Ground Deformation Monitoring Research Articles

A network of multi-sensor stations for continuous monitoring of ground motion and deformation

Abstract Recent technical developments in seismic instrumentation have made the recording of ground motion possible with higher fidelity—extending both the frequency band and dynamic range—and with progressively smaller space and time gaps. This is achieved by dense networks, continuous recording and real-time data acquisition. The trend is towards the emplacement of networks of multi-sensor stations (MSSs) to record ground motion in the whole frequency domain. A new digital telemetered network of such MSS stations in Italy is being planned by the Istituto Nazionale di Geofisica (ING). It will include strong-motion accelerographs, broadband seismographs and geodetic GPS receivers. An MSS network offers several advantages as compared with the separate installation of different types of instrumentation. It is more cost effective, allowing the sharing of expenses related to sites and telemetry. It will allow more comprehensive collection of data, by giving each sensor the possibility of triggering acquisition from the entire set after detection of an event. It will encourage co-operation among earth scientists. Through its real-time-oriented design, it will constitute an important device for the rapid quantification of earthquakes and, possibly, detection of earthquake precursors.

Five years of ground deformation monitoring on axial seamount using a bottom pressure recorder

The results of five years of pressure measurements collected by a Bottom Pressure Recorder deployed within the summit caldera of Axial Seamount, on the central Juan de Fuca Ridge, are interpreted for monotonic offsets related to vertical seafloor movements. Results indicate five significant subsidence events during the observation period (3.7 years), or an average of 1.35 events per year. The displacements range from 3–10 cm with durations of 10–14 days. These results indicate an active volcanic system and can be viewed as positive evidence for future attempts at seafloor volcanic observatories and response to active events on oceanic ridge crests. Advanced instrumentation based on multiple sensors has been deployed at the site to reinforce the results of this pilot experiment.

Mechanisms of magma eruption and emplacement at Mt Etna between 1989 and 1992

THE present eruption sequence of Mt Etna began during September 1989 with extensive fracturing, and culminated in 1991–93 with the largest lava eruption this century1. Here we present measurements of gravity and ground deformation which, in conjunction with seismic data, provide a detailed record of the processes occurring before and during eruption. These reflect a complex interplay between magma injection into the paths of lowest compressive stress through the volcano flanks, forcing elastic deformation, and fracture growth in response to gravitational stresses. We find that after the end of the eruptive activity in 1989, fractures in the south-southeast flank of the volcano remained unfilled until some months before the 1991–93 eruption. These results indicate the utility of microgravity monitoring, as the precursory processes that we identify were largely undetected by seismic and ground deformation monitoring.

Real time monitoring of vertical ground deformations during eruptions at Piton de la Fournaise

Tilt changes related to a summit dike emplacement at Piton de la Fournaise volcano have been recorded by a 4 continuously recording electronic tiltmeters network during the April 18, 1990 eruption. The real‐time processing of these data at the Observatory allows to calculate during the emplacement the relative position of the inflation center from the tilt vectors supplied by the nearby stations. Two main steps have been identified: an initial subvertical intrusion step, followed by a lateral dike migration from the summit cone. This monitoring allows to calculate a high average intrusion velocity of about 0.2 m.sec−1 which can related to the highly fracturated pattern of the summit area.

A geodetic network in the Novarupta area, Katmai National Park, Alaska

A small geodetic network was established in 1989 and 1990 to monitor ground deformation in the Novarupta area, Katmai National Park, Alaska. Slope distances and zenith angles for three lines were repeated in 1990. A comparison of the two surveys indicates changes that are within the error of the measurements. Mean mark‐to‐mark slope distance changes are 1.17±1.46 ppm. Two benchmarks were added to the network in 1990 to configure a five‐endpoint braced quadrilateral centered about the Novarupta dome.

Ground deformation monitoring at Mt Etna; evidence for dyke emplacement and slope instability

Ground deformation monitoring at Mount Etna has successfully enabled the sub-surface paths of two shallow feeder-dykes to be traced, and their dimensional parameters, depths and departures from the vertical, to be constrained. Determinations of horizontal movement have recorded the displacement, by 3.6 m (maximum), of a rim segment of the Valle del Bove flank caldera, due to dilation associated with dyke emplacement adjacent to the rim over the period 1981 to 1989. Small-scale collapse along this rim segment, associated with the opening of the 1989 fissure, points to increasing slope instability, and highlights the potential for major failure and debris avalanche generation.

Eruptive mechanism on Piton de la Fournaise volcano associated with the December 4, 1983, and January 18, 1984 eruptions from ground deformation monitoring and photogrammetric surveys

Two photogrammetric aerial surveys were carried out over the summit area of the basaltic shield volcano Piton de la Fournaise (Indian Ocean), one survey in 1981 and the second one in 1984. During this time, only two eruptions occurred; both the December 4, 1983, and the January 18, 1984, eruptions opened fissures on the southwestern part of the summit. Because of the slight and erratic ground deformation measured on the dry-tilt network, by continuous tilt stations, and within the geodetic network between 1981 and November 1983, and between February 1984 and June 1984, ground deformations revealed by the two photogrammetric surveys can be essentially associated with the two eruptions. Large displacement vectors were obtained, up to 40 cm. Horizontal displacement vectors indicate a northeastward ground deformation of the southwestern part of the cone where the effusive vents opened. A more diffuse uplift along the main fracture zones that cut the volcanic edifice was observed, while east of these fractures only slight—less than 10 cm—and opposite displacements were observed. This displacement field can be associated with the main geological structures of the dome of Piton de la Fournaise volcano. Some consequences of the observed displacement field may be outlined for the volcanic observational ground deformation networks. To estimate the displacement field revealed by the photogrammetric surveys, a simple model of dyke intrusion has been computed. This model is based on dislocations and takes into account the main fracture zones. Good agreement is observed between computed and observed data in the area of the effusive vents. Some disagreement remains in the northwestern part of the survey area, where horizontal deformations are small and erratic, and also in the northern part, where an uplift was observed that can be associated with the northern active fracture zones.

Temporary Tunnel Support Using Jet‐Grouted Cylinders

The construction is described of the Monte Olimpino 2 tunnel in northern Italy. The location of the tunnel is such that it spans alternating geologic conditions. This paper describes the problems created by the alluvial and glacial deposits that led to the use of jet-grouting as part of the excavation and construction process. It then discusses the construction experience and measurements made to monitor ground deformation and strains during the construction process. The jet grouting proved to be a viable alternative to more conventional techniques of soft ground tunneling. The precompression of the soil by the soil-cement cylinders makes jet-grouting a useful construction technique wherever soil stabilization is required.

Ground deformation associated with mining a remnant footwall pillar in a surface coal mine

ABSTRACT Data is presented from a ground deformation monitoring program conducted at a surface coal mine over a two year period. Monitoring focused on the mining of a coal measures sequence on a footwall slope. Mining plans had aimed to isolate a remnant pillar block, 12500 m2 in plan, carrying a 132kV pylon tower and thus to maintain the integrity of a cross-pit power line. Pillar deformation which arose from shear on inclined strata is reviewed and related to the excavation sequence by strike cut and dip cut mining, involving remedial backfill buttressing. The monitoring program was based principally on an array of borehole inclinometers, supported by ground tiltmeter and conventional survey methods. The paper discusses the performance of the monitoring practice and strategy adopted in this mine production environment.

Deformation monitoring of the ostler fault zone, South Island, New Zealand

The Ostler Fault Zone is a zone of Quaternary reverse faulting along the western margin of the Mackenzie Basin, South Island, New Zealand. During investigations for and construction of the Ohau-A Power Project, a number of survey patterns were designed and installed to monitor ground deformation in the area of the fault zone. A triangulation network, two long and two short levelling routes, and six tilt levelling patterns have been installed at various times since 1964. Analysis of the levelling data shows that vertical deformation has occurred within the ault zone at a near constant rate for the last 20 years. Deformation consists of buckling about fault traces at a maximum rate of uplift of approximately 1 mm/yr. The vertical deformation preserved over 16,000–25,000 yrs has a similar rate although it is dominated by displacement across the fault zone as a whole. Analysis of triangulation data spanning up to 100 years indicates that significant horizontal strain is occurring, with the axis of relative contraction approximately perpendicular to the trend of the fault.

Seismic and ground deformation monitoring in the seismogenetic region of the southern Apennines, Italy

The 23 November, 1980, magnitude ( M s ) 6.9 southern Italy earthquake has been associated by Crosson et al. (1986) with a faulting system consisting predominantly of two high-angle sub-perpendicular normal faults forming the corner of down-dropped crustal block. This model was inferred from a detailed analysis of the space-time distribution of aftershocks and their focal mechanisms, some features of the acceleration field, the focal mechanism of the main shock and the observed displacements along two leveling profiles. Quantitative modelling of ground displacements (which amounted to up to 70 cm) was performed through a forward analysis, with a dislocation source consisting of several fault segments, and by inverse methods with a priori information, allowing the parameters of a multiple rectangular fault system to be retrieved. Quantitative modeling of vertical displacements observed in a nearby seismic region showed a similar mechanism, with coseismic movements of up to 12 cm, which are consistent with uniform dislocation on two rectangular faults, coinciding with the focal mechanism, location and magnitude of the earthquakes which occurred in August 1962 ( M s = 5.7, 6.1 and 5.4 ). A system for monitoring ground displacements and related seismic activities has been in place since 1982 in this region, and has recently been improved on the basis of our knowledge of the seismic source mechanisms. The aim is to study the stress regime during interseismic phases, to gain a better understanding of the physics of earthquake processes and to discover any patterns in earthquake occurrences in the seismogenetic regions of the Apennines.

Developments in volume and ground deformation monitoring

Abstract Two techniques are presented. In the first one a laser optical range finder is developed for measuring and setting our distances to undefined inaccessible objects. The use of the developed prototype in quarry, stope and open pit surveying is investigated. In the second technique a differential subsidence meter is developed for high precision measurement of the ground deformation due to mining. The developed prototype is capable of measuring the progressing ground tilt with an accuracy of a fraction of a second.

On the Development of Shallow Hydraulic Fractures as Viewed Through the Surface Deformation Field: Part 1-Principles

Summary An overview of the surface tiltmeter method of mapping shallow hydraulic fractures is presented. The paper is intended to provide the analytical background to a companion paper (Part 2, see Page 411) in which a number of case histories are discussed from the viewpoint of fracture growth and consolidation characteristics. In this paper we discuss the nature of the observations, outline the theoretical framework within which the data typically are interpreted, and comment on the significance of the various fracture parameters resolved. A summary review of other fracture-mapping technologies is presented also. Introduction Recently a considerable research effort has been directed toward the formulation of practical guidelines for predicting and possibly controlling the geometrical characteristics of hydraulic fractures. Substantial advances have been made through theoretical and laboratory studies of the various mechanisms thought to play a role in determining fracture geometry. Yet, the degree to which the results of these idealized simulations apply to commercial-scale in-situ hydraulic fractures remains uncertain. A fundamental problem is presented by the inherent difficulty in "observing" the geometry adopted by fractures formed in situ. This has impeded the collection of a substantial observational database pertaining to the form of commercial-scale hydraulic fractures against which the predictions of the various theories can be tested. The establishment of such an observational framework is an essential step in assimilating the large volume of information now available. During the past few years, the surface tiltmeter technique has been used to monitor ground deformations associated with the formation of a number of shallow [less than 450 m (1,500 ft] hydraulic fractures. From the data it is possible to infer broad-scale features of the geometrical development of the fracture. Quantitative estimates of fracture closure behavior following shut-in also can be obtained. In Part 2 we review the shallow-fracture data collected to date from the viewpoint of fracture growth and consolidation behavior. We begin by presenting a summary review of the fracture mapping technology currently in use or under development within the energy industry. The most detailed descriptions of in-situ hydraulic fractures have been obtained through mineback operations. In these experiments, fractures formed with dyed or tagged fluid subsequently are excavated to expose the decorated surfaces along which fluid penetration of the formation has occurred. Clearly this direct approach to penetration of the formation has occurred. Clearly this direct approach to fracture mapping is rarely possible in commercial operations, and it is usually necessary to deduce fracture geometry indirectly through measurements made either downhole or at the earth's surface. The passive seismic technique has enjoyed considerable success in mapping fractures formed in crystalline rocks, and the method shows practical potential for application to hydrocarbon reservoirs. The underlying principle is to determine the path of fluid penetration into the medium from the hypocentral distribution of microseismic emissions stimulated as a result of increased pore, pressure. A number of methods of sampling the seismic radiation field have been used, including surface-based microseismic arrays, and downhole-based hydrophones or three-component geophones. Systems for detecting the change in surface electrical potential that accompanies fracture formation have been developed although a satisfactory strategy for data interpretation remains to be established. Examples of fracture mapping using active seismic methods, in which an artificial seismic source is used to irradiate the fracture, have been presented by Stewart et al. and Aki et al. Overton has rigorously investigated the potential of oriented downhole superconducting magnetic systems (SQUIDS) potential of oriented downhole superconducting magnetic systems (SQUIDS) for detecting the distribution of magnetic tracer fluid injected into the fracture. JPT p. 406

Relationships between Recent Middle East and African Earthquakes

THE almost random occurrence of earthquakes seems to rule out any useful method of forecasting, at least until we understand and can closely monitor earth mantle processes. The probable influence of known natural factors like earth and ocean tides in triggering earthquakes is not sufficiently determining to be a forecasting aid. More hopeful are the results of ground deformation monitoring in the vicinity of such fault zones as the San Andreas, where patterns are emerging between ground deformation and ensuing local earthquakes1.