (x,y,z) are input into a

spreadsheet, and travel times are entered into a software program

(3DTOM) for reconstruction. Finally, contouring software is used

to smooth the tomograms and add text and mine openings for final

presentation and interpretation.

Tests Results

Spokane Research Laboratory personnel have conducted six seismic

tomography surveys at three deep underground mines. The

technique has proven to be reliable in each survey, and high

stresses in the mapped rock masses have corresponded with later

seismic activity, rock bursts, and major ground falls. In all the

surveys, low-velocity areas corresponded with fractured rock and

mine openings, and high-velocity areas corresponded with unmined

pillars above stopes, corners of pillars, and known areas of high

stress.

Summary

Spokane Research Laboratory personnel have developed a PC-based

seismic tomography method that is reliable, cost-effective, and

efficient. This method is capable of mapping and monitoring large

rock masses. This technology can be used to identify high-stress

conditions that may result in ground failure and affect the safety of

miners.
Example of seismic tomogram

For More Information

Contact—

Doug Scott

Spokane Research Laboratory

NIOSH

E . 315 Montgomery

Spokane, WA 99207

(509) 534-8056

FAX (509) 354-8099

E-mail dus3@niosh7.em.cdc.gov

Ted Williams

Spokane Research Laboratory

NIOSH

E . 315 Montgomery

Spokane, WA 99207

(509) 354-8060

FAX (509) 354-8099

E-mail tew9@niosh7.em.cdc.gov

Mention of any company name or product does not constitute

endorsement by the National Institute for Occupational Safety and

Health.

To receive additional information about mining issues or

other occupational safety and health problems, call 1-800-

35-NIOSH (1-800-356-4674), or visit the

NIOSH Home Page on the World Wide Web at

http://www.cdc .

gov/niosh/homepage.html 

As of October 1996 the safety and health research

functions of the former U.S. Bureau of Mines are

now located in the National Institute for

Occupational Safety and Health (NIOSH).