EVALUATING TECHNIQUES FOR MONITORING ROCK FALLS AND SLOPE
 

STABILITY
 

Edward L. McHugh and Jami M. Girard
 

Spokane Research Laboratory, National Institute for Occupational Safety and Health, Spokane, WA
 
ABSTRACT 

While less than 1% of reported accidents are associated with slope 

stability problems, slope failures were responsible for about 15% of 

all U.S. surface mine fatalities between 1995 and 2001. Small 

rockfalls, which may involve hand-sized rocks weighing only a few 

kilograms, can cause fatal injuries to workers away from the 

protection of large machinery. Massive highwall failures containing 

a million cubic meters of material or more can be fatal even for 

heavy equipment operators. As part of an ongoing study at the 

Spokane Research Laboratory of the National Institute for 

Occupational Safety and Health, several remote-sensing technologies are being evaluated as tools to monitor slopes for hazards and 

to assess slope stability. Field tests of a hyperspectral imager were 

conducted to assess its value for improving geologic maps of potentially unstable alteration zones on mine slopes. An interferometric 

radar device capable of detecting very small displacements on 

slopes awaits final assembly and field testing. Computerized monitoring methods using images from digital and video cameras are 

being assessed for application to mine slope surveillance. These 

and other techniques will eventually provide new tools to augment 

current methods for monitoring ground control hazards in mines. 

INTRODUCTION 

The mission of the Spokane Research Laboratory, National Institute 

for Occupational Safety and Health, is mine safety and health 

research. A current project involves enhanced recognition of slope 

stability hazards at surface mines. Since 1995, 34 miners have died 

in slope failure accidents at surface mines in the United States. 

Shovel operators and drillers suffered the greatest numbers of 

fatalities. Fatal slope failures range from small, hand-sized rocks 

weighing only a few kilograms to massive highwall failures containing a million cubic meters or more of material. Small rockfalls 

generally cause fatal injuries to workers on foot who are away from 

the protection of large machinery. Large rockfalls are responsible 

for fatal injuries to virtually anyone who ventures under or on top 

of unstable slopes, including the operators of large equipment. 

A principal objective of the project is to improve the means for 

detecting conditions in open-pit mines that could lead to catastrophic slope failure. Three types of remote sensing technology—hyperspectral imaging, interferometric radar, and photographic image analysis—are being developed as tools to help recognize 

potential hazards at mine sites. These technologies have been 

developed mainly for airborne or satellite instruments, but their 

spatial resolutions are not normally sufficient to be very useful in 

addressing local ground control problems in surface mines. Adaptation and simplification of these technologies for use with groundbased platforms could provide a cost-effective means to reduce 

injuries from the catastrophic failure of mine slopes. 

HYPERSPECTRAL IMAGING 

Every material on or off the Earth’s surface reflects light in a 

characteristic pattern; the manner in which light of different 

wavelengths is reflected or absorbed from each material is known 

as its reflectance spectrum. By filtering reflected light to specific 

wavelengths (colors in the visible part of the spectrum), images can 

be created that enhance our ability to differentiate materials. 

Multispectral imaging makes use of a few broad wavelength bands 

in the electromagnetic spectrum, primarily visible and infrared. 

Hyperspectral imaging refers to obtaining reflectance spectra for the 

region being imaged over a large number of discrete, contiguous 

spectral bands. A contributing factor in many highwall failures is 

the presence and distribution of mechanically incompetent, clayrich altered rock in pit walls. Most alteration minerals have 

characteristic absorption features that can be recognized with hyperspectral imaging and thus can be used to help identify these 

weakened, altered rocks. Hyperspectral imaging has been shown to 

provide objective information for mineral identification and to help 

in the inherently subjective process of geologic mapping. 

Field instruments are commercially available that can analyze 

light reflected from a single point within a scene, but an imaging 

system can record spectral reflectance data for an entire scene, 

adding a spatial dimension to the data. High-resolution spectral 

imagery can assist in remotely compiling objective geologic maps 

of mine highwalls and other hazardous slopes (1). A ground-based 

spectral imager could provide— 

C An alternative to high-cost, logistically complex airborne and 

satellite spectral imagery, 

C More capability than nonimaging (point by point) 
spectrometers currently being used to augment mapping, and 

C Flexibility in timely acquisition of images at various scales and 

view angles. 

The output of a hyperspectral imager is a stack of images over 

a spectral range, referred to as an image cube, where the image is in 

two spatial dimensions plus a third, or spectral, dimension 

identified by its wavelength (figure 1). A radiant energy value is 

recorded for each data point (pixel) in the image for every 

wavelength sampled so that a spectrum is collected for each pixel 

in the image. 

Spectro-Polarimetric Imager 

Field trials were conducted of a prototype ground-based spectral 

imager developed by Denes at Carnegie Mellon Research Institute 

(2). The instrument collects spectral images at visible and nearinfrared wavelengths to generate a data cube that can be processed 

by standard hyperspectral techniques. An acousto-optical tunable 

filter (AOTF) produces a diffracted beam at the specified 

wavelength that is passed to a monochrome charge-coupled device 

(CCD) camera. An external host computer controls the filter and 

collects an image at each selected wavelength through a framegrabber board (3). A liquid-crystal, variable-phase retarder provides optional polarimetric control for the imaging system. 

Software in the host computer sets the spectral range, spectral 

bandwidth, and polarization parameters for data collection. The 

controller can switch at 30 microseconds per spectral band over 

wavelengths of 450 to 1000 nm, normally at 10-nm intervals. In 

sweep mode, the AOTF sequences through the full spectral range 

of the instrument, collecting the maximum amount of information 

in a scene. In switching mode, the instrument alternates between a 

small number of parameter settings for quick collection of specific 

bands for the sampled scene. The switching mode can be programmed to obtain band ratio images in real time. 

Data Collection and Preprocessing 

A test site at a surface mine in California was selected because the 

diagnostic spectral features of its rare-earth mineral assemblage are 

in the spectral range of the present instrument. Twelve hyperspectral images were collected, including pit highwalls, outcrops, 

drill core from the ore zone, and hand samples. Example images are 

shown in figure 2. Additional images of hand specimens under 

artificial light were collected in the laboratory. These images were 

used as an aid in calibrating and evaluating the instrument. Thin 

sections were made of both ore and host rocks to assist in 

interpreting spectral images. 

The mineral deposits occur as carbonatite veins associated with 

potassium-rich igneous rocks that intruded a block of Precambrian 

metamorphic rocks. The carbonatites, apparently of magmatic 

origin, consist mainly of calcite, dolomite, ankerite, and siderite. 

Bastnaesite, a fluorocarbonate of lanthanide series metals and the 

principal rare-earth-bearing mineral present, constitutes 5% to 15% 

of much of the ore body and locally exceeds 60% (4). Rare-earth 

elements of the lanthanide series (cerium, lanthanum, neodymium, 

europium, and others) produced at the mine have been used in 

petroleum- and pollution-control catalysts, specialty glasses and 

magnets, and television and computer monitor phosphors. 

Diagnostic spectral characteristics of the rare-earth assemblage 

correspond to published spectra of neodymium oxide, with at least 

eight distinctive absorption features between 450 and 1000 nm. 

Spectral reflectance data for the deposit were published by Rowan 

and others (5). 

The data consist of 47 bands from 480 to 940 nm in 10-nm 

intervals each collected at four polarizations. Data preprocessing 

and most of the subsequent analysis was done using basic 

procedures from ENVI software (Research Systems, Inc., Boulder, 

CO).1

1

Mention of specific products and manufacturers does not imply endorsement by the National Institute for Occupational Safety and Health. 

 For each scene, original raw data comprising 190 bitmap 

files were compiled into data cubes. Original data sets consisted of 

640- by 480-pixel images. Each analyzed scene contained a white 

card and a black card that were used to correct for path radiance and 

to normalize the data to relative reflectance. 

For this study, reflectance spectra of rare-earth ore from the 

same site were measured to evaluate the images. The spectra were 

collected with a full-range spectroradiometer (Analytical Spectral 

Devices, Inc.) at 1-nm resolution and resampled to 10-nm 

resolution. These profiles had very good agreement with published 

spectra and U.S. Geological Survey library profiles for neodymium 

oxide (6). The most diagnostic absorption features occur at 

approximately 740, 800, and 870 nm. 

Data were compiled into image cubes, and basic analysis was 

done for each of the scenes. A sampling of single pixels from the 

resulting data cube showed numerous spectral profiles that are 

related to the expected rare-earth mineral assemblage. Spectra in 

figure 3 show a range of correlation with the spectra measured from 

bastnaesite ore with the spectroradiometer. Some of the profiles 

match nearly all the characteristic absorption features for the ore, 

and several show the asymmetry at the 790- to 800-nm feature. An 

artifact in the images is a set of near-vertical to diagonal light and 

dark stripes apparently resulting from radio frequency (RF) noise at 

the AOTF boundary. 

Image Analysis 

A first classification of the normalized images was based on two 

endmembers defined as regions of interest for the ore zone (lower 

portion) and host rocks (upper portion) within the image. The 

result was quite reasonable, leaving about 20% of the image 

unclassified. A cross-track illumination correction was applied to 

the original images to reduce the effect of RF striping and an 

apparent x-axis gradient in the original images. The corrected 

images yielded an improved classification, including larger areas of 

ore endmember correlation in the left side of the image (figure 4). 

Several other classification methods and settings, including 

noise reduction transformations and predetermined matched 

filtering analyses, showed no particular improvement. A pure pixel 

analysis using endmember spectra from the image resulted in classification of only very small areas within the scene. 
Figure 1.—Hyperspectral image cube showing relation to spectra; top image is a 

false-color composite of rocks and vegetation. 

Figure 2.—Imager examples showing (A) drill core, (B) vegetation and outcrop, (C) 

rock hand samples, and (D) wall in main pit. 
Figure 3.—Sampling of spectra from main pit west wall that 

correlate with bastnaesite. 

Figure 4.—Classification of pit wall image using spectra 

derived from image and cross-track illumination correction; 

white areas are classified as ore. 

Basic analysis was also done on hyperspectral images collected 

under artificial illumination in the laboratory. The results show that 

the method can be used to characterize scenes at different scales 

where pixel dimension is controlled by magnification and distance 

to the object. The images in figure 5 show mineral samples from a 

distance of about 3 m. 

Figure 5.—Classification of rock sample in the laboratory; 

spectra from a region of bastnaesite mineral specimen 

(black spot) were used to classify scene. Bastnaesite is 

shown as white areas in both mineral specimen and mixed 

ore sample. 

An averaged spectrum from the elliptical 

“region of interest” highlighted on the small specimen of nearly 

pure bastnaesite on the left of each image was used to classify the 

scene using the matched filtering routine. Bastnaesite in both the 

mineral specimen and the mixed ore sample on the right are 

highlighted. 

The field tests clearly illustrated the capabilities of the method 

for collecting hyperspectral images and discriminating materials, 

validating the concept of remotely characterizing the mineral 

composition of mine slopes. However, the spectral range of the 

instrument tested limits its application in identifying geologic 

materials. In addition, the signal-to-noise ratio in the data limited 

detailed classification of the images. Ground truthing and feature 

extraction based on image-derived spectra were essential for reliable 

mapping of geologic samples and scenes. 

A spectral imager in the 1.0 to 2.5 micrometer part of the 

infrared spectrum, where spectral features for clays and other 

minerals are more distinctive, would be more useful for mapping 

clay-altered rocks. Such an imager would assist in creating 

objective geologic maps of mine highwalls and other hazardous 

slopes from a safe position as well as help define faults, shear 

zones, and fracture systems in mine slopes and characterize drill 

core, rock chips, and hand samples in the field and in the laboratory. Thermal infrared imaging offers some promise of helping to 

distinguish geologic structures in mine walls. Differential cooling 

that results from water saturation levels and flows in wall rocks 

could be mapped to reveal geologic faults, contacts, and altered 

zones. Research will be conducted in this area with ground-based 

instruments. 

INTERFEROMETRIC RADAR 

The Spokane Research Laboratory has supported development of a 

ground-based radar at Brigham Young University (BYU). The 

technology stems from research efforts by Long at BYU to build a 

relatively low-cost interferometric synthetic aperture radar (InSAR) 

that can be deployed in light aircraft and operated at low altitudes. 

Project funding facilitated field tests of a ground-based version of 

the radar system to detect and measure displacement on mine 

slopes. Displacement measurements will be used to track mass 

movement of failing slopes in surface mines and possibly to warn 

of imminent catastrophic collapse. 

Synthetic aperture radar (SAR) uses electronic techniques to 

create a very long virtual antenna by processing pulsed signals from 

a real antenna as it moves past a target area. Because the width of 

the radar beam is inversely proportional to the length of the transmitting antenna, this long virtual antenna allows high-resolution 

imagery. SAR systems in airborne or earth orbit platforms have 
been used to generate high-quality digital elevation maps of the 

earth’s surface. InSAR techniques compare images collected at 

different times but at nearly the same viewing angle to map 

displacements in the time-lapse SAR images. The technique has 

been used to map ground displacement, at resolutions measured in 

millimeters caused by natural hazards such as earthquakes, volcanic 

activity, and landslides (7-8), and subsidence caused by the extraction of ground water, oil and gas, or minerals (9-11). These 

pioneering studies have generated enormous interest in the earth 

science community because they point to an entirely new way to 

study the surface of the earth. SAR data from the Earth Resource 

Satellite-1 (ERS-1) have recently been used to measure ice stream 

velocity in Antarctica, co-seismic displacement associated with the 

Landers and other earthquakes in California, and volcanic deformation on Mt. Etna that may be preparatory to a major eruption. This 

is the first time that the velocity or displacement fields associated 

with these phenomena have been captured with virtually complete 

spatial coverage. 

The compact InSAR developed at BYU’s Center for Remote 

Sensing is intended to make imaging radar inexpensive enough for 

small-scale mapping projects (figure 6). 

Figure 6.—InSAR image of urban Logan, UT, area. 

The system was funded in 

part by a NASA grant (12) and has been used recently in 

collaboration with the U.S. Geological Survey to periodically 

collect images of a landslide area in Colorado. 

Prism-based survey networks, extensometers, and other 

standard instruments sample movement only at discrete points on 

a slope. These instruments are effective in very accurately recording both the direction and magnitude of displacements, but sample 

points need to be strategically placed where motion will occur. 

Displacements between sample points may be underestimated or 

missed altogether. One answer to this undersampling problem 

when covering large areas is a new generation of scanning laser 

instruments that can generate three-dimensional models of mine 

slopes without reflector prisms. Laser scanners have been used 

successfully in large surface mines, but their range and accuracy can 

be impaired by differences in the reflectivity of the rock, the angle 

of the rock face, weather, or dust. Manufacturers of prismless range 

finders generally claim a range of 500 m or less. Radar can cover 

large surface areas for true two-dimensional monitoring. In 

addition, radar is able to function day and night in almost any 

weather condition, and atmospheric dust or haze have little effect. 

Radar’s active transmit/receive mode of operation provides an 

advantage over passive optical methods that depend on solar or 

other illumination. 

A slope monitoring radar capable of detecting hazards needs a 

much shorter repeat time and better ground resolution than airborne 

and satellite systems can provide. Ground-based interferometric 

radar systems have been designed to monitor displacement of 

unstable slopes: a two-dimensional scanning system was proposed 

to monitor slopes above highwall coal mine operations in Australia 

(13); a ground-based system has been integrated with other devices 

in a landslide monitoring network in France and has been used for 

temporary monitoring of active volcanoes (14). 

Natural Hazards Radar System 

BYU’s Center for Remote Sensing developed a stationary groundbased natural hazards radar system capable of detecting and 

measuring slope movements. Assistance by NIOSH led to a variation of the system for use in monitoring mine slopes (15). Although 

radar technology has been widely used for the last 50 years, only in 

the last 10 years have advancing computer capabilities provided the 

low-cost processing power needed for interferometric computation. 

The widespread adoption of modern wireless communications 

devices has resulted in practical integrated circuits for microwave 

frequencies that can be used in task-specific radar applications. The 

BYU instrument evolved from one designed for the Canadian 

National Railroad to test the application of interferometric radar to 

the detection of rock falls and washouts along rail corridors (16). 

The system consists of a radar transmitter, two receivers, three 

antennas, and a computer controller (figure 7). The transmitter 

produces a 10-GHz pulse with a 300-MHz linear-frequencymodulation (LFM) waveform. In an LFM or “chirp” waveform, 

frequency is increased linearly over the duration of the pulse. The 

frequency difference between the transmitted pulse and the 

returning echo captured at the receiving antennas is proportional to 

the distance traveled to the target. These difference frequencies can 

be divided into range cells with a resolution of 2 to 3 m at a range 

of 300 to 400 m. Interferometry is employed to refine this distance 

measurement by using two identical receive antennas a short 

distance apart. By comparing the phase difference in the received 

signal at each antenna, precision of the measurement in each range 

cell is narrowed to a fraction of the signal wavelength (about 3 cm 

at 10 GHz). With a 7° beam-width transmitting antenna, the system 

has a 12-m footprint at a range of 100 m. 
Figure 7.—Stationary array, displacement detection radar 

Field Tests 

Initial tests of a prototype interferometric instrument showed that 

the technology is workable at relatively low cost. One technical 

obstacle was to generate a sufficiently stable and linear LFM signal. 

A new instrument will include a fully coherent oscillator and an 

improved chirp transmitter to maximize long-term stability. In a 

second approach, the airborne SAR system was modified to 

function as a fixed differential phase monitor for experiments in 

detecting mine slope displacements. Outdoor tests are being 

conducted with this instrument, and slope monitoring experiments 

at a suitable mine site are planned for spring of 2002. 

PHOTOGRAPHIC IMAGE ANALYSIS 

Digital camera and computer technologies provide tools to derive 

far more information from images than was possible just a few years 

ago. Small differences between pairs of images can be readily 

detected, changes can be quantified in pixel counts or area 

percentages, and images are time stamped for easy sequencing and 

animation. These capabilities can be used to enhance mine slope 

monitoring. McVey and others (17) used a 35-mm camera and 

carefully positioned reflectors to measure deformation over time in 

an underground mine. Processed film was used to measure 

deflections to a resolution of 0.5 mm, but the use of reflectors adds 

substantial complexity to the installation process and limits analysis 

to sites with reflectors. Corthésy and others (18) described a differencing technique using before and after digital images to detect rock 

displacements in an underground mine. The results were similar to 

those described here except that the use of artificial illumination in 

the underground environment may simplify noise filtering. 

Photographic change detection can be used in a time lapse 

mode to record such things as bench loading, fracture development, 

creep and mass movements, and fallen rock sources. A real-time 

slope monitoring system using low-cost video cameras can be used 

to generate rock fall warnings where workers could be at risk. 

Photographic Change Detection 

High-resolution digital images collected at regular intervals or 

before and after events can be sequenced and compared to determine total displacement and rates of change. The human eye is 

highly adapted to detect movement, allowing change between 

images to be readily seen simply by flipping from one to the other. 

Sequences of images can be viewed as animations that depict scene 

changes from beginning to end. Computer analysis can begin with 

one of several relatively inexpensive image-processing software 

packages (e.g., Adobe Photoshop, JASC Paintshop) that provide 

layering or mathematical operators for image manipulation. In a 

subtraction or differencing operation between two nearly identical 

images, intensity values for corresponding pixels from each image 

are subtracted, resulting in a low value (dark) if the pixels are the 

same in the two images. For any pixels that have different intensity 

values between the two images, i.e., if there is a change, the result 

will be bright. 

A change-detection experiment demonstrated one method of 

using photographic techniques to identify and characterize rock falls 

on mine slopes. A rock fall event was set up by positioning rocks 

at the top of a highwall. As the rock fall was triggered, a sequence 

of images was collected using a digital camera in time-lapse mode. 

The first image in figure 8 shows the rocks at the top of the highwall, the second image shows the rocks falling, and the third shows 

the difference image with a threshold adjusted to enhance changes. 

Movement of vegetation in the wind caused the noise at the top of 

the difference image. 

The digital camera used to collect these images will capture an 

image in standard time-lapse mode at intervals as short as 1 min. 

This repeat time is too slow to record details of rock falls. Capture 

rates as fast as 2 sec/frame are possible, but built-in processing time 

for each image before another can be collected may result in events 

being missed. This method is best suited to long-term monitoring 

where images might be collected once a day or less often. 

Photographic change detection requires that image sequences 

be nearly exact in position and view in order to reveal changes. The 

most reliable technique is to secure the camera in position and move 

it only after all images have been collected. However, software is 

available that can process images that were collected from slightly 

different camera positions (19-21). The process involves spatially 

adjusting one image to match scale, rotation, and parallax differences of a second image. Fiducial or ground control points in the 

two images are inserted to control the registration process (figure 

9). The adjusted image can then be compared visually or with a 

commercial photo editor. 
Figure 8.—In a rockfall experiment, rocks positioned at the top of a highwall are shown in the left image, the rocks falling 

are shown in the middle image, and the difference image with a threshold adjusted to enhance changes is shown on the 

right. 

Figure 9.—Fiducial points used for spatial registration of 

image pairs 

It is clear that the digital camera will work quite well for timelapse monitoring of slopes, but video cameras will likely be a better 

choice for warning of slope movements in hazard zones. 

Real-Time Monitoring 

Real-time monitoring of a mine slope to avoid injuries from falling 

rocks requires a very fast response time, so that a worker at risk is 

given enough time to get out of the way or find cover. Surveillance 

devices are available that can monitor a specified area and detect 

motion in a video image. These devices were developed in the 

security industry and are designed to signal an alarm when motion 

is detected and record images of the scene as long as motion is 

taking place. This technology is being tested for application to 

mine slope monitoring. 

A basic surveillance system built around hardware and software 

from Strategic Vista, Inc., Markam, ON, was tested in the laboratory and in limited field trials. The system includes a color 1/3-inch 

(0.85-mm) CCD video camera with 480 lines of resolution, automatic iris, and a power zoom lens. The camera is connected by means 

of an external frame grabber to the USB port on a laptop computer. 

The computer, with a Windows 98 operating system, runs programs 

that display real-time and recorded video images. 

The system can be set to record video frames and sound an 

audio alarm when motion is detected. Detection is accomplished by 

algorithms that monitor the intensity value of each pixel in the 

scene. When sufficient change is detected, video frames containing 

the motion are recorded to the computer hard drive and the alarm is 

activated. Sensitivity of the trigger is adjustable from 0% to 100% 

of the view area. Captured images are 320 by 240 pixels. A region 

can be selected within the field of view where motion will trigger 

the record and alarm functions; this allows motion to occur in other 

parts of the view without triggering unwanted alarms. This system 

can also be configured to view the scene remotely through an 

Internet connection and to dial a designated telephone number. 

Laboratory tests showed that motion is readily detected and 

recorded. During a continuous test over a 40-hour period, more 

than 100 individual events triggered the system, capturing a total of 

15.6 min of video at about four frames/sec. Individual events 

ranged from less than 1 sec to more than 3 min in duration. The 

resulting files totaled 26.7 Mb of hard drive memory. Other tests 

showed that very slow motion within the detection area would not 

trigger the alarm and that when detected motion stopped, the alarm 

would cease even though elements within the scene had changed 

from the original image. 

Field tests in a local gravel pit showed that rock fall events 

could be captured by the system (figure 10). The system was set up 

to monitor highwalls between10 and 17 m high at ranges of 30 to 

80 m. Rocks at least 15 cm were dropped from the top of the slopes 

to generate minor rock falls. The dropped rocks and rock falls were 

sufficient to trigger the detection system, sounding an audio alarm 

and recording images of the rock fall. In one 13-min interval of 

continuous monitoring, seven motion events were detected, 

including each of three induced rock falls. The rock fall events 
lasted from 3.1 to 4.0 sec. Event times and durations are shown in 

figure 11. 

Figure 10.—Spokane Rock Products pit and video changedetection camera 

 

Figure 11.—During this 13-min interval, about 25 sec of video 

was recorded for seven motion events that included three 

rock falls, each lasting less than 4 sec. In addition to warning workers in the vicinity of hazardous 

rock slopes, recorded video images will allow safety investigators 

to reconstruct a rock fall and help identify areas of unstable ground. 

One big advantage of motion-triggered video monitoring is that 

only short intervals of recorded images need to be reviewed in the 

process. One problem with the current setup is that the laptop’s 

LCD screen is difficult to use in bright outdoor light. The cameras 

need to be suitable for use in bright to dim lighting conditions and 

in adverse weather. 

CONCLUSIONS 

A portable ground-based hyperspectral imager in the 1- to 2.5­

micrometer range could provide objective information to assist in 

the inherently subjective process of geologic identification and 

mapping. It would be a higher-resolution alternative to expensive 

airborne and satellite imagery. Such an instrument would have 

more capability than the nonimaging (point by point) spectrometers 

currently used to augment mapping. However, a practical field 

instrument is not available. Optical components in the required 

wavelengths are costly, and development awaits adequate funding. 

Ground-based interferometric radar has the potential of 

measuring displacements over large areas of mine highwalls at 

unprecedented resolution. The technology has been deployed at 

some locations. Development will continue, and practical systems 

are likely to be expensive but available in the near future. 

Digital and video cameras have already proven valuable for 

recording mine slope conditions. Computer tools using time-lapse 

and motion sensing methods can provide means to document slope 

failures and warn workers of rock falls. Basic development work 

will look to streamline the framing, capturing, and processing of 

digital images. 

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