Damage Delineation in Structures Using Laser 

Vibrometry And Remote Excitation 

Peter Swanson1

1

Research Geophysicist (pswanson@cdc.gov), 

and John Rettkowski2

2

Engineering Technician 

National Institute for Occupational Safety and Health 

Spokane Research Laboratory 

315 E. Montgomery Ave. 

Spokane, WA 99207 

USA 

ABSTRACT 

As part of a research program to reduce injuries and fatalities in the U.S. mining workforce, NIOSH is investigating the potential use 

of noncontact vibration measurements to identify hazardous ground fall conditions. In the present study, the effectiveness of 

remote vibration sources in exciting resonant responses in damaged structures is examined. Initial experiments were performed on 

a damaged concrete slab in a controlled environment. Vibration response was measured with a laser Doppler vibrometer using 

both single-point and two-dimensional array recordings. Excitation sources included direct mechanical impacts to remote portions 

of the slab and a remotely located industrial demolition hammer. Surficial damage was effectively delineated with both sources

through visualization of vibration response in spectral bands selected to discriminate wave propagation from large-scale slab 

resonance effects. 

INTRODUCTION 

The unexpected fall of ground remains one of the largest sources of injuries and fatalities in underground mines [1]. Visual 

inspection, followed by roof “sounding,” are the primary means by which miners evaluate the stability of overhead rock structures in 

their workplaces. As part of NIOSH’s mission to find ways to reduce hazards to workers, the feasibility of measuring remote 

vibrations using laser techniques is being explored as a means of improving, expanding, and automating procedures for mine roof

inspection. 

With the age-old method of roof sounding, miners provide direct mechanical impacts to roof rock and listen for the hollow drummy 

sound that indicates the presence of a loose block of rock. As impact-generated vibrations interact with nearby low-stiffness 

fracture interfaces and imperfect interfaces between rock layers, loose blocks of rock resonate weakly and produce airborne sound 

through seismic-acoustic wave conversion. Removing a miner from beneath the hazard requires not only measurements of 

remote (noncontact) responses, but also remote vibration stimulation. The feasibility of delineating damaged structures through the 

combination of remote stimulation and remote (noncontact) response measurements is the focus of this investigation. For 

operational ease in developing and testing the proposed methods, a damaged concrete slab was selected to serve as a convenient 

analog to the blocky roof rock structure encountered in underground mines. 

Additional background information on previous studies of loose roof rock vibrations in underground mines, analyses of expected 

low-order resonant frequencies of slabs with varying boundary conditions, and reviews of laser Doppler and full-field vibration

techniques applied to large structures can be found in [2]. 

Techniques for detecting defects in concrete structures include impulse response (IR) [3] and impact-echo (IE) [4] testing and 

modal analysis. The impulse-response and impact-echo techniques are both essentially drive-point response measurements, as is 

the miners’ roof sounding, where a stimulus is applied and a response is measured in the same, or nearly same, location. IE uses 

a low-amplitude, high-frequency source (often a spring-loaded or solenoid-driven ball bearing) to obtain a frequency-domain 

measure of the compression wave's travel time between the outer surface and internal planar discontinuities or external boundaries 

that exhibit an acoustic impedance mismatch. IR uses a larger amplitude, lower frequency impact source (e.g., instrumented 
hammer) that stimulates more of a low-frequency global deformation response. Determination of this global response through 

identification of the structure’s natural frequencies, mode shapes, and damping forms the basis of experimental modal analysis [5]. 

Both the IR and IE methods can be used to characterize the vibration response over two-dimensional surface structures by making

measurements over a two-dimensional grid. Damaged areas, or regions requiring further investigation, can readily be recognized

through anomalies in vibration response. This same approach is applied in this study to evaluate the integrity of a concrete slab. 

However, the advantage we seek is elimination of the requirement to occupy each target site physically for both source and 

response measurements by using a single, remote, excitation point and numerous noncontact response measurements. 

INSTRUMENTATION

Vibration response was measured with a portable, battery-powered, laser Doppler 

vibrometer (Figure 1).

Figure 1.–Portable laser vibrometer, 

dynamic signal analyzer, battery pack,

and data collection computer. 

 The vibrometer (LDV) is an optical interferometer that 

measures relative motion between the LDV and target via a Doppler shift carried by 

the scattered return signal. The unit outputs analog and digital measures of ground 

velocity over a frequency range of 0 to 22 kHz with a resolution of 0.05 mm/s. Unlike 

automated scanning vibrometers, the portable LDV does not have a speckle tracking 

feature to help compensate for the poor reflective qualities of rough surfaces. Speckle 

tracking helps maintain high signal-to-noise ratios to reduce momentary signal 

dropouts [6]. To minimize the time required to collect suitable time periods of dropoutfree data, each target site was given a small area of retro-reflective treatment. 

To detect and be able to compensate for movement of the tripod-mounted LDV, an 

accelerometer was attached to the unit in line with the beam direction. The accelerometer outputs a velocity signal that is recorded with the LDV signal. However, none of 

the data presented in this paper were actually corrected for tripod motion. 

A 1.4-kg hammer with an integrated load cell was used as the first impact source. A 0- 

to 2-kHz force signal is typically generated by the hammer when equipped with a hard 

plastic tip. The second vibration source was a standard industrial-grade demolition 

hammer (5 kg) equipped with a 7.5-cm diameter flat steel bit. It was used to strike a 

2.5-cm-thick steel plate at the bottom of a 25-cm hole dug into the loose soil at a 

distance of 4 m from the edge of the slab. 

Signals were recorded with a 24-bit, 4-channel dynamic signal analyzer powered by 

the laptop data acquisition computer’s USB port. Data collection windows for the 

sources investigated in this study ranged from 5 to 10 seconds with sampling rates of 

48.8 microseconds. 

(a) (b) 

Figure 2.─(a) Concrete damage that has penetrated through to the slab’s surface, exposing more-competent material (field of 

view is 90 cm across). (b) Near-surface damage and aggregate characteristics exposed at edge of slab (penny for scale) 

CONCRETE SLAB TEST STRUCTURE

The 1.8- by 1.9- by 0.25-m concrete slab was about 23 years old at the time of the experiments. It was located on the shady north 

side of a building where most of it was exposed to snow and rain. Surficial damage was obvious in places where a thin outer layer 
had become completely detached from the bulk of the slab (Figure 2a) and exposed a deeper, more-competent, rough concrete 

surface. The thickness of the detached layer in the exposed area was up to one to two aggregate particle diameters. Figure 2b

shows a closeup of similar damage on the edge of the slab outside the measurement array. 

The relative positions of the slab, LDV, and 

two different vibration source locations (A, 

B) are shown in Figure 3. 

A

B

Figure 3.─Slab test setup with LDV and source positions A, B 

Source location B 

is positioned 4 m from the edge and 1 m 

below the top of the slab. Each test sequence utilized a single stationary source 

position and roving response measurements. The angle between the slab normal 

and direction of the LDV measurements 

varied as a function of target position on the 

slab. Consequently, uncorrected measured 

velocities contain systematic variations in 

relative amounts of vertical and horizontal 

motion. 

RESULTS

Qualitative Sounding Test 

A conventional qualitative sounding test was first used to assess the spatial 

variation in apparent slab competence. A 1.5-m-square measurement grid 

was laid out with target points spaced every 0.15 m. Grid points are 

indicated in Figure 4 as alpha-numeric row-column pairs (e.g., F6). 

 

Figure 4─Manual sounding results. 

Competent (S) sites (white symbols), 

damaged/loose (L) sites (black symbols), 

intermediate (H) response (no symbols) 

 

 Each

target position was repeatedly tapped with a wooden rod or other suitable 

object. The acoustic response was separated into three categories—S, L, 

and H. The S category corresponded to what was perceived as being 

unquestionably solid and competent. L referred to a marked low-frequency 

response, and H was intermediate where the response was perceived to 

contain relatively high frequencies in comparison to L. An H designation 

encompassed a surprisingly wide range in acoustic response. Such an 

evaluation is very qualitative and is biased by an individual’s tendency to use 

the most recent sounding measurements as a reference point instead of an 

absolute reference. S (solid) and L (loose) targets are marked in Figure 4 

with white and black circles, respectively, and H (intermediate) targets are 

not marked. 

The lowest frequency audible response (L) was observed in the vicinity of

visual slab damage. However, in places where the surface layer of concrete

was totally removed by the damage to expose a deeper, more-competent

surface, the sounding response was more intermediate. The area of slab

exhibiting predominately solid (S) response (rows A-C) was under the

protective roof overhang of an adjacent building. 

Impact Hammer Applied to Edge of Slab 

Examples of transient vibrations recorded by the LDV as a result of four sequential hammer impacts at source site A (Figure 3) are 

shown in Figure 5. J4 and J5 target locations were denoted as competent and damaged sites, respectively, in the manual 

sounding tests. The raw velocity response consists of an initial high-amplitude, high-frequency phase with abrupt first arrival 

followed by a decaying low-frequency phase. The effect of damage is manifested as an enhancement of the transient highfrequency response. 
Figure 5─LDV vibration signals from hammer 

impacts on competent (S) and damaged (L) 

targets J4 and J5. 

The velocity of propagation of the initial phase was estimated to be 

~2800 m/s from the slope of plots of measured travel time versus 

propagation distance. Such a value is less than the compressional, or 

longitudinal, wave velocities typically reported for good-quality concrete 

(3500-4500 m/s). As the predominant amount of energy recorded at the 

targets is less than 2 kHz, the longitudinal wavelengths are long 

compared to slab thickness. Therefore, the measured first-arrival likely 

represents a guided wave mode; however, a longitudinal plate wave in 

poor-quality concrete cannot be ruled out. The initial transient phase is 

herein referred to as a propagation phase to distinguish it from the 

energy of larger-scale resonant modes that continue through later times. 

Signals from sequential impacts were observed to be very repeatable at 

a given target site, but the response from target to target exhibited 

significant variation. To illustrate the spatial variation in the time domain 

response, velocity waveform profiles were obtained over two grid lines 

(column 4, row J). Each waveform in Figure 6 represents a time-domain 

averaged stack of 10 individual impacts, each with a duration of 0.04 s. 

Figure 6─Profiles showing time series for 

column 4 and row J targets for impact hammer 

at position (A) 

Note that this time window is a factor of 40 less than that shown in 

Figure 5 and therefore primarily represents the initial propagation phase. 

A general correlation is apparent between locations of significant damage, 

as indicated visually and by the sounding results, and enhanced vibration 

response. The highest amplitudes are observed in the area (J5, J6, J7) 

immediately surrounding the region where damage has breached the 

surface to reveal the more-competent concrete structure below. 

A comparison of raw velocity spectra observed at competent and 

damaged targets J4 and J5 is shown in Figure 7. 

Figure 7─Power spectra for LDV signals recorded at 

targets J4 (competent = dashed line) and J5 (damaged 

= continuous line) 

 A notably elevated 

response is observed at the damaged site, in comparison to the solid 

target, over the frequency band of 600 to 1500 Hz. To account for 

variation in the source input, frequency response, or mobility, functions 

were created using the measured excitation and response signals for the 

J4 and J5 targets (Figure 8). 

Figure 8─Mobility for targets J4 (competent = dashed 

line) and J5 (damaged = continuous line) 

The mobility shows regions of both highfrequency and low-frequency response. The 600- to- 3000-Hz band is 

largely produced by the initial transient propagation, while the 10- to 100-

Hz band is mostly due to larger-scale resonant response. As was 

apparent in the time-domain signals and raw spectra, waveforms 

measured at the J5 damage site show an elevated high-frequency 

response in comparison to the more competent J4 target. A less notable, 

but still significant, elevated response is also apparent in the lowfrequency portion of the spectrum. 
One way to visualize the spatial variation in damage is to map some measure of vibration response into a color scheme and 

superimpose it onto the grid/object. For example, the measure could be simple RMS average particle velocity values from the time 

series or various band-restricted spectral measures of amplitude, power, mobility, etc. In general, high values of these response 

measures correspond to areas that sound hollow with tapping, and low values correspond to areas that sound most competent. 

Figure 9a shows such a map produced using power spectral density (PSD) integrated over a range of 10 to 5000 Hz. The 

maximum spread in response values in each image of Figure 9 is mapped into the full color spectrum. The results from the manual 

sounding map from Figure 4 are also superimposed on Figure 9a and reproduced in Figure 9b. 

Figure 9─Vibration response due to impact hammer excitation. (a) Summation of PSD over 

10 Hz to 5000 Hz, (b) correlation with manual sounding results, (c) summation over 200- to 

5000- Hz, (d) summation over 10- to 200-Hz band. 

The area of highest vibration response is not found at sites of greatest damage. The area of greatest damage, where the surface 

has been breached to reveal the more-competent underlying structure (H5, I5-7), exhibits an intermediate vibration response. 

Immediately adjacent to this area is a region that displays no obvious visual signs of damage, yet has the greatest velocity 

response (e.g., rows I-K). This region is interpreted as an area where delamination has taken place, but the damage has not yet 

breached the surface. 

The region above the area of greatest damage (rows F-H), which contains L response targets from manual sounding, does not 

exhibit as strong an anomalous vibration response. The reason for this discrepancy is likely attributable to bias in the qualitative 

and cursory sounding measurements and points to the need for independent measures of material damage. 

A broad area of elevated response is also observed over the entire lower right-hand corner of the slab (Figure 9a). From the time 

series (Figure 6), it is quite apparent that the elevated response in this area is low frequency in nature. By integrating the power 

over different passbands, we can largely isolate the propagation phase (200 to 5000 Hz) (Figure 9c) from lower-frequency slab 

resonance modes (10 to 200 Hz) (Figure 9d). Corrections for geometric spreading, attenuation, and source radiation pattern were 

not applied to any of the data in this paper. The high-amplitude, low-frequency velocity response is enriched in the 30- to 40-Hz 

band compared to other slab quadrants. From its lack of symmetry, it is not thought to represent a typical low-order resonance

mode in a uniform plate. Possible explanations for this anomalous response include (i) the slab is not uniformly supported on, or 
attached to, its underlying foundation, (ii) the foundation material is more compliant in this corner, (iii) slab thickness is reduced, (iv) 

large-scale delamination yielding an effective reduced slab thickness, and (v) reduced confinement at slab edges in this corner. 

Several of these explanations (i, ii, v) may apply as the slab is on a steep hillside with bedrock exposed, and the high-response 

area of the slab lies on the outer downhill corner. 

By processing response data over different spectral bands, different scales of anomalous dynamic response can be identified and

analyzed with respect to structural health and performance. 

Demolition Hammer at Remote Soil Location 

The demolition hammer source (Figure 10) was applied at source position B (Figure 3) for a 

duration of 5 s for each LDV target position. 

Figure 10─Demolition 

hammer vibration source 

Raw velocity spectra from the J4 and J5 target 

positions are shown in Figure 11. 

Figure 11─Vibration response at solid (J4, dashed line) 

and loose (J5, continuous line) targets due to 

demolition hammer excitation at source position (B) 

The 43-Hz percussion frequency is clearly seen in the

dominant 43.6-Hz spectral peak along with its associated harmonics. Notable elevation in 

velocity response over the frequency range of 200 to 2000 Hz was observed at J5 and other 

damaged target sites in comparison to competent targets such as J4. 

Power spectral density values across the measurement grid were integrated over the 200- to 

2000-Hz frequency band and mapped into a color scheme (Figure 12). 

Figure 12─Vibration response (200 to 2000 Hz) 

due to demolition hammer excitation 

Good agreement with 

the results shown in Figure 9c is apparent. Exceptions are evident at certain damaged target 

positions. These positions experienced additional permanent damage during a series of direct 

target impact tests conducted in between the impact-hammer and demolition-hammer tests. 

These direct impact response tests are still being analyzed and will be reported elsewhere. The 

measured vibration amplitudes produced by the demolition hammer source were significantly 

lower than those produced by the impact hammer. As a result, analysis of the low-frequency 

response (10 to 200 Hz) did not reveal the same broad asymmetric response pattern in the 

lower right corner as was observed with the demolition hammer. Other sources of vibration, 

such as tripod motion, dominated and obscured any such pattern if present. A problem that 

developed with the LDV-mounted accelerometer data during these tests did not allow 

compensation for tripod motion. 

SUMMARY AND CONCLUSIONS

A series of experiments was performed on a concrete slab to (i) determine the suitability of laser vibrometry to identify anomalous 

dynamic properties indicative of material/structural damage and (ii) determine whether remote vibration excitation sources are able 

to stimulate such anomalous responses. Vibration sources included an instrumented impact hammer striking the edge of the slab 

away from target locations and an industrial demolition hammer striking a metal plate in soil several meters away from the slab. 

Responses were measured with a portable, tripod-mounted laser vibrometer manually scanned across a two-dimensional grid over 

a range of several meters. Due to the poorly reflecting surface set at oblique angles to the beam, a reflective treatment was used 
on target locations to maintain a sufficient signal-to-noise ratio to minimize signal dropouts. The speckle tracking feature of 

automated scanning vibrometer systems and/or signal processing methods have not yet been evaluated to determine to what 

extent this barrier to more practical applications can be overcome. Spatial variation in vibration response was analyzed by 

overlaying two-dimensional velocity response maps over the structure image and comparing the results with a similar map of 

qualitative sounding measurements. 

The LDV response measurements from the impact hammer tests were loosely interpreted in terms of a propagation phase and a 

resonance phase, where the initial elastic transient provided a relatively high-frequency excitation compared to the later arriving, 

lower frequency modal response. By separating the vibration response into low- (10 to 200 Hz) and high- (200 to 5000 Hz) 

frequency bands, two types of anomalous vibration signatures were observed. Surficial material damage was delineated in the 

high-frequency band. In the low-frequency band, an area of unusually high response was observed that may reflect an anomaly in

slab foundation support. High-frequency response maps obtained with the impact and demolition hammers were in agreement and 

largely consistent with qualitative sounding results and areas of damage. 

ACKNOWLEDGMENTS 

The authors would like to acknowledge the able assistance of Andy Caley (NIOSH) in both collecting the field data used in this 

study and in performing experiments during the course of technique development. 

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