Electronic Distance Measurement Research Articles

Absolute distance measurement system using a femtosecond laser as a modulator

The generation of broadband microwave frequency comb from a femtosecond pulse train by direct photodetection opens the possibility for high-accuracy length measurements of long distances. We demonstrate a relatively simple realization of this measurement principle: an electronic distance measurement system based on a time-of-flight approach, driven by a femtosecond fibre laser source as a modulator. By the evaluation of the phase shifts of two distinct comb frequencies, a coarse and a fine measurement of the absolute distance can be performed. The range of the measurement system is demonstrated up to a length of 100 m. The experimental comparison of the femtosecond laser system with a conventional reference counting interferometer shows a precision better than ±10 µm at 100 m, corresponding to a relative measurement uncertainty of 1 × 10−7 L. The limiting factors for the measurement uncertainty of the system are theoretically investigated and shown to be of the same order of magnitude.

Thermal expansion-contraction and slope instability of a fumarole field inferred from geodetic measurements at Vulcano

Between 1987 and 1993, fumarole temperatures at the Fossa crater of Vulcano (Italy) were characterized by the highest values since the 1920’s, increasing from about 300°C in 1987 to 690°C in May 1993, before decreasing to 400°C by 1996–1997. During 1990, Vulcano’s Electronic Distance Measurement (EDM) network was expanded to provide more detailed coverage of the northern sector of the Fossa crater and, in particular, to monitor the movement of the northern flank the Fossa cone. Measurements, carried out between 1990 and 1994, showed shortening by about 6 to 7 cm along baselines measured to a small section of the northern rim. Over the following four years these baselines showed a slow extension by about 3 cm, to gradually recover part of the previous deformation. We believe that the shortening and lengthening of the EDM baselines was respectively due to the increasing and decreasing temperature of the rock body lying close to the deforming area. This caused thermal expansion, followed by contraction. The positive movement of the rim was not completely matched by a negative recovery, suggesting that a non-recoverable sliding movement was also responsible for some of the shortening of the baselines. We verified our hypothesis by calculating the expected dilatation of a rock body, as a function of the volume of rock heated and its thermal expansion coefficient, and compared the expected deformation to that observed. The geodetic investigation showed that the unstable portion affects a small length of the rim (about 100 m long) and involves a volume of about 0.8 × 106 m3. However, this zone lies directly above a particularly unstable portion of the flank, as well as the main village and port on the island.

Spatio-temporal distribution of magma intrusions inducing repeated earthquake swarms off the east coast of the Izu peninsula, central Japan, for 1973–1998

A time-dependent model for magma intrusion associated with repeated earthquake swarm activities off the east coast of the Izu peninsula was developed from precise levelling, electronic distance measurements (EDM) and global positioning system (GPS), and sea level observation data for the period 1973–1998. Significant deformations were observed in the inter-swarm period, suggesting the existence of a preparatory process for repeated earthquake swarm events. The geometry of a dyke-like tensile crack at a depth of 13 km from deformation detected was estimated by precise levelling and GPS for the inter-swarm period, using a genetic algorithm. A large crack plane was considered; it had a combination of estimated deep tensile crack with a shallow dyke detected using the hypocentre distribution and calculated time-dependent inversion, which was carried out to estimate the temporal changes in the volumes of the crack in the period 1973–1998. Akaike's Bayesian information criterion (ABIC) was used to choose the smoothing hyperparameters for the inversion. The characteristics of the estimated temporal change in magma volume were as follows. A rapid and large magma intrusion occurred at a depth of about 15 km between 1976 and 1977. Despite a decrease in its expansion rate, a tensile crack at a depth greater than 8 km continued to inflate between 1978 and 1988. The swarm activity started just one year after the rapid and large intrusion around the 15 km depth. In addition, the tensile crack at the depth of about 15 km beneath the internal region of the eastern Izu peninsula expanded in the inter-swarm period and deflated in the swarm period. This result suggests that magma was temporarily reserved in a dyke-like body beneath the eastern part of the Izu peninsula at a depth of 15 km in a preparatory process for the earthquake swarm. These features strongly show that estimating the spatio-temporal distribution of this deep crack provides useful information for long-range predictions of volcanic earthquake swarms off the east coast of the Izu peninsula.

Ultra-wideband ranging precision and accuracy

This paper provides an overview of ultra-wideband (UWB) in the context of ranging applications and assesses the precision and accuracy of UWB ranging from both a theoretical perspective and a practical perspective using real data. The paper begins with a brief history of UWB technology and the most current definition of what constitutes an UWB signal. The potential precision of UWB ranging is assessed using Cramer–Rao lower bound analysis. UWB ranging methods are described and potential error sources are discussed. Two types of commercially available UWB ranging radios are introduced which are used in testing. Actual ranging accuracy is assessed from line-of-sight testing under benign signal conditions by comparison to high-accuracy electronic distance measurements and to ranges derived from GPS real-time kinematic positioning. Range measurements obtained in outdoor testing with line-of-sight obstructions and strong reflection sources are compared to ranges derived from classically surveyed positions. The paper concludes with a discussion of the potential applications for UWB ranging.

Modern Surveying Techniques In National Infrastructural Development: Case Study Of Roads

Proper planning for physical development requires maps, prepared by surveyors, showing the locations of existing features, infrastructures and nature of the terrain. The study defined surveying and a surveyor. The purpose is to show convincingly that surveying is very important in National development. Previous works were reviewed. The reviewed works showed many applications of surveying in Infrastructural Development. The methodology examined the contributions and necessity of surveying in National Infrastructural Development. It exposed various applications of surveying, and various methods of surveying used. Modern surveying techniques and measuring equipment available and in use were discussed. It was made clear that Highway Planning should be preceded by a massive gathering of geo-referenced data. The necessary materials are aerial photographs, Satellite images, Global Positioning System (GPS) equipment, Electronic Distance Measuring (EDM) equipment, Total Station, etc. Ways that Remote Sensing help to make highway construction easier were discussed. The findings indicate that applications of modern surveying techniques are very necessary in physical planning and infrastructural development. The techniques make planning and implementation easy, straight forward and more accurate.. Keywords: Remote Sensing, Global Positioning Systems, Orthophoto map, Infrastructural Development, Geo-referenced. JORIND Vol. 6 (1) 2008: pp. 13-13

Slope–channel coupling between pipes, gullies and tributary channels in the Mocatán catchment badlands, Southeast Spain

AbstractThe dispersive nature of the highly sodic silts of the Triassic‐rich unit of the Góchar formation plays a fundamental role in the erosion of the Mocatán catchment badlands in Almería, where a rejuvenating pipe and incised channel network is rapidly evacuating slope materials. Referring to concepts of medium‐ and long‐term landscape evolution, and incorporating contemporary thoughts on the role of connectivity in system evolution from the geomorphological literature, this paper attempts to develop a conceptual model of the way geologic, topographic, material property and ecological factors combine to explain the complex geomorphological evolution of the site.An electronic distance measurement (EDM) survey was undertaken using a Leica TC3100, to produce a detailed topographic map. This database was supplemented by geomorphological, geological and ecological data derived from ground survey and remote sensing, and further morphometric analysis undertaken. Preferred orientations of channel segments, and the topographic distribution of pipe‐roof‐collapse features and outfalls in relation to known stratigraphic controls, suggests that, once coupled to the slope‐base channel, pipe networks develop in a systematic, sequential way. A wave of incision moving up the main channel reconnects channels with slopes, and the resulting increased hydraulic gradients on sideslopes encourage extensive deep pipe development for the first time in these materials. Once major pipe development is possible, three‐dimensional pipe networks enlarge and then collapse to form an extensive, partially coupled steep‐sided gully network. From this perspective, the coupling of the pipe to a rejuvenating channel is a significant intrinsic threshold event and the main reason that badlands have developed locally in these dispersive materials. It is concluded that erosion in this landscape will only be suppressed after a considerable period of slope‐base stability, which could allow a gradual loss of sodium from the surface by leaching or organic exchanges. Both theoretical and management implications are explored. Copyright © 2007 John Wiley & Sons, Ltd.

Flank instability on Mount Etna: Radon, radar interferometry, and geodetic data from the southwestern boundary of the unstable sector

Understanding Etnean flank instability is hampered by uncertainties over its western boundary. Accordingly, we combine soil radon emission, interferometric synthetic aperture radar (InSAR), and electronic distance measurement (EDM) data to study the Ragalna fault system (RFS) on the SW flank of the volcano. Valuable synergy developed between our differing techniques, producing consistent results and serving as a model for other studies of partly obscured active faults. The RFS, limited in its surface expression, is revealed as a complex interlinked structure ∼14 km long that extends from the edifice base toward the area of summit rifting, possibly linking northeastward to the Pernicana fault system (PFS) to define the unstable sector. Short‐term deformation rates on the RFS from InSAR data reach ∼7 mm yr−1 in the satellite line of sight on the upslope segment and ∼5 mm yr−1 on the prominent central segment. Combining this with EDM data confirms the central segment of the RFS as a dextral transtensive structure, with strike‐slip and dip‐slip components of ∼3.4 and ∼3.7 mm yr−1, respectively. We measured thoron (220Rn, half‐life 56 s) as well as radon, and probably because of its limited diffusion range, this appears to be a more sensitive but previously unexploited isotope for pinpointing active near‐surface faults. Contrasting activity of the PFS and RFS reinforces proposals that the instability they bound is divided into at least three subsectors by intervening faults, while, in section, fault‐associated basal detachments also form a nested pattern. Complex temporal and spatial movement interactions are expected between these structural components of the unstable sector.

Attenuated retroreflectors for electronic distance measurement

Methods are described for attenuating solid glass and hollow retroreflectors, without introducing optical path length modifications, for electronic distance measurement. Construction of a prototype novel-design hollow retroreflector is described.

Tropospheric Delay Estimation for Pseudolite Positioning

Pseudolites, ground-based GPS signal transmitters, can significantly enhance the GPS satellite geometry or can even be an independent positioning system. However, as pseudolites are very close to the receivers, error effects are different from the traditional GPS and should be considered and modeled in a different way. Tropospheric delay is one of the largest error sources in pseudolite positioning, as pseudolite signal propagates through the lower troposphere which is very difficult to be modeled due to spatial variations in atmosphere. The objective of this research is to analyse pseudolite tropospheric delay modelling methods and to select the optimal tropospheric delay models for different applications. Several methods to estimate the tropospheric delay for pseudolite positioning are introduced and compared. One approach is to utilize single-differenced GPS tropospheric models. Another one is to compute the tropospheric delay as a function of the local refractivity along the pseudolite signal path. The ratio method used for Electronic Distance Measurement (EDM) can also be applied to estimate tropospheric delay. Experiments with simulation and real flight test data are conducted in this study to investigate the proposed methods. The advantages and limitations of each method are analysed. The mode defined by RTCA and its modification are suitable for a low elevation and short range application, such as LAAS and local ground based applications. Models derived from single-differenced NMF and Saastamoinen models perform well in long range and high elevation but have a big bias in low elevation. And the model derived from the Hopfield model performs relatively well in all the range and elevation.

An increment of volcano collapse: Kinematics of the 1975 Kalapana, Hawaii, earthquake

Earthquakes on Kilauea Volcano can be even more hazardous than volcanic eruptions. The most recent large earthquake was the 1975 Kalapana quake, which generated a locally damaging tsunami, over 8 m of horizontal surface displacement and 3 m of subsidence at the coast. The seismic magnitude estimates for this earthquake range from Ms 7.2 to Mw 7.7. The complexity of the seismic data is matched by the complex surface deformation that was observed with a combination of leveling, tilt and Electronic Distance Measurements (EDM). The geodetic data shows evidence of collapse of the caldera due to magma withdrawal, localized subsidence over the rift zone, normal faulting associated with shallow slumping along the Hilina Pali, and displacement on the basal detachment fault. We model the summit caldera, rift zones and basal detachment fault as dislocations in an elastic half-space to better quantify the sources of deformation related to this devastating event. Using inversion techniques that allow us to let the data constrain the geometry and magnitude of these sources, we find 0.04 km 3 of magma withdrawal at the summit, between 3–5 m of opening along the rift zone and 7.1 m of slip along the basal detachment at 8.3 km depth. Models that allow for finer spatial resolution of slip on the detachment show that the largest slip occurred west of the seismically recorded earthquake hypocenter, along the region of the fault below the coast, and that the majority of the fault slip occurred south of the region of microseismicity. Our best-fitting model has a geodetic moment of 4.1 × 10 20 Nm (M w 7.7), which is consistent with tsunami models and recent analysis of long period seismic data. The residual displacements of sites located in the hanging wall of the Hilina Pali slump system, which were not used in the inversion of the other sources, suggest that triggered shallow slumping contributed several meters of horizontal displacement to the coseismic displacement at these sites.

Volcano geodesy and magma dynamics in Iceland

Here we review the achievements of volcano geodesy in Iceland during the last 15 years. Extensive measurements of crustal deformation have been conducted using a variety of geodetic techniques, including leveling, electronic distance measurements, campaign and continuous Global Positioning System (GPS) geodesy, and interferometric analysis of synthetic aperture radar images (InSAR). Results from these measurements provide a comprehensive view of the behavior of Icelandic volcanoes. Between inflation, intrusion, and eruption episodes, volcanoes are likely to deflate or show no sign of seismic activity. Subsidence rates are often in the range of a few millimeters to a few centimeters a year, reducing progressively with time since the last eruption or intrusion at the volcano. Subsidence can be caused by cooling and contraction of magma, outflow of magma, it can be related to plate spreading. Volcano subsidence or lack of deformation is often interrupted by episodic magma flow towards near-surface locations. Such magma recharge has been observed geodetically at Hengill, Hekla, Eyjafjallajökull, Katla, Grímsvötn, and Krafla volcanoes, with inflow inferred to last from a few months up to two decades. In the last 15 years, five volcanic eruptions, three intrusive events and two > M6 earthquakes have occurred. In recent years, the Grímsvötn and Katla volcanoes have exhibited continuous inflation of a few centimeters per year, which at Grímsvötn culminated in an eruption on 1 November 2004. Hekla and Torfajökull volcanoes have inflated at rates an order-of-magnitude less. Subsidence is occurring presently at the Askja and Krafla volcanoes. Within the period of geodetic measurement, signals consistent with no deformation are typical for most of the 35 active volcanoes in Iceland.

Anatahan, Northern Mariana Islands: Reconnaissance geological observations during and after the volcanic crisis of spring 1990, and monitoring prior to the May 2003 eruption

Anatahan island is 9.5 km east–west by 3.5 km north–south and truncated by an elongate caldera 5 km east–west by 2.5 km north–south. A steep-walled pit crater ∼1 km across and ∼200 m deep occupies the eastern part of the caldera. The island is the summit region of a mostly submarine stratovolcano. The oldest subaerial rocks (stage 1) are exposed low on the outer flanks and in the caldera walls. These include thick (∼10 m) and thin (2–3 m) lava flows, well-indurated tuffs, and scoria units that make up the bulk of the island. Rock compositions range from basaltic andesite to dacite, and most are plagioclase-phyric. On the steep north and south flanks of the volcano, these rocks are cut by numerous east–west-oriented, few-hundred-m-long lineaments of undetermined origin. Indurated breccias unconformably overlie scarps cut into stage 1 units low on the south flank. Intermediate-age eruptive units (stage 2) include caldera-filling lava flows and pyroclastic deposits and, on the outer flanks, vents and valley-filling lava flows. The youngest pre-2003 volcanic unit on Anatahan (stage 3) is a hydromagmatic surge and fall deposit rich in accretionary lapilli. Prior to 2003, this unit was found over almost the entire island, and in many places original depositional surfaces and outcrops could be found in high-energy environments along the coast, indicating a young (but undetermined) age. During reconnaissance visits in 1990, 1992, 1994, and 2001, geothermal activity (fumaroles as well as pits with boiling, sediment-laden pools) was observed in the southern part of the pit crater. In March and April 1990, increased local seismicity, a large regional earthquake, and reported increased fumarolic activity in the pit crater prompted evacuation of Anatahan village, at the west end of the island. Our first field investigation took place in late April 1990 to assess the level of volcanic unrest, conduct reconnaissance geological observations, collect rock and geothermal water samples, and set up a geophysical monitoring network. Results at this time were inconclusive with respect to determining whether the activity was anomalous. Water in some of the geothermal pits within the pit crater was boiling, and pH values as low as 0.7 were recorded in the field. An electronic distance measurement (EDM) network was installed, and over a ∼1-week period, up to 9 cm of extension occurred across some lines but not others. Seismicity was characterized by intermittent local earthquakes but no sustained swarms or tremor. A brief visit in June 1990 revealed that the shallow lake near the boiling pits was gone, but activity in the pits themselves was similar to that of April 1990. Only minor extension had occurred along a single EDM line since the previous visit, and no earthquakes >M2.5 occurred during the visit. Subsequent 1- to 2-day visits occurred in October 1990, May 1992, May 1994, and June 2001. Activity within the geothermal pits was relatively constant during every visit, although during this 11-year period the level of the water in each pit decreased. In June 2001, a ∼50-m-wide region of mud pots and steaming ground in the central part of the geothermal area had developed. No geologic evidence, however, suggested that an eruption would occur < 2 years afterward. Most of the EDM lines showed slight extension between late 1990 and 1992, followed by very gradual contraction from 1992 to 2001. A more extensive seismic-monitoring system was installed on the Northern Mariana Islands during these visits, and it recorded a small seismic swarm at Anatahan from May to July 1993. The telemetry component of the seismic equipment broke prior to 2001 and had not been repaired by the time of the May 2003 eruption, so no precursory seismic data were recorded to indicate pre-eruption unrest.

Contrasting triggering mechanisms of the 2001 and 2002–2003 eruptions of Mount Etna (Italy)

Mount Etna produced two significant eruptions in 2001 and 2002–2003, which we have analysed using geological, seismic and deformation data. These eruptions showed some similarities, such as the activating of two magmatic plumbing systems (central–lateral and eccentric), but they differed in their triggering mechanisms. While the 2001 eruption was largely the result of the emplacement of a N–S eccentric dike (independent from the central conduits) consistent with E–W regional extension, the 2002–2003 eruption occurred in response to a major flank slip on the eastern and southeastern sides of the volcano. This is demonstrated by the spatial and temporal distribution of seismicity and deformation preceding and accompanying the two eruptions. During the months prior to the 2001 eruption, most epicenters were concentrated on the southern flank, at depths of 5–15 km below sea level. During the 4 days before the eruption, earthquake hypocenters migrated to shallower levels (from 5 km bsl. upward) indicating the emplacement of the eccentric dike. This is confirmed by the patterns of ground fracturing observed in the field and deformation documented by electronic distance measurements (EDM). In contrast, the months before the 2002–2003 eruption were characterised by shallower seismicity, mainly concentrated along the active faults bordering the slipping flank sector. Flank slip accelerated in September 2002 and a second, more vigorous acceleration of flank slip occurred on 26–27 October 2002, accompanying the opening of eruptive vents. The very short (2 h) seismic crisis preceding the onset of eruptive activity stands in neat contrast with the 4 days of intense seismicity before the 2001 eruption. Subsequently, flank slip-deformation extended all over the eastern and southeastern flanks of the volcano, causing serious damage in this sector. The events of 2001–2003 can be seen as a continuous chain of intimately interacting processes including regional tectonics, magma accumulation and eruption, and flank instability. In this scenario the 2001 eruption led to increased flank instability that subsequently accelerated and culminated with the massive flank slip, which in turn facilitated the 2002–2003 eruption. This sequence of events points to a long-term feedback mechanism between magmatism and flank instability at Etna.

Multidirectional retroreflector assembly with a common virtual reflection point using four-mirror retroreflectors

A brief survey of retroreflector designs and applications is presented. A novel multidirectional (as opposed to omnidirectional) retroreflector concept is proposed by the author. A practical four-mirror retroreflector subassembly with a common virtual reflection point (thus eliminating the Abbe error), is described. Applications include: multilateration with interferometers, laser trackers, and electronic distance measurement surveying instruments; hidden point measurements to inaccessible locations; an extension of the concept to include optical targets for theodolite work; and an extension to other radiation sources, e.g., microwaves and acoustics. Example configurations are given.

The use of surface monitoring data for the interpretation of landslide movement patterns

The Tessina landslide is a large, seasonally active slope failure located on the southern slopes of Mt. Teverone, in the Alpago valley of NE Italy, consisting of a complex system that has developed in Tertiary Flysch deposits. The landslide, which first became active in 1960, threatens two villages and is hence subject to detailed monitoring, with high quality data being collected using piezometers, inclinometers, extensometers, and through the use of a highly innovative, automated Electronic Distance Measurement (EDM) system, which surveys the location of a large number of reflector targets once every 6 h. These systems form the basis of a warning system that protects the villages, but they also provide a very valuable insight into the patterns of movement of the landslide. In this paper, analysis is presented of the movement of the landslide, concentrating on the EDM dataset, which provides a remarkable record of surface displacement patterns. It is proposed that four distinct movement patterns can be established, which correspond closely to independently defined morphological assessments of the landslide complex. Any given block of material transitions through the four phases of movement as it progresses down the landslide, with the style of movement being controlled primarily by the groundwater conditions. The analysis is augmented with modelling of the landslide, undertaken using the Itasca FLAC code. The modelling suggests that different landslide patterns are observed for different parts of the landslide, primarily as a result of variations in the groundwater conditions. The model suggests that when a movement event occurs, displacements occur initially at the toe of the landslide, then retrogress upslope.

Surveying co-located space-geodetic instruments for ITRF computation

A new and comprehensive method is presented that can be used for estimating eccentricity vectors between global positioning system (GPS) antennas, doppler orbitography and radiopositioning integrated by satellites (DORIS) antennas, azimuth-elevation (AZ-EL) very long baseline interferometry (VLBI) telescopes, and satellite laser ranging (SLR) and lunar laser ranging (LLR) telescopes. The problem of reference point (RP) definition for these space-geodetic instruments is addressed and computed using terrestrial triangulation and electronic distance measurement (EDM) trilateration. The practical ground operations, the surveying approach and the terrestrial data processing are briefly illustrated, and the post-processing procedure is discussed. It is a geometrically based analytical approach that allows computation of RPs along with a rigorous statistical treatment of measurements. The tight connection between the geometrical model and the surveying procedure is emphasized. The computation of the eccentricity vector and the associated variance–covariance matrix between an AZ-EL VLBI telescope (with or without intersecting axes) and a GPS choke ring antenna is concentrated upon, since these are fundamental for computing the International Terrestrial Reference Frame (ITRF). An extension to RP computation and eccentricity vectors involving DORIS, SLR and LLR techniques is also presented. Numerical examples of the quality that can be reached using the authors’ approach are given. Working data sets were acquired in the years 2001 and 2002 at the radioastronomical observatory of Medicina (Italy), and have been used to estimate two VLBI-GPS eccentricity vectors and the corresponding SINEX files.

Development of an impact sound diagnosis system for tunnel concrete lining

In Japan, various non-destructive test techniques are in use for evaluating the soundness of tunnel concrete lining. Among these evaluation methods, impact sound diagnosis by human inspectors is most widely used because it is the easiest to conduct and requires only a minimum of equipment. Impact sound diagnosis by human inspectors, however, has several problems. First, since two or more persons conduct impact sound diagnosis, inspection results tend to vary among inspectors. Second, quantitative judgement required for impact sound diagnosis is difficult to make and the evaluation process is not recorded. Finally, impact sound diagnosis forces inspectors to work in a tunnel, which has an unfavorable working environment for long time. In order to solve these problems, the authors developed ''Sonic Meister'', the impact sound diagnosis system for tunnel lining concrete. This system has a number of characteristics: (1) The equipments are mounted on an eight-ton truck because mobility is considered to be an important requirement; (2) a general-purpose arm robot has been adopted to speed up the inspection process; (3) the impact sound unit has been developed to apply stable impact forces to concrete; and (4) manpower requirements have been reduced dramatically by mechanizing the inspection process. The newly developed impact sound diagnosis system has made it possible to evaluate the soundness of concrete speedily, safely and accurately. To evaluate the soundness of a concrete lining, the impact sound diagnosis system pushes the five-hammers unit, which is equipped with a parallel array of five hammers spaced 30 cm apart, against the concrete wall and activates the five hammers successively at 0.2 s intervals. After collecting the sound generated during the 0.1-s period after each impact, the system analyzes the collected sound signals by using a digital signal processor in real time and displays the results. These results are stored in real time together with raw sound data. Then, the system moves the impact sound unit by 30 cm in the transverse direction and repeats the same procedure. By using this method, the system is capable of inspecting concrete surfaces at a rate of 250-350 m2/h, covering 30 cm grid spaces one after another. The system controls the movement of the multi-hammer unit by using data received from cross-section measurement system and an electronic distance measuring (EDM) instruments system. On the basis of the information thus obtained, the system automatically avoids obstacles such as lighting equipment, piping and fans while conducting impact sound diagnosis. Since the robot mounted on the truck sways back and forth violent, the system is equipped with outriggers to reduce this swaying. Since the outriggers have non-motorized wheels, the vehicle can be moved forward without raising the outriggers. This design has helped to speed up the inspection process. To investigate the nature of sound generated by cavities and exfoliation areas, a series of impact sound diagnosis was conducted using specimens prepared for the examination. The concrete blocks used as test specimens were provided with artificial cavities and the exfoliation. In the tests, three parameters were measured: vibration of concrete surface (measured with an accelerometer), hammer impact force and impact sound. Comparison of impact sound and surface vibration showed that they show similar waveform tendencies although there are differences in degree. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.

Water-level changes induced by local and distant earthquakes at Long Valley caldera, California

Distant as well as local earthquakes have induced groundwater-level changes persisting for days to weeks at Long Valley caldera, California. Four wells open to formations as deep as 300 m have responded to 16 earthquakes, and responses to two earthquakes in the 3-km-deep Long Valley Exploratory Well (LVEW) show that these changes are not limited to weathered or unconsolidated near-surface rocks. All five wells exhibit water-level variations in response to earth tides, indicating they can be used as low-resolution strainmeters. Earthquakes induce gradual water-level changes that increase in amplitude for as long as 30 days, then return more slowly to pre-earthquake levels. The gradual water-level changes are always drops at wells LKT, LVEW, and CH-10B, and always rises at well CW-3. At a dilatometer just outside the caldera, earthquake-induced strain responses consist of either a step followed by a contractional strain-rate increase, or a transient contractional signal that reaches a maximum in about seven days and then returns toward the pre-earthquake value. The sizes of the gradual water-level changes generally increase with earthquake magnitude and decrease with hypocentral distance. Local earthquakes in Long Valley produce coseismic water-level steps; otherwise the responses to local earthquakes and distant earthquakes are indistinguishable. In particular, water-level and strain changes in Long Valley following the 1992 M7.3 Landers earthquake, 450 km distant, closely resemble those initiated by a M4.9 local earthquake on November 22, 1997, during a seismic swarm with features indicative of fluid involvement. At the LKT well, many of the response time histories are identical for 20 days after each earthquake, and can be matched by a theoretical solution giving the pore pressure as a function of time due to diffusion of a nearby, instantaneous, pressure drop. Such pressure drops could be produced by accelerated inflation of the resurgent dome by amounts too small to be detected by the two-color electronic distance-measuring network. Opening-mode displacement in the south moat, inferred to have followed a M4.9 earthquake on November 22, 1997, could also create extensional strain on the dome and lead to water-level changes similar to those following dome inflation. Contractional strain that could account for earthquake-induced water-level rises at the CW-3 well is inconsistent with geodetic observations. We instead attribute these water-level rises to diffusion of elevated fluid pressure localized in the south moat thermal aquifer. For hydraulic diffusivities appropriate to the upper few hundred meters at Long Valley, an influx of material at temperatures of 300°C can thermally generate pressure of 6 m of water or more, an order of magnitude larger than needed to account for the CW-3 water-level rises. If magma or hot aqueous fluid rises to within 1 km of the surface in the eastern part of the south moat, then hydraulic diffusivities are high enough to allow fluid pressure to propagate to CW-3 on the time scale observed. The data indicate that seismic waves from large distant earthquakes can stimulate upward movement of fluid in the hydrothermal system at Long Valley.

254 poster Correlation between pelvic patient movement derived from electronic portal images and external focus-skin distance measurements

254 poster Correlation between pelvic patient movement derived from electronic portal images and external focus-skin distance measurements

Strains and displacements in the Mam Tor landslip, Derbyshire, England

The spectacular Mam Tor landslip, near Castleton, Derbyshire, formed over 3000 years ago on an oversteepened slope left after the last ice age. A section of the Namurian Edale mudstones and overlying Mam Tor sandstones has collapsed, leaving an 80 m high scar on the eastern side of Mam Tor. The slip is about 750 m long and nearly 300 m wide and has the old main road from Manchester to Sheffield built across it. The slipped mass is in a state of year-on-year creep motion, which over the 190 years since the construction of the highway has led to extensive damage to the road, culminating in its closure in 1979. Since 1996 we have carried out annual monitoring by electronic distance measurement of the movement of a network of some 30 stations on the slipped mass. The average movement rate of the whole mass is about 10 cm a −1 , with the central region moving significantly faster, at almost 50 cm a −1 . Thus substantial readjustments of mass are taking place within the landslip. Local vertical displacements are systematically related to horizontal offsets, and the ratio of the two allows inference of the local attitude of the basal slip surface. The development of surface morphological features (pressure ridges and irregular topography, shear offsets on the highway, bulging of the road surface) reflects the lateral variations in displacement rate. Comparison of our survey of the present-day road position with that recorded in the topographical survey of 1880 shows a total 40 m downhill displacement of the highway over 122 years. This is consistent with extrapolation of our measured displacement rates back to 1880. Displacement rates within this period are, however, clearly higher than during the past 3000 years. There is a fairly clear correlation between vertical and horizontal displacement rates and annual variations in rainfall, with accelerated displacements following winter rainfall above a critical threshold level. Using survey points to define nodal points of a network of triangles, we have analysed the distribution of strain within the slipped mass. This revealed a pattern of continuous strain variations comparable with that found in flowing glaciers. In the lower part of the slip, horizontal strains are contractional and triangle areas are decreasing, causing some uplift of the ground relative to the general downhill flow on the basal slip surface. In the uphill part, strains are extensional and triangle areas are increasing. The new data allow an estimate of the time-dependent rheological behaviour of the sheared mudstone in the basal shear zone. The shear stress vs. shear strain rate relationship is very non-linear, with significant shear strain rates occurring only at shear stress levels within 10% of that required for rapid (catastrophic) shearing.