Tiltmeter Data Research Articles

Seismogenic stress field beneath Mt. Etna (South Italy) and possible relationships with volcano-tectonic features

Shallow shear-type seismic activity occurring beneath the Etna volcano during 1990–1995 has been analysed for hypocenter locations, focal mechanisms and stress tensor inversion. The results have been examined jointly with Electronic Distance Measurements and tiltmeter data collected in the same period and reported in the literature. Significant seismicity located in the upper 10 km was found to be confined to the time intervals in which ground deformation data indicated inflation of the volcano edifice (e.g., the periods preceding the December 1991–March 1993 and August 1995–March 1996 eruptive phases). The shocks mostly occurred in a sector approximately centered on the crater area and elongated in the East–West direction. The causative seismogenic stress shows a low-dip East–West orientation of σ 1. In agreement with existing knowledge on relationships between local fault systems and magma uprise processes, the shallow seismicity in question is tentatively explained as being due to lateral compression by magma inside a nearly North–South system. The volcano deflation phase revealed by Electronic Distance Measurements and tilt data during the 1991–1993 major eruption was not accompanied by any significant shear-type shallow event. Below the depth of 10 km, the North–South prevailing orientation of σ 1 reflects the dominant role of the regional stress.

A seafloor long‐baseline tiltmeter

Long‐term monitoring of seismicity and deformation has provided constraints on the eruptive behavior and internal structure and dynamics of subaerial volcanoes, but until recently, such monitoring of submarine volcanoes has not been feasible. Little is known about the formation of oceanic crust or seamounts, and we have therefore developed a stand‐alone long‐baseline tiltmeter to record deformation on active seafloor volcanoes. The instrument is a differential pressure, two‐fluid sensor adapted for use on the seafloor, combined with an autonomous data logger and acoustic navigation/release system. The tiltmeter can be installed without use of remotely operated vehicles or manned submersibles and, to first order, is insensitive to noise driven by temperature or pressure gradients. We recorded 65 days of continuous data from one of these tiltmeters on Axial Seamount on the Juan de Fuca Ridge during a multidisciplinary experiment that included ocean bottom seismographs, magnetotelluric instruments, and short‐baseline tiltmeters. After instrument equilibration the 100‐m‐long tiltmeter provided a record with long‐term drift rates of 0.5–5 μrad day−1 and higher frequency variations of the order of 5–10 μrad. Comparison with records of subaerial volcanic tilt shows that this instrument can discriminate volcanic deflation events, though none occurred during our deployment, a conclusion supported by nearby short‐baseline tilt and bottom pressure recordings. The short‐ and long‐baseline data constrain volcanic inflation of Axial Seamount to be below 0.5–1 μrad day−1 during mid‐1994. Analysis of the long‐baseline tilt data in conjunction with electric field, temperature, and short‐baseline tiltmeter data shows that high‐frequency signals are largely driven by ocean currents. Improved coupling between the tiltmeter and seafloor should reduce this noise, improve stability and drift, and further enhance our ability to record tilt related to active submarine volcanism.

Tiltmeter data inversion with continuous, non-uniform opening distributions: A new method for detecting hydraulic fracture geometry

Abstract Tiltmeter technology has long been an important fracture diagnostic tool. Tiltmeters are extremely good at determining fracture azimuth and dip. However, details about fracture opening variations have not been exploited in the past, partly because of a lack of data resolution to justify more work. The work presented here extends tiltmeter inversion modeling by solving for a non-uniform fracture opening distribution (fracture opening can vary with height and length). This more detailed fracture geometry can give clues to fracture growth processes such as asymmetric wing development and multi-zone fracture propagation. Theoretical calculations show the type of fracture geometry variation that can be imaged, and the technique is demonstrated interpreting field data from a shallow reservoir in California. The motivation for extending tiltmeter analysis methods is to take advantage of newly developed field instruments that can be installed at greater depth to image fractures to as deep as 10,000 ft.

Tiltmeter data inversion with continuous, non-uniform opening distributions: A new method for detecting hydraulic fracture geometry

Tiltmeter data inversion with continuous, non-uniform opening distributions: A new method for detecting hydraulic fracture geometry

Tilts, strains, and ground-water levels near an earth fissure in the Mimbres Basin, New Mexico

Vertical deflection profiles derived from microradian-sensitivity tiltmeter data collected over a 1 yr period suggest that an earth fissure in the subsiding Mimbres Basin is located near the inflection point of an evolving surficial fold of infinitesimal amplitude. Water levels beneath the study site also fell steadily during the period of observation. Calculated curvature profiles, which reflect the sign and relative magnitude of fiber strain along the ground surface, further imply that the fissure was within a zone of contraction throughout the study period. Observed fissure-perpendicular tilts and the location of the fissure at an inflection point rather than along an anticlinal crest are consistent with a mode II fracture (shearing mode crack or fault) rather than a mode I fracture (opening mode crack or joint). Tilting parallel to the fissure further suggests a component of mode III (tearing-mode crack) displacement during the study period. Thus, the three-dimensional displacement, strain, and stress fields around the fissure were considerably more complicated than those suggested by previous fissure monitoring studies and suggest that the fissure may have been formed in compression as a shear crack rather than in tension as an extensional crack. On the basis of the occurrence of right steps in the trace of the fissure, we speculate that our data may reflect contraction across one of the steps as a consequence of left-lateral slip along the fissure.

Shear-wave polarizations and subsurface stress directions at Lost Hills field : Winterstein, D F; Meadows, M A GeophysicsV56, N9, Sept 1991, P1331–1348

Two closely spaced nine-component VSPs recorded in the Lost Hills oil field in the southern San Joaquin Valley of California show that the polarization of the fast shear (S) wave is aligned with the direction of maximum horizontal compressive stress as determined from analysis of tiltmeter data. Tiltmeters monitored fractures that were hydraulically induced in or near the VSP wells. When fractures were induced in one of the VSP wells, fracture azimuths as determined from tiltmeter data were N53{degree}E and N56{degree}E, respectively, in two different depth zones; in the same zones the fast S-wave polarization directions were N58{degree}E and N59{degree}E. When fractures were induced in a well 537 ft from the second VSP well, tiltmeter data indicated a mean fracture strike of N59{degree}E, while the fast S-wave polarization direction in that case was N40{degree}E. S-wave polarization directions were determined by minimizing energy on off-diagonal components of the 2 {times} 2 S-wave data matrix, accomplished by computationally rotating sources and receivers by the same angle. S-wave polarizations from two concentric rings of offset VSPs were consistent in azimuth with one another and with polarizations of the near offset VSP. This consistency argues strongly for the robustness of the S-wave polarization techniquemore » as applied in this area. The S-wave polarization pattern in offset data fits a model of vertical cracks at N55{degree}E in a weakly transversely isotropic matrix with infinite-fold symmetry axis tilted at 10{degree} from the vertical toward N70{degree}E.« less

Shear-Wave Polarizations and Subsurface Stress Directions at Lost Hills Field

Two closely spaced nine-component VSPs recorded in the Lost Hills oil field in the southern San Joaquin Valley of California show that the polarization of the fast shear (S) wave is aligned with the direction of maximum horizontal compressive stress as determined from analysis of tiltmeter data. Tiltmeters monitored fractures that were hydraulically induced in or near the VSP wells. When fractures were induced in one of the VSP wells, fracture azimuths as determined from tiltmeter data were N53{degree}E and N56{degree}E, respectively, in two different depth zones; in the same zones the fast S-wave polarization directions were N58{degree}E and N59{degree}E. When fractures were induced in a well 537 ft from the second VSP well, tiltmeter data indicated a mean fracture strike of N59{degree}E, while the fast S-wave polarization direction in that case was N40{degree}E. S-wave polarization directions were determined by minimizing energy on off-diagonal components of the 2 {times} 2 S-wave data matrix, accomplished by computationally rotating sources and receivers by the same angle. S-wave polarizations from two concentric rings of offset VSPs were consistent in azimuth with one another and with polarizations of the near offset VSP. This consistency argues strongly for the robustness of the S-wave polarization techniquemore » as applied in this area. The S-wave polarization pattern in offset data fits a model of vertical cracks at N55{degree}E in a weakly transversely isotropic matrix with infinite-fold symmetry axis tilted at 10{degree} from the vertical toward N70{degree}E.« less

Real-time Seismic Amplitude Measurement (RSAM): a volcano monitoring and prediction tool

Seismicity is one of the most commonly monitored phenomena used to determine the state of a volcano and for the prediction of volcanic eruptions. Although several real-time earthquake-detection and data acquisition systems exist, few continuously measure seismic amplitude in circumstances where individual events are difficult to recognize or where volcanic tremor is prevalent. Analog seismic records provide a quick visual overview of activity; however, continuous rapid quantitative analysis to define the intensity of seismic activity for the purpose of predicing volcanic eruptions is not always possible because of clipping that results from the limited dynamic range of analog recorders. At the Cascades Volcano Observatory, an inexpensive 8-bit analog-to-digital system controlled by a laptop computer is used to provide 1-min average-amplitude information from eight telemetered seismic stations. The absolute voltage level for each station is digitized, averaged, and appended in near real-time to a data file on a multiuser computer system. Raw realtime seismic amplitude measurement (RSAM) data or transformed RSAM data are then plotted on a common time base with other available volcano-monitoring information such as tilt. Changes in earthquake activity associated with dome-building episodes, weather, and instrumental difficulties are recognized as distinct patterns in the RSAM data set. RSAM data for domebuilding episodes gradually develop into exponential increases that terminate just before the time of magma extrusion. Mount St. Helens crater earthquakes show up as isolated spikes on amplitude plots for crater seismic stations but seldom for more distant stations. Weather-related noise shows up as low-level, long-term disturbances on all seismic stations, regardless of distance from the volcano. Implemented in mid-1985, the RSAM system has proved valuable in providing up-to-date information on seismic activity for three Mount St. Helens eruptive episodes from 1985 to 1986 (May 1985, May 1986, and October 1986). Tiltmeter data, the only other telemetered geophysical information that was available for the three dome-building episodes, is compared to RSAM data to show that the increase in RSAM data was related to the transport of magma to the surface. Thus, if tiltmeter data is not available, RSAM data can be used to predict future magmatic eruptions at Mount St. Helens. We also recognize the limitations of RSAm data. Two examples of RSAM data associated with phreatic or shallow phreatomagmatic explosions were not preceded by the same increases in RSAM data or changes in tilt associated with the three dome-building eruptions.

The use of tiltmeters to study the dynamics of Antarctic ice-shelf grounding lines

AbstractNew tiltmeter data are presented from Doake Ice Rumples on Ronne Ice Shelf, Antarctica. Five sites which showed a tidal ice-shelf flexure have been analysed using an elastic beam model to investigate the variation of flexure amplitude with distance from the grounding line. An earlier study on Rutford Ice Stream which also used an elastic model required an ice thickness much less than that observed. Reworking the Rutford Ice Stream data suggests that this greatly reduced ice thickness is not required, given the current sparse data coverage. The elastic model is used to improve the estimated grounding-line position on Rutford Ice Stream. Some of the difficulties in modelling ice-shelf flexure and locating grounding lines are discussed.

The use of tiltmeters to study the dynamics of Antarctic ice-shelf grounding lines

Abstract New tiltmeter data are presented from Doake Ice Rumples on Ronne Ice Shelf, Antarctica. Five sites which showed a tidal ice-shelf flexure have been analysed using an elastic beam model to investigate the variation of flexure amplitude with distance from the grounding line. An earlier study on Rutford Ice Stream which also used an elastic model required an ice thickness much less than that observed. Reworking the Rutford Ice Stream data suggests that this greatly reduced ice thickness is not required, given the current sparse data coverage. The elastic model is used to improve the estimated grounding-line position on Rutford Ice Stream. Some of the difficulties in modelling ice-shelf flexure and locating grounding lines are discussed.

Fracture Geometries Consistent With Ttiltmeter Data And Pressure And Tracer Communication In The Glisp 1-1 Stimulation

Abstract The Gregoire Lake In-situ Steam Pilot (GLISP) in the Athabasca tar sands, includes three producing u:ells surrounding an injection well. The authors modeled tiltmeter records of the fracturing treatment of the central injection well, and pressure and tracer communication from the central injection well to the producing wells, and investigated how these analyses support various fracture geometry scenarios. Several different fracture geometry scenarios have been examined. Theorencal surface tiltmeter vectors for these scenarios are compared with observed vectors. Also, theoretical time delays based on propagation of pressure transients and tracer material have been predicted for these scenarios, and compared with observed delays. The results of these comparisons are expressed in the form of a "range" of fracture geometries that are most consistent with measured tiltmeter data in the field, and pressure and tracer time delays in the three production wells. Interpretations within this range include complex fracture systems with one or two fracture components rising steeply from tire base of the McMurray Formation into the overlying depleted zone. Dominant horizontal fracturing does not appear to occur. Introduction The Gregoire Lake In-Situ Steam Pilot (GLISP), in the Athabasca tar sands, includes three producing wells, P-l, P-2, and P-3 surrounding an injection well, I-1 (Fig. la). On February 5, 1987, the injection well I-1 was fractured with about 114 m3 (30 000 gallons) of linear gel without proppant, to communicate with the producing wells. The cased well was fractured through notches near the bottom of the McMurray Formation. Profiles of bottomhole treating pressure and rate are shown in Figure 1b. A wide variety of diagnostic information has been evaluated in regard to the geometry of the induced fracture by MacKenzie, et al. (1). Several different fracture geometries are ranked, in order, according to how consistent they are with these data. The most probable fracture geometry is a U-shaped fracture, or variations of this generic shape, such as a non-symmetric U-shaped fracture, an L-shaped fracture, or an elongated bowlshaped fracture. The goal of this paper is to examine how consistent are three totally independent diagnostic observations of fracturing (tiltmeters and pressure, tracer communication). Delays in pressure transient peaks at the three production wells varied from 1.1 to 3.6 hours (from beginning of fracture). Hot water and tracers were injected after the treatment to assess communication with the other wells in the pilot The time to maximum tracer at the producing wells was 3 to 6 days. Several different fracture geometries scenarios have been examined. Using an in-house code, theoretical surface tilt meter vectors for these scenarios are compared with observed vectors. Also, theoretical time delays based on propagation of pressure transient and tracer material have been predicted for these scenarios, and compared with observed delays. The results of this analysis are expressed in the form of a "range" of fracture geometries that are most consistent with these data.

Fracture Stimulation In The Glisp Tar Sands Pilot-Case History

Abstract This paper reviews fracture stimulation of the McMurray formation in the Gregoire Lake In Situ Steam Pilot Project (GLISP), in northeastern Alberta. The purpose of the stimulations was to effect interwell communication between the injection and production wells prior to steam injection. In situ stresses measured at shallow reservoir depths, indicated a possibilityfor horizontal fractures, a very desirable prerequisite/or the potential success of the experimental pilot. Analyses of data collected during and following the final fracture stimulation in the pilot area on the I-I central injection well (i.e. tiltmeter data, bottomhole pressures, temperature profiles, tracer surveys, production response, laboratory experience, field observations) and fracture theory indicate that a very complex fracture geometry resulted with both horizontal and vertical components. These data were used to address an array of nine possible fracture geometries to determine the more probable fracture configurations). Interwell communication by hydraulic fracturing was achieved over short distances but not over the full well spacing of the pilot. Introduction Amoco Canada Petroleum Company Ltd., Petro-Canada, Inc., and the Alberta Oil Sands Technology and Research Authority (AOSTRA) are participants in an in situ experimental recovery pilot to investigate the feasibility of producing the heavy oil reserves in the McMurray formation in the Gregoire Lake area of the Athabasca tar sands deposit. This pilot known asthe Gregoire Lake In-Situ Steam Pilot or "GLISP" is located as shown in Figure 1. The pilot area, shown in Figure 2, was selected on the stratigraphy seen in the C-l abandoned core hole as being the preferred lithology in the general area. Three confirmation wells (D-l, D-2 and P-l) were drilled. Log evaluation indicated that the P-l well had the most suitable reservoir properties and therefore it was selected as the central injection well for the pilot. One observation well (O-1) was drilled and three producing wells were planned to be drilled triangularly spaced around this injection well (see original pilot configuration, Fig. 2). Drilling the three production wells was deferred pending the possible fracture stimulation of the central injection well. In addition, a tiltmeter survey was planned to monitor fracture evolution and better define the optimum locations for the wells. Injectivity tests run on the three confirmation wells indicated that fracturing would be required to accomplish communication between wells. Concurrent with the injectivity tests, in situ stress tests were also conducted to obtain fracture design data. The majority of these stress tests indicated closure pressures approximating the overburden pressure. In some tests there was also evidence of a much lower closure pressure. These data suggested the strong possibility of a horizontal fracture which would be preferred for the pilot. However, there was the distinct possibility of a vertical fracture, or more likely, a complex fracture geometry with both horizontal and vertical components. The P-l central injector was fracture stimulated through perforations at the base of the McMurray formation.

Tiltmeter Observations From Doake Ice Rumples, Ronne Ice Shelf, Antarctica (Abstract)

New tiltmeter data from Doake Ice Rumples on Ronne Ice Shelf are presented. The tiltmeters detected flexing of the ice shelf close to the grounding line, due to tidal forces. In earlier studies on Rutford Ice Stream, flow was from grounded ice into the floating ice shelf. In contrast, the area studied on Ronne Ice Shelf exhibits flow from the ice shelf on to grounded ice rumples. The data span 5 km of the up-stream ice shelf and much of the grounded ice. Approximately 30 d of continuous tilt data are available for one site on the ice shelf. Analysis of this record can determine the dominant tidal frequencies present. A number of shorter records cover periods normally of a few days. These were obtained from sites both up-stream and down-stream of the long-term site. They have been used to investigate the variation in tilt amplitude with distance from the grounding line. The approximate position of the grounding line was located from the position of strand cracks and by using surface-elevation and ice-thickness data. The tiltmeters helped to confirm this position. Over grounded ice, tidal flexing is clearly present up to about 2 km from the grounding line. Beyond this it appears to be absent. The signal recorded by tiltmeters on grounded ice farther than 2 km from the grounding line is attributed, at present, to a temperature dependency of the tiltmeters. The flexing of ice shelves at tidal frequencies has previously been treated as an elastic problem. Available models which use beam theory require the use of a time-dependent function and a reduced “effective” ice thickness in the elastic modulus. On preliminary analysis, data from the ice shelf and grounding line of Doake Ice Rumples appear to be consistent with these theories. More detailed analysis and interpretation is required in order to confirm the level of agreement and to determine possible variations due to the “reversed” nature of the flow direction.

Tiltmeter Observations From Doake Ice Rumples, Ronne Ice Shelf, Antarctica (Abstract)

New tiltmeter data from Doake Ice Rumples on Ronne Ice Shelf are presented. The tiltmeters detected flexing of the ice shelf close to the grounding line, due to tidal forces. In earlier studies on Rutford Ice Stream, flow was from grounded ice into the floating ice shelf. In contrast, the area studied on Ronne Ice Shelf exhibits flow from the ice shelf on to grounded ice rumples.The data span 5 km of the up-stream ice shelf and much of the grounded ice. Approximately 30 d of continuous tilt data are available for one site on the ice shelf. Analysis of this record can determine the dominant tidal frequencies present.A number of shorter records cover periods normally of a few days. These were obtained from sites both up-stream and down-stream of the long-term site. They have been used to investigate the variation in tilt amplitude with distance from the grounding line.The approximate position of the grounding line was located from the position of strand cracks and by using surface-elevation and ice-thickness data. The tiltmeters helped to confirm this position. Over grounded ice, tidal flexing is clearly present up to about 2 km from the grounding line. Beyond this it appears to be absent. The signal recorded by tiltmeters on grounded ice farther than 2 km from the grounding line is attributed, at present, to a temperature dependency of the tiltmeters.The flexing of ice shelves at tidal frequencies has previously been treated as an elastic problem. Available models which use beam theory require the use of a time-dependent function and a reduced “effective” ice thickness in the elastic modulus. On preliminary analysis, data from the ice shelf and grounding line of Doake Ice Rumples appear to be consistent with these theories. More detailed analysis and interpretation is required in order to confirm the level of agreement and to determine possible variations due to the “reversed” nature of the flow direction.

Tidal and secular tilt from an earthquake zone: thresholds for detection of regional anomalies

Tiltmeter data from an array of three boreholes at the Charlevoix observatory in the Charlevoix seismic region of Que´bec have been analysed for evidence of tectonically related signals. The secular tilt is dominated by water table induced effects which can be substantially removed by linear regression of the water level on the tilt. Short-term (days to months) anomalies are shown to be detectable at the 0.3 μrad to 1 μrad level depending on the depth of the measurement. Long-term changes in the linear drift as small as 0.1 μrad/yr would be detectable in all of the boreholes. Large spatial anomalies in the mean tidal admittance among boreholes preclude its use in refining either models of the regional crustal structure or the adjacent marine tide distribution. Strongly coherent time variations in the tidal admittance among the observations of all the major tidal constituents are shown to be generated by corresponding variations in marine tidal loading in the St. Lawrence estuary. Diurnal band variations are closely correlated with the tide gauge data. The semi-diurnal constituents show a weaker correlation because of the complex spatial pattern within the estuary of time variations in this band. Inspection of the residual admittance variations for the M 2 and O 1 constituents demonstrates that the thresholds for detecting tectonic tidal tilt anomalies are ±2% and ±5–8%, respectively. The level of earthquake activity in the Charlevoix seismic zone throughout the period of the borehole tiltmeter experiment was sufficiently low that no significant tilt anomalies were expected or were undeniably detected.

Inversion of tiltmeter data obtained during two hydraulic fracturing experiments (In French) : Julien, P; Saleh, B; Blum, P A; Despax, D Rev Fr GeotechN31, 1985, P55–63

Inversion of tiltmeter data obtained during two hydraulic fracturing experiments (In French) : Julien, P; Saleh, B; Blum, P A; Despax, D Rev Fr GeotechN31, 1985, P55–63

Eruption Prediction Aided by Electronic Tiltmeter Data at Mount St. Helens

Telemetry from electronic tiltmeters in the crater at Mount St. Helens contributed to accurate predictions of all six effusive eruptions from June 1981 to August 1982. Tilting of the crater floor began several weeks before each eruption, accelerated sharply for several days, and then abruptly changed direction a few minutes to days before extrusion began. Each episode of uplift was caused by the intrusion of magma into the lava dome from a shallow source, causing the dome to inflate and eventually rupture. Release of magma pressure and increased surface loading by magma added to the dome combined to cause subsidence just prior to extrusion.

Modelling ocean-loading effects on tidal gravity in Australia

Summary. Tidal gravity measurements were recorded in Australia and Papua New Guinea between 1974 and 1976 as a cooperative programme between the International Centre for Earth Tides, Bruxelles, the Department of Geodesy at the University of New South Wales and the Government of Australia's Bureau of Mineral Resources, Geology and Geophysics, Canberra. This series of observations was analysed for the effect of ocean loading on tidal gravity with a view to modelling these effects as a function of space and time in the Australian region. In the absence of reliable tidal models for S2, K1 and P1 tidal constituents, it was decided to restrict the analysis to the M2 wave as the observations were only taken over four-month periods at many of the sites. The ocean-tide models used for the M2 wave in the present study were two models prepared by Hendershott and one by Zahel. The ocean-tide loading effects were computed using each of the three tidal models at every one of the nine sites occupied in Australia and Papua New Guinea using a simplified model of the crustal response as an alternative to representation by the set of load deformation coefficients h′n, k′n. The objective was the development of a model for the Earth response which can generate the ocean loading effect on tidal gravity at any point on the Australian mainland so that it is adequate for correcting quasi-radial distance measurements to extraterrestrial sources in Earth orbit for the effect of Earth tides to ±2 cm, in conjunction with adequate supplementary tiltmeter data. It is shown that a ten-parameter representation of the crustal response is adequate for modelling the deformation of the Earth tide by ocean loading at any site in Australia with a resolution of ±2 μgal provided extrapolation is not performed over distances greater than 103 km. This is assessed as being of sufficient accuracy for all purposes in high-precision geodesy. It was not possible to establish the stability of the model over long periods of time in excess of a few years in the absence of a continuous recording of tidal gravity over sufficiently long periods of time. The separation of the actual radial deformation due to ocean loading from the effect on tidal gravity could not be achieved in the absence of reliable tiltmeter data in the region which were only available at Cooney Observatory, Armidale.