Feasibility of using laser-based vibration measurements to detect 

roof fall hazards in underground mines†

Peter Swanson‡

 

National Institute for Occupational Safety and Health (NIOSH) 

Spokane Research Laboratory, 

Spokane, WA USA 

ABSTRACT 

One of the primary methods for analyzing roof stability in underground mines is the age-old method of “roof sounding” 

where a miner taps on the roof and listens for the hollow sound of loose blocks of rock. This paper looks at the feasibility 

of using noncontact laser-based vibration measurements to detect roof fall hazards with the ultimate vision of improving, 

expanding and automating procedures for mine roof inspection. Vibration measurements made on loose blocks of rock in 

underground mines are summarized and compared to estimates of fundamental resonance frequencies for rock slabs of 

the size responsible for highly hazardous “skin failures.” Both laser Doppler vibrometry and full-field interferometric 

methods are examined and are considered to be feasible methods for detecting anomalous vibrations in loose roof rocks. 

Results from simple laboratory experiments using laser vibrometry demonstrate some of the proposed application 

concepts. While considered a challenge to move these techniques from the laboratory to heavy industrial or outdoor 

environments, the potential for success in the current application is enhanced by the reduced requirements of qualitative 

analyses. 

 

Keywords: Roof falls, ground falls, ESPI, laser vibrometry, mining, roof sounding, TV holography 

1. INTRODUCTION 

Unexpected falls of ground remain one of the largest sources of injuries and fatalities in underground mines throughout 

the world. While safety statistics reveal that U.S. mines are among the safest, there are still approximately 10 to 15 

fatalities and 700 to 1000 injuries each year due to ground falls.1,2 This feasibility assessment is being conducted as part 

of the mission of the National Institute for Occupational Safety and Health (NIOSH) to find ways to reduce hazards to 

worker populations. 

The first part of the paper describes mine roof conditions, roof falls, vibration characteristics of loose roof rocks, and 

some of the accelerometer-based tools that have been used to probe and identify them. The vibration response of loose 

slabs composed of different common rock types is estimated and compared to observations. A number of laser-based 

vibration and deformation measurement studies pertinent to the present application are reviewed. The last part presents 

the results of simple laboratory tests to demonstrate the application concepts and outlines proposed optical methods to 

delineate regions of anomalous vibration associated with loose roof rock. 

2. THE UNDERGROUND WORKPLACE 

2.1 The nature of mine roof 

Mine roof conditions vary significantly because of differences in mining methods and geology. For example, roof 

heights can range from 1 to 30 m high (2 to 4 m is typical), and roof spans can extend from 1 m to greater than 20 m. 

Roof surfaces can range from flat, smooth, and of a single composition requiring no support to incompetent, 

unconsolidated material requiring steel arches and lagging or complete suspension through installation of mesh and roof 

bolts. In many U.S. mines, typical support is provided by 1- to 2-m-long roof bolts installed every 1 to 1.5 m. Metal 

straps and timber are added where supplemental support is required. As mining is an evolutionary process and rock 

 

†

 Pres. 5th Intl. Conf. on Vibration Measurements by Laser Techniques, Ancona, Italy, June 18-21, 2002. 

‡ pswanson@cdc.gov; phone 1 509 354-8072; fax 1 509 354-8099; National Institute for Occupational Safety and Health, 

Spokane Research Laboratory, 315 E. Montgomery Ave., Spokane, WA 99207 USA 
properties are rate sensitive, mine roof can deform and fail in a time-dependent fashion. Roof conditions in certain 

geologic structures also exhibit sensitivity to weathering and seasonal variations in moisture content. Thus, mine roof 

that is safe today is not necessarily safe tomorrow. 

2.2 Roof fall characteristics

Gravity-driven mine roof failure results from the time-dependent loss of cohesion across fracture, joint, and geologic 

interfaces. Roof fall volumes range from small pieces of rock to complete failure of the roof and roof-support system. 

Several studies have documented how fatality and injury rates relate to roof fall characteristics. Ninety-eight percent of 

the roof fall injuries in U.S. coal mines in 1997 involved failure with a thickness, or extent into the roof, of 0.3±0.2 m.1

These are termed “skin failures” and often involve failure of the roof between roof supports. Fatalities, however, 

occurred more often with massive roof falls (2.8±2.3 m thick) during this study period. More than 70% of all roof falls, 

as well as 70% of the roof falls associated with fatalities, in S. African coal mines had a thickness of 0.5 m or less.3

 Thus, 

in this initial study, the emphasis is on detecting loose blocks of roof rock with thicknesses of 0.5 m or less.

3. ANOMALOUS VIBRATION BEHAVIOR OF LOOSE ROOF ROCKS 

3.1 Methods used to assess roof stability 

Visual examination is the primary method used to assess the condition of a mine roof, followed by roof sounding. In 

conventional roof sounding, a miner taps the roof with a scaling bar, sounding tool, or other object and listens to the 

response. Elastic waves generated by the tap interact with the near-surface roof rock and are converted back into sound 

waves through seismic-acoustic coupling. Roof response may also be felt with the fingertips if the roof is within reach. 

There are two end-member responses: (1) the rock mass is competent and acts like a stiff infinite half-space and (2) a 

wholly or partially detached block of rock resonates like a free body. The “drummy” or hollow response that the miner 

uses to identify and eliminate loose blocks of roof rock is closer to the latter. Reduced stiffness (acoustic impedance) 

along the loose block boundaries provides the flexibility (reflection) that allows the loose rock mass to resonate for a 

short time upon excitation. 

This manual inspection method takes place as new roof surfaces are created. With time, however, roof conditions can 

deteriorate. A noncontact inspection tool for imaging anomalous roof vibrations from a remote location could help 

identify dangerous conditions in this work environment and lead to improvements in mine safety. A two-dimensional 

map showing qualitative relative surface displacements, or vibration amplitudes, may allow loose blocks of rock to be 

identified in a manner similar to the empirical way a miner contrasts the sound of safe and unsafe roof. 

3.2 Vibration characteristics of loose roof rocks

Several attempts have been made to quantify the vibration characteristics of safe and unsafe roof. In these studies, an 

accelerometer or geophone (velocity sensor) was mounted to the mine roof, an impact was applied to the roof up to 

several tens of centimeters away from the sensor, and the signals were recorded for analysis. Measurements on both 

competent and drummy roof were then compared. These studies showed that vibration frequency was lower, amplitude 

was greater, and duration was longer than for competent mine roof. 

Summerfield4

 investigated coal mine roof and found that drummy-sounding roof rock generated low-frequency vibration 

amplitudes that were twice as large as observed in solid roof. This effect was most pronounced between 200 and 1000 

Hz. Similarly, the duration of vibration in drummy roof was found to be roughly twice that for solid roof. de Montille 

and Weber5

 found loose slabs to vibrate at frequencies ranging from several to 450 Hz. Palmer and Czirr6 examined 

vibration spectra from loose slabs in a coal mine and found enhanced spectral peaks in the 200- to 600-Hz range with 

some response below 200 Hz as well. Hanson7

 investigated vibration characteristics of loose roof rocks in both hard rock 

and coal mines. A notable increase in response in the 0.2- to 1-kHz range was observed in the hard rock mines and from 

a few to 250 Hz in a coal mine. Most of these studies noted the influence of sensor mounting and details of the impact 

force on the results. 

A slightly different approach was used by Brennecke and Gallagher.8

 Following Obert,9 their method inolved a single 

fixed geophone recording site and multiple tap/impact sites. The geophone was attached to solid roof rock and located as 

far away as 75 m from an impact site. Impacts on solid rock produced signal amplitudes that were up to 10 times as high 

as impacts on loose rock. Instead of recording the anomalous vibration response of the loose rock directly (drive-point 

response), these measurements sensed the reduced ability to transmit seismic energy into the surrounding medium due to 

the rock mass decoupling (transmittance). 
Use of sound wave recordings to discriminate between competent and loose roof rock has been investigated4,5 and 

rejected5

 as microphone data were difficult to interpret due to extraneous airborne noise in the confines of underground 

space. It is worth noting that the seismic-to-acoustic coupling in this process is very inefficient. Because of the large 

acoustic impedance mismatch between air and rock, the signal heard by a miner has less than 0.05% of the energy 

reflected back into the rock. Thus, there would appear to be advantages to recording surface vibrations directly. 

The effect of crack-like defects on the low-frequency vibration response of mining structures has been investigated using 

both physical and numerical models.10,11 With the introduction of defects, nodal frequencies shift to lower values, which 

is consistent with field observations and the expected effect of a reduction in system stiffness. 

3.2 Instruments to detect loose roof rock 

Several attempts have been made to construct an instrument for industrial use by nonexperts to detect loose blocks of 

rock. They consist of a sensor, a method of applying a nearby impact, and circuitry connected to the sensor to 

discriminate the signal. Hanson7 developed a prototype that compared vibration power spectra in the 200- to-1000-Hz 

band and the 3000- to 3500-Hz band. Field tests in which the sensor was held against the rock with a spring-loaded pole 

showed that large values of this ratio are indicative of roof rock instability. A similar instrument was developed and 

tested by Siva Kumar et al.12 They contrasted relative amounts of energy in three different bands (600-1200, 1800-2400, 

3000-3600 Hz). In both of these studies, a silicon-based couplant was used between the sensor and the roof. Altounyan 

and Minney13 reported on trials of a similar prototype that was based on the rate of decay of impact-induced vibrations. 

Following an area-specific calibration, these data were used to indicate questionable roof sites. 

With each of these approaches, there is essentially a duplication of the same procedures that a miner follows in a 

conventional sounding test (manual handling of a tool, local application of an impact, and subsequent sensing of the local 

response). As the most sophisticated hardware and software available are still no match for the powerful integrating 

process carried out by a miner’s senses and brain, and as this natural data collection and processing would still 

automatically be used by a miner during operation of such electronic systems, it is difficult to foresee widespread 

adoption of this approach. 

3.4 Slab resonance frequencies 

We estimated the frequencies of the lowest order resonance mode of rock slabs to determine their sensitivity to rock 

composition and boundary conditions and to compare with field observations. Figure 1 shows several possible loose roof 

rock configurations and their approximate boundary conditions. These include (1) a slab completely separated from the 

host rock and held in place solely by geometrical constraints, (2) a cantilevered slab attached by only one of its four 

lateral edges, and (3) a slab formed by separation along a bedding plane but still attached on all four edges. 

For simplicity, we consider a square plate with edge length a and uniform thickness h. The frequency of the first bending 

mode (n = 1) is given by14 

Figure 1.−Three types of loose slabs that pose a roof fall hazard and their approximate boundary conditions. 
 _________ 

F = (K1/2π)*√(D/ρ*h*a4

), (1) 

where D, plate stiffness, is given by 

D = Eh3

/12(1-ν2). 

E is Young’s modulus, ν is Poisson’s ratio, ρ is mass density, and K1 is a constant for the first bending mode. Values of 

K1 for the boundary conditions of Figure 1 are 19.74, 3.49, and 35.98, respectively. 

Equation 1 is evaluated for several common types of roof rock with 

properties listed in Table 1. Due to natural variability, material 

properties for specific rock types can vary significantly. Specific 

values in Table 1 were chosen to help identify the range of expected 

vibration behavior. Fundamental frequencies are plotted against plate 

thickness in Figure 2 for a range of slab sizes expected in underground 

environments. Data for the different rock types are shown in hachured 

bands for each slab size. Quartzite and coal values are found on the 

upper and lower bounds, respectively. For a given slab size, the first mode frequency varies by a factor of three for the 

common roof rock materials. Variation due to the different boundary conditions (not shown) is one order of magnitude. 

Figure 2.−Resonance frequencies of roof slabs (simply supported plates) 

With such variation, it is not likely that frequency measurements alone will allow loose block dimensions to be estimated 

accurately. A similar conclusion was reached in several of the roof vibration measurement studies. The frequencies 

reported in those field observations and the most commonly observed roof fall slab thicknesses are highlighted in the box 

in Figure 2. The slabs within this box represent reasonable estimates of the sizes of actual roof falls. 

The estimates of slab vibration frequencies delineate the range to be expected in the field and provide a constraint for the 

laser vibration measurement and forced-vibration excitation procedures. 

Table 1. Properties of roof rock 

Rock type E (GN/m2

) ν ρ (kg/m3

) 

Quartzite 65 0.16 2700 

Limestone 50 0.25 2400 

Sandstone 30 0.22 2250 

Shale 15 0.27 2100 

Coal 3.5 0.40 1500 
4. VIBRATION EXCITATION SOURCES 

Three basic types of excitation are considered: impact, forced vibration, and ambient vibration. 

Miners’ mechanical impacts are known to be effective in exciting anomalous vibrations. While response amplitudes vary 

considerably from blow to blow, the frequency content remains similar.4,7 For remote operation and automation of this 

inspection process, other impulsive sources should be explored, such as dynamic pulses from water jets, lasers, and/or 

compressed air. 

For discussion purposes, the following distinction is made between forced and ambient vibrations. A forced vibration is a 

continuous, periodic vibration specifically imposed to assist in roof and rib inspection. Ambient vibration includes 

background earth noise as well as all other continuous and discontinuous sources of ground vibration in the mining 

environment. The latter include conveyor belts, mining machinery (mobile and fixed), light and heavy diesel-powered 

mobile equipment, trains, hoists, drilling equipment, ventilation fans, motors, and pumps. Many of these noise sources 

are in proximity to active mining areas where the greatest number of roof fall injuries and fatalities occur. At close range, 

most of these sources are likely to provide effective excitation. A number of these ambient sources could be utilized as 

controlled forced-vibration sources. Other sources to consider include arrays of loudspeakers that take advantage of 

enclosed-space resonance, pneumatic-powered hammers, rotating eccentric masses, etc. 

Certain types of mining machinery produce low-frequency vibrations that travel much farther than those produced by 

higher frequency sources. As an example, ground velocity was measured at a longwall mine with and without the 

longwall operating. Amplitude spectra for signals recorded at an underground site 600 m from the longwall are shown in 

Figure 3. Signals were obtained using a local microearthquake monitoring system.15 The two large peaks at ~4 Hz 

dominated the spectra at surface and underground sites separated by as much as 2.5 km. This long wavelength excitation 

source operates for long periods of time and affects large areas. While such low frequencies may excite low-order 

resonance modes in very large but thin slabs, another potential use is in providing small periodic strains to examine 

quasi-static deformation response. 

Figure 3.−In-mine ambient background vibration spectra. Longwall on (gray line) and off (black line).

5. LASER-BASED VIBRATION MEASUREMENT METHODS 

5.1 Potential advantages of noncontact vibration sensing

Noncontacting laser vibration methods offer several potential advantages: (1) The miner does not have to stand 

immediately below hazardous roof to measure vibration response, (2) vibration response can be measured on roofs that 

exceed the miner’s reach, (3) wide areas can be scanned or imaged instead of having to be measured at a single point, 

yielding a much more effective, timely, and thorough evaluation of the workplace, (4) stability evaluations can 
conceivably be made automatically and continually updated in the most dangerous areas while a miner continues to 

work, (5) large areas of a mine may be evaluated on a regular or less frequent basis at a small fraction of the effort 

required to do the same job by conventional sounding techniques, and (6) periodic surveys can be made at the same 

location without sensor contact repeatability problems. 

5.2 Lasers and mine safety 

One issue that should be considered in methane-rich mining environments is the possibility of laser-induced ignition. 

Experiments conducted by NIOSH examined minimum power levels required to ignite methane-air mixtures.16 While 

U.S. federal safety regulations that govern intrinsically safe equipment in underground mines do not contain specific 

guidance on lasers and other optoelectronic components, the European Committee for Electrotechnical Standardization 

(CENELEC) has suggested limits of 150 mW or 20 mW/mm2

 for open-beam lasers where methane gas and coal dust 

mixtures may be present.17 Higher-power lasers can stay below these limits through beam expansion. As with all 

electronic equipment, issues related to permissibility, or operation of electronic devices within potentially explosive 

environments, must be addressed before such technology can be used within exhaust airflows in methane-rich and coaldust-laden mine atmospheres. 

5.3 Previous work on application to large surfaces in industrial environments 

5.3.1 Scanning laser Doppler vibrometry: Numerous examples of the use of laser Doppler vibrometry (LDV) in 

analyzing the response of large structures are found in the literature and are not reviewed here. However, two recent 

studies with particular relevance to the present evaluation are noted. Scanning LDV systems (SLDV) have been 

demonstrated to be effective in identifying buried land mines. Sabatier and Xiang18 irradiated a soil- and gravel-covered 

land mine target area with pseudorandom 80- to 300-Hz acoustic waves. The mismatch in elastic properties between 

soil/gravel and buried land mines resulted in an anomalous vibration response that was detected by the SLDV. 

In the second study, large controlled defects beneath a plaster surface on a brick wall were identified by Drdacky et al.19

through excitation by forced vibration and sensing with a SLDV. Smaller defects, which have reduced flexibility, were 

more difficult to detect through excitation of the wall via mechanical shaking. 

5.3.2 Full-field interferometric techniques: Key considerations20-22 in moving full-field laser interferometric systems 

from measurements of small objects in the laboratory to large surfaces in industrial or outdoor environments are (1) 

maximization of available laser light and (2) minimization of extraneous vibration during recording. High-power lasers, 

reflective coatings, and more sensitive cameras help achieve the former, while stable optics, common path 

interferometers, short exposures using pulsed or chopped continuous wave (CW) lasers, and synchronization of object 

illumination and camera exposure with object vibration help achieve the latter. 

Several studies in which full-field techniques were applied to large surfaces and/or which sought to identify anomalous 

vibration characteristics are described below. Unless otherwise indicated, use of the general term ESPI23 (electronic 

speckle pattern interferometry) implies TV holography or the 25 to 30 frames per second recording of speckle 

interference patterns by video or CCD camera. This form of ESPI is most suitable for mine safety inspections. 

Buried land mine targets have also been detected using full-field interferometric techniques. Gaul and Plenge24 recorded 

distortions of transient elastic wave fields as they interacted with simulated land mine targets buried in sand. A 1-joule 

pulsed ruby laser illuminated a 4-m2

 area in double-exposure holographic interferometry. Both mechanical impacts and 

forced vibration (100 to 500 Hz) were used as excitation sources. Christnacher et al.25 also imaged impact-excited land 

mine targets buried in sand with holographic interferometry and pulsed ESPI. 

Several studies have examined deformation of plaster walls. Facchini22 analyzed quasi-static deformation in a 10-m2

section of a plaster wall using a CW laser with 1 to 2 W output. Shearing, out-of-plane, and in-plane configurations were 

used. A desensitized in-plane ESPI arrangement (60 micrometers per fringe) was used by Jacquot et al.26 to characterize 

quasi-static rotational movements in a similar wall. An area of approximately 12 m2 was illuminated using a CW laser 

with 800-mW output. Plaster detachments in large historical murals were identified by Gülker et al.27 using acoustic 

stimulation and ESPI. A 150-mW diode laser was used to illuminate a 2-m2

 area. Vibration frequencies were swept from 

50 to 700 Hz in 10-Hz steps. Vibration amplitudes were kept as small as 10 nm to avoid damaging the murals, and 

reference beam modulation was used to increase measurement sensitivity. Anomalous vibration areas were highlighted 

on the monitor with time-varying brightness (i.e., flickering). 
A 3-m2

 area of a reinforced concrete wall containing defects was subjected to vibration excitation and studied with ESPI 

by Facchini22 using a 1- to 2-W CW laser. Christnacher28 used a 1-joule pulsed laser to analyze deformation in a cracked, 

reinforced-concrete wall with double-exposure holography and pulsed ESPI. Surface displacements were generated 

through impact excitation. 

Vibration modes of a 3.4-m-tall liquid storage tank were recorded using holographic interferometry by Trolinger et al.29 

A 1-joule pulsed ruby laser was used at distances ranging from 6 to 20 m. Forced vibration utilized sinusoidal and white 

noise signals. When the excitation frequency was equal to a specified resonance frequency, the nodal/fringe patterns 

were reproducible. A variable fringe pattern was observed when the excitation frequency was other than a resonant 

frequency. White-noise excitation produced a fringe pattern dominated by a prominent resonant mode. 

Periodic deformation of solid earth due to tidal forces was recorded with real-time holographic interferometry by 

Takemoto.30 A 1-m2 surface area of a wall in an underground tunnel was coated with a retro-reflective substance and 

illuminated continuously with a 50-mW He-Ne laser. After the initial hologram film plate was developed and returned to 

its original position, a CCD video camera and a time-lapse video tape recorder were used to record the real-time fringe 

patterns. The conditions were observatory quality with the optical elements affixed to massive steel supports attached to 

concrete footings. Temperature, pressure, and humidity variations were minimal. An updated version of the system uses 

ESPI. 

An example of clear sharp holographic interferometry fringes obtained on a large vibrating steel railway bridge is given 

in Steinbichler et al.31

Active compensation for movements of a real-time ESPI system was demonstrated by Galanulis et al.32 The method uses 

a compact Michelson inteferometer to determine relative motion between the ESPI head and the object. This information 

is then used to modify reference phase shifting in the real-time out-of-plane ESPI system. The ability to compensate for 

instrument movement is critical to moving ESPI and SLDV systems toward being practical industrial inspection tools. 

However, when only qualitative comparisons are required, such as in delineation of spatial variations in vibration 

response, compensation may not be as critical. 

These studies document the feasibility of using full-field interferometric methods on large surfaces outside of the 

laboratory environment. Although many of these examples use holographic interferometry with pulsed lasers, the 

number of CW ESPI applications on large surfaces continues to increase.

6. DEMONSTRATION OF MEASUREMENT CONCEPTS 

A series of vibration measurements was made in the laboratory using an LDV to illustrate a few of the concepts to be 

evaluated in the underground environment. The first test contrasted the impact response of competent mine roof (Figure 

4A) with that of roof with loose rock (Figure 4B). A geophone (velocity sensor G1) was attached to the concrete model 

roof for comparison. Low-amplitude movements recorded by a geophone (G2) placed on the LDV support structure 

(Figure 4A) indicated the source of the low-frequency rumble in the LDV output. 

The second lab test illustrates a slightly different approach to sensing the presence of loose roof rock. In this case, the 

recording site was stationary and multiple impact sites were distributed about an area. As a convenient analog to roof 

sounding, we conducted a common stud-finding test by knocking on a wooden frame wall covered with gypsum board 

(Figure 5). The difference in sound when tapping at stud locations (the analogy to stiff, competent roof) and locations 

without studs (less stiff, loose rock) was not unlike the differences heard in mine roof sounding tests. In both cases, the 

differences were sometimes subtle but recognizable. Tapping on the wall directly above a stud resulted in signals (Figure 

5A) that were higher in frequency, lower in amplitude, and shorter in duration compared to impact sites between the 

studs (Figure 5B). This result paralleled the audible response. As the distance between the impact and LDV measurement 

site increased, signal amplitude decreased. However, the contrast in relative signal amplitude, frequency, and duration 

between taps at stud and nonstud locations persisted. In this test, the zones of low stiffness were well coupled with the 

rest of the structure. If impacts are made directly on a zone of low stiffness that is also decoupled from the rest of the 

structure (e.g., 
Figure 4.−Vibration response due to impact on (A) competent (simulated) roof and (B) loose roof. Signals are from laser 

vibrometer (LDV), geophones (G1, G2), and impact hammer (F). 

Figure 5.−Time-series response measured with LDV at a single site due to impacts at multiple sites. Gypsum covered wooden stud 

wall used as analog of mine roof with spatial distribution of competent and loose rocks. (A) Impact on stud (stiff competent roof) and 

(B) impact between studs (low stiffness loose rock). Similar results were observed with LDV aimed at stud and nonstud sites. 

Figure 1), then overall signal amplitudes measured away from the impact site are expected to be much smaller than 

amplitudes for impacts at competent sites. This behavior was observed in mine roof sounding tests in the field.8

 

When the LDV signals in these simple analog tests had high signal-to-noise ratios, it was fairly easy to discriminate 

between “loose slab” and “competent roof” by simple visual examination of the time series. However, as the signal-tonoise ratio decreased, it became more difficult. The same was not true with the audible signal. Thus, the LDV signal was 

converted back into an audible signal to see if the processing available in the ear and brain allowed continued 

discrimination at low signal-to-noise ratios. After amplifying the LDV output and using it to drive audio speakers, the 

stud finder test was repeated. The resulting output provided a sufficient reproduction of the sound such that blindfolded 

listeners could recognize the type of impact (stud or nonstud) with high accuracy, even with low-amplitude signals in a 

noisy environment. 
7. DISCUSSION 

Field measurements demonstrate that loose roof rocks exhibit anomalous vibrations during conventional roof sounding 

tests. The vibrations have larger amplitude, lower frequency, and longer duration in comparison to measurements made 

on competent roof. LDV and full-field interferometric techniques are well suited to delineating anomalous vibration 

conditions. To the extent that such anomalies identify hazardous ground conditions, these techniques can be used to 

recognize these hazards. The real issues are practical matters. Which method of stimulation should be used? Should 

LDV, ESPI, or holographic interferometry be used as the measurement method? How do we develop practical methods 

of inspection for the mine environment? 

Conventional film-based double-exposure holographic interferometry with a pulsed laser can produce acceptable results 

on large surfaces under any excitation conditions, but the delayed feedback of film-based processing is not a realistic 

choice for this application. On the other hand, a single-point LDV used in conjunction with a miner manually striking the 

roof meets no operational or engineering barriers, but it does not significantly improve a miner’s own ability to assess 

roof hazards. 

Selection of an optical vibration measurement technique is usually based upon on the type of excitation to be used. 

Vibrations of interest are typically inherent in the structure or easy to excite in a controllable fashion. In the current 

application, there is uncertainty as to which method of stimulation would be most effective. While roof impacts are 

clearly effective in discriminating between competent and loose roof rock, the rapid transient response limits the use of 

available optical vibration measurement methods. Ambient ground motion resulting from drilling and other nearby 

vibration sources is no doubt of sufficient amplitude and of appropriate frequency to be useful in discriminating loose 

roof rocks. However, control over amplitude, frequency, duration, and positioning of vibration sources during inspection 

is highly desirable. Forced-vibration sources provide this control. Additional work is clearly needed to characterize the 

vibration response of ambient sources as well as potential active forced-vibration sources in the mine environment. 

We briefly consider possible measurement methods to use with the different vibration sources. 

7.1 Laser vibrometry

A single-point LDV can sense and discriminate the response of a miner’s localized mechanical impact. Of all the 

potential advantages of noncontact vibration measurements, removing the miner from underneath potentially loose roof 

rock is most critical. The simple laboratory tests described here suggest that if LDV signals are converted to audible 

signals, they can be used to discriminate between loose roof rock and competent roof, just as stud and nonstud locations 

behind drywall can be discriminated. Therefore, without any sophisticated data processing, use of an LDV would allow a 

miner to be removed from beneath hazardous roof, at least for the purpose of sensing vibration response. Special 

conditions such as high roof and high-noise areas could benefit from this simple application of LDV and extend a 

miner’s ability to evaluate roof stability. 

To move toward improving and automating the inspection process, the response should be obtained over a field of view 

that is larger than that sampled by an individual roof tap. Two possible configurations to increase the sampling area 

include use of a single, fixed impact site with multiple scanned response sites and a fixed response measurement site 

with multiple impact sites (e.g., Figure 5). Use of manual or automated scans with an LDV in conjunction with a forcedvibration source could increase inspection coverage and rate of inspection. 

LDV units are now being manufactured in fairly durable portable configurations. With the handy point-and-shoot 

measurement style and the ability to determine impact response, LDV is likely to find some role in mine safety 

inspections. However, collecting vibration response data over large surfaces is likely faster with full-field interferometric 

techniques. 

7.2 ESPI 

From an inspection standpoint, the ability to visualize full-field vibration patterns using the TV holographic form of 

ESPI is very attractive. In addition, simple qualitative analyses are adequate for this application. The main practical issue 

relates to laser power and the size of the inspected area. A pulsed laser provides sufficient light to record vibration mode 

shapes on large surfaces, even those lacking retro-reflective treatments. They also produce extremely short light pulses 
capable of capturing impact response. However, their use may be limited to off-shift inspections or limited-access areas 

because of safety considerations. High cost further limits their use in many practical applications. 

Vibrations can still be recorded on large surfaces at long distances using CW lasers as reported in the studies cited 

earlier. Of course, the greater the surface reflectivity, the larger the view area. Because of the speed with which ESPI 

images are obtained, large areas can be covered rapidly with a roving CW laser system even if the individual areas are 

smaller than areas scanned with a pulsed laser. 

Forced vibration is the preferred method of excitation so that frequencies can be swept through a range that includes 

resonance frequencies of loose blocks. At resonance, the contrast between the vibration response of loose blocks and that 

of surrounding competent rock is maximized. A dynamic shearography33 configuration may allow such contrasting 

behavior to be delineated in a qualitative fashion while minimizing the influence of extraneous vibrations. 

Since ambient vibrations are omnipresent, a significant benefit would be derived from their use as an excitation source. 

ESPI’s ability to adjust fringe sensitivity23 to the level of vibration amplitude and phase provides a way of adapting to the 

diverse and time-varying character of ambient mine vibration. To take full advantage of ESPI’s flexibility requires a 

signal synchronized to the object vibration. As this signal is not generally available with ambient vibration, the analog 

output from an LDV or electromechanical transducer may be used to provide this synchronization. 

7.3 Limitations 

While numerous examples of the use of ESPI and holographic interferometry outside of the laboratory environment were 

described, these environments were quite tame compared to certain areas of active mining operations. These mine areas 

and their attendant activities are associated with elevated levels of dust, dripping water, strong air turbulence, and fog. 

The performance of optical measurement methods is adversely affected by these conditions, and these techniques would 

not be expected to be applied in such areas or during these activities. 

8. CONCLUSIONS 

Loose roof rocks associated with roof fall hazards in underground mines exhibit anomalous vibration characteristics. 

Laser vibrometry and full-field interferometric techniques are well suited to delineating regions of anomalous vibrations. 

To the extent such anomalies identify hazardous ground conditions, these techniques could be used to recognize these 

hazards. Ambient ground motions and potential sources of forced vibration excitation in mining environments need 

further characterization prior to field evaluation tests. While there are many difficulties in extending these techniques 

from the laboratory to larger surfaces in industrial or outdoor environments, the potential for success in the current 

application is enhanced by the reduced requirements of qualitative analyses. 

ACKNOWLEDGEMENTS 

The author wishes to acknowledge discussions with Professor O. Løkberg, Department of Physics, Norwegian 

University of Science and Technology, Trondheim, Norway, and Dr. J. Trolinger and Dr. D. Weber, Metrolaser, Irvine, 

CA, about the feasibility and technical details of making full-field interferometric measurements on large surfaces under 

difficult conditions. 

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