High-precision Gravity Research Articles

Changes in instrumental sensitivity of some feedbacksystems used in LaCoste-Romberg gravimeters

High-precision gravity measurements and records are used to investigate geodynamic processes. Theamplitude of the gravity changes induced by these phenomena is of the order of 10-8 g to 10-9 g, where the global average of g (gravity) is 9.80 m s-2. Instrument sensitivities up to 10-9 are therefore required to detect gravity changes. As changes in instrument sensitivity can reduce the repeatability of measurements and affect the phase and amplitude of recorded gravity signals, the accurate calibration of gravimeters in high-precision gravimetry is topical. The stability of the calibration factors in two LaCoste-Romberg gravimeters has been investigated through different calibration methods. The main results show that (a) the application of multiple calibration approaches is a powerful constraint on instrumental response and identifies real or fictitious changes in instrument sensitivity; (b) a comparison between spring and superconducting gravimeters is the most suitable way of almost completely determining the transfer function of spring gravimeters in the tidal band. Moreover, a correlation was detected between stronger changes in instrument sensitivity and the occurrence of seismic events. Thus mechanical perturbation of spring gravimeters during earthquakes can affect the stability of their calibration factors.

Study of the Earth’s free core nutation by tidal gravity data recorded with international superconducting gravimeters

By stacking high-precision tidal gravity observations obtained with superconducting gravimeters at six stations in China, Japan, Belgium, France, Germany and Finland, the local systematical discrepancies in the parameter fitting, caused by atmospheric, oceanic tidal loading and the other local environmental perturbations, are eliminated effectively. As a result, the resonance parameters of the Earth’s free core nutation are accurately determined. In this study, the eigenperiod of free core nutation is given as 429.0 sidereal days, which is in agreement with those published in the previous studies. It is about 30 sidereal days less than those calculated in theoretical models (about 460 sidereal days), which confirms the real ellipticity of the fluid core of the Earth to be about 5% larger than the one expected in assumption of hydrostatic equilibrium. The quality factor (Q value) of free core nutation is given as about 9543, which, compared with those determined before based on the body tide observations, is much larger, but more close to those obtained using the VLBI observations. The complex resonance strength is also determined as (−6.10×10−4, −0.01 ×10−4)°/h, which can principally describe the deformation characteristics of an anelastic mantle.

Local seasonal gravity changes in the lower rhine embayment, Germany

Repeated high resolution gravity measurements with LaCoste and Romberg gravity meters have been carried out for nearly two years in the Lower Rhine Embayment, Germany. In the long run, the observations aim at detecting long term gravity changes that are associated with surface subsidence due to groundwater withdrawal in a nearby browncoal mining area. Concurrently, short term gravity variations on local scales are also investigated to find out their influence on long term gravity observations. For that reason, two local networks with extensions of a few kilometers each have been set up, one of them across an active fault where presently large height changes are occurring (up to 2.2 cm/a), the other one in a stable intra-block site showing no changes of height. The gravity data, so far, do not reveal any significant long term changes. However, seasonal variations of up to 500 nms−2 are observed between stations on both sides of a steeply dipping tectonic fault. This is where significant height changes due to ground water withdrawal occur, while gravity differences between stations on the same side of that fault as well as the observations in the other network remain constant throughout the observation period. The physical reason for the gravity variations is not yet clear. We assume, that different types of vegetation and soil cause seasonal changes in soil moisture, and thus in subsurface density. This study emphasizes that stations for repeated high precision gravity observations (aiming e. g. to detect seismogenic precursory anomalies) may be largely influenced by local, effects of presumably hydrological origin and that networks even on the 100 m scale can considerably suffer from systematic distortions.

Three years of high precision gravity measurements at the gravimetric station of Brasimone - Italy

From August 1995 up to now, at the Enea Research Center of Brasimone, in the Italian Apennines between Bologna and Florence (Italy: 44º07'N, 11º.07'E, 890 m height), the superconducting gravimeter GWR model TT70 number T015 has been continuously recording the variation of the local gravity field, in the frame of the Global Geodynamics Project. The gravimetric laboratory, being a room of the disused nuclear power plant of Brasimone, is a very stable site, free from noise due to human activities. Data blocks of several months of continuous gravity records have been collected over a time span of three years, together with the meteorological data. The gravimeter has been calibrated at relative accuracy better than 0.3% with the aid of a mobile mass system, by imposed perturbations of the local gravity field and recording the gravimeter response. The results of this calibration technique were checked by two comparison experiments with absolute gravimeters performed during this period: the first, in May 1994 with the aid of the symmetrical rise and fall gravimeter of the Institute of Metrology Colonnetti of Turin, and the second in October 1997 involving an FG5 absolute gravimeter of the Institute de Physique du Globe of Strasbourg. The gravimeter signal was analysed to compute a high precision tidal model for Brasimone site. Starting from a set of gravimetric and atmospheric pressure data of high quality, relative to 46 months of observation, we performed the tidal analysis using Eterna 3.2 software to compute amplitudes, gravimetric factors and phases of the main waves of the Tamura catalogue. Finally a comparison experiment between two of the STS-1/VBB broadband seismometers of the MedNet project network and the gravity records relative to the Balleny Islands earthquake (March 25, 1998) were analysed to look for evidence of normal modes due to the free oscillations of the Earth.

Deployment Test Methods for a Large Deployable Mesh Reflector

The deployment characteristicsof large structures must be evaluated by ground tests. A deployment test system is described that enables high-precision gravity canceling, in contrast to the conventional radial support test method (hanging a ree ector by cables suspended from a high ceiling ). A large deployable ree ector composed of hexagonal pyramid-shaped modules is used in the deployment test. The ree ector is deployed by a coil spring force and controlled by motors and cables for a quasistatic stable deployment motion. A magnetically suspended slider test system was developed for accurate deployment tests. We cone rm that hanging at the pantograph bar is the most suitable test cone guration to simulate microgravity conditions, according to one-sided model experiments and deploying forceanalyses using mechanism analysissoftware. We also establish that a vertical support method, using thedevelopedmagneticallysuspendedsliders,enableshigh-precisiondeploymentfora largedeployablemesh ree ector.

Gravity changes and present-day dynamics of the island of Pantelleria (Sicily Channel—Italy)

The island of Pantelleria is an active volcano located in the Sicily Channel (southern Italy), occurring in the middle of a continental rift system. Since the 1980's the island has been periodically surveyed by means of geodetic and geophysical methods to monitor the regional and local volcanic dynamics. Also, high-precision gravity measurements were started in 1990. The present paper is an analysis of the time-space gravity changes. Gravity measurements were carried out on a network presently formed by twenty stations. The gravity network was fully surveyed in June 1990 and June 1995 and partially surveyed in September 1993. Two absolute gravity stations were established in 1993 to provide a reference system and to check for long-term variations. The areal distribution of the gravity changes during the 1990–1995 time interval, obtained in the thirteen stations of the 1990 network, is strongly similar to the Bouguer anomaly field and to the large-scale features of the basement of the island. Otherwise, gravity changes are directly correlated with the Bouguer anomaly and inversely correlated with the altimetric variations. Comparison with the geological setting suggests that the present activity may be ascribed to the influence of the geodynamics of the Sicily Channel.

Gravity modelling along a Lithoprobe seismic traverse, northern Grenville Province, western Quebec

High-precision gravity data were collected along Lithoprobe seismic reflection lines in the northern part of the Grenville Province, in western Quebec. An interpretation is presented for line 52, which starts some 60 km southeast of the Grenville Front, traverses the parautochthonous Reservoir Dozois Terrane including the allochthonous slice of the Réservoir Cabonga Terrane, and ends near the town of Mont-Laurier, in the allochthonous Mont-Laurier Terrane. On the regional scale, the Bouguer gravity anomaly is consistent with the interpretation of the seismic reflection data. It supports crustal thinning southward of the Grenville Front, under the Cabonga allochthon. This thinning may be related to postorogenic extension. The gravity modelling shows dramatic thinning of the lower crust and suggests that extension was accommodated by extrusion of the lower crust. The gravity modelling also requires a steep boundary between the Réservoir Cabonga and the Réservoir Dozois terranes extending to ~ 15 km. The geometry of the Baskatong ramp derived from gravity data is also consistent with the seismic interpretation. This supports the suggestion that the Baskatong ramp is a major discontinuity along which Proterozoic terranes were accreted. In the Réservoir Cabonga Terrane in the northern part of the profile, the residual gravity anomalies (short wavelength variations) are related to outcropping mafic intrusions. Modelling of these anomalies complements the seismic reflection data, which did not image the base of the intrusions. The interpretation calls for three small distinct gabbroic bodies that extend no deeper than 3 km. The total volume of the intrusions is ~ 3000 km3.

The rupture zone of Cascadia great earthquakes from current deformation and the thermal regime

An important but poorly known part of the earthquake hazard at near‐coastal cities of western North America from southern British Columbia to northern California is from great thrust earthquakes on the Cascadia subduction zone. Although there have been no such events in the historical record, there is good geological evidence that they have occurred in the past. The downdip landward limit of the seismogenic or seismic rupture zone on the subduction thrust fault has been estimated for the whole Cascadia margin from (1) the locked zone from dislocation modeling of current deformation data, and (2) the thermal regime, taking the downdip limit of seismic behavior on the fault to be controlled by temperature. The geodetic data include ten leveling lines, tide gauges at six locations along the coast, one high precision gravity line, seven horizontal strain arrays, and a continuously recording Global Positioning System (GPS) network. There is present uplift for most of the coast at a rate of a few millimeters per year, decreasing inland, and shortening across the coastal region at about 0.1 µstrain/yr (i.e., 10 mm/yr over a distance of 100 km). The present interseismic uplift is consistent with the great earthquake coseismic subsidence inferred from buried coastal salt marshes and other paleoseismicity data. The modeled width of the locked zone that is taken to be accumulating elastic strain averages 60 km fully locked, plus 60 km transition (90 km fully locked with no transition gives similar interseismic deformation). It is wider off the Olympic Peninsula of northern Washington and narrower off central Oregon to northern California. This unusually narrow downdip extent compared to many other subduction zones is a consequence of high temperatures associated with the young oceanic plate and the thick blanket of insulating sediments on the incoming crust. The variations in the modeled locked zone from geodetic data correspond well to variations along the margin of downdip temperatures on the fault as estimated from numerical thermal models, taking the maximum temperature for the fully locked seismogenic zone to be 350°C with a transition zone to 450°C. The temperatures on the subduction thrust fault and thus the downdip extent of the seismogenic zone depend on five local subduction parameters: (1) the age of the subducting plate, (2) the plate convergence rate, (3) the thickness of insulating sediments on the incoming crust, (4) the dip angle profile of the fault, and (5) the thermal properties of the overlying material. The landward limit to the seismogenic zone, extending little if at all beneath the coast, limits the ground motion from great subduction earthquakes at the larger Cascadia cities that lie 100–200 km inland. The narrow width also limits the earthquake size but events of magnitude well over 8 are still possible; the maximum depends on the along‐margin length. If the whole Cascadia margin seismogenic zone fails in a single event, empirical fault area versus magnitude relations give earthquakes as large asMw=9.

Fourth International Comparison of Absolute Gravimeters

In 1994, from 24 May to 8 June, the fourth International Comparison of Absolute Gravimeters was held at the Bureau International des Poids et Mesures, Sèvres, France. Eleven absolute gravimeters were operated at five sites and then compared by means of a high-precision gravity network which was itself observed throughout the period. The results demonstrate that absolute gravimetry can be carried out to an accuracy of 3 μGal to 4 μGal, which represents a substantial improvement since the first international comparison (1981, 10 μGal). The relatively low rate of improvement, one third of an order of magnitude in fifteen years, is certainly limited by current technology for measuring small time and space intervals. A notable finding of the comparison was an unsuspected source of systematic error which can be eliminated by new hardware or corrected through a correction algorithm.

Constraints on the usefulness of gravimetry for detecting precursory crustal deformations

The detectability of gravity changes arising from preseismic processes in the source region of an earthquake depends on the nature of the process, the magnitude of the impending event, the distance from the hypocentre and the background noise level. Using typical background spectra from recording spring-type gravimeters, superconducting gravimeters and long-term gravity network surveys, we have calculated the feasibility of detecting gravity precursors to thrust-type earthquakes. As a result of the 1/ f 2 nature of the gravity noise power spectral density the detectability of precursors by recording gravimeters depends on the rate of build-up (amplitude over rise time) as well as on the ultimate amplitude of the preseismic anomaly. Ruling out site-specific fluid effects, the maximum possible amplitude of the precursory gravity signal can be estimated for both preseismic dilatant volume expansion and preseismic dip-slip as a function of earthquake magnitude and depth. Combining these results shows that detection of gravity precursors over a wide range of possible rise times (minutes to years) would require a network of ten to fifteen recording superconducting gravimeters tied by regular bi-weekly high-precision gravity measurements distributed over a 30 × 30 km area where there was a reasonable expectation of seismic events in the range M = 6 to 7 at depths of no more than 10–20 km. If monitoring were restricted to a search for short-term precursors (minutes to hours) over a subduction zone, an unconnected network of superconducting gravimeters could be expected to detect gravity effects resulting from precursory aseismic slip prior to an M = 6 event under certain conditions.

Ordovician Carbonate Buildups: Potential Gas Reservoirs in the Ordos Basin, Central China

The Ordos basin of central China covers an area of about 25,000 km{sup 2}. A series of eastward moving overthrusts developed along its western flank, but most of the basin consists of a stable slope that dips westward less than one degree. The basin contains sediments from Sinian to Middle Ordovician and from the Middle Carboniferous to Cretaceous. Its evolutionary history is similar to that of the Alberta basin. Recently drilled wildcat wells have produced commercial gas flows that are closely associated with Ordovician carbonate buildups and a weathered surface between the Ordovician and Carboniferous. Most of the buildups consist of agal mounds; however, some Middle Ordovician reefs developed in the western portion and along the southern margin of the Ordos basin. More than 200 buildups were delineated using seismic stratigraphic techniques. They can be divided into four distinct types. The growth and distribution of buildups were controlled by sea-level fluctuations. The interpretations made in this study were based on the integration of results from a variety of analyses including vertical profiling, differential interformational velocity analysis, amplitude versus offset comparisons, G-log analysis, seismic modeling techniques, and high-precision gravity surveys. The best gas prospects are the Ordovician carbonate buildups distributed aroundmore » the basin's central uplift. The delineation of carbonate buildups and the demonstration that they are associated with commercial gas flows open the gate for future gas exploration in this area.« less

Eighteen years of observations on the Fennoscandian land uplift gravity lines

In order to investigate the variation in gravity caused by the Fennoscandian land uplift, the Nordic countries have established 4 profiles with near-zero gravity differences along the latitudes 65°, 63°, 61° and 56° N. High-precision gravity measurements have been repeatedly made on these lines since 1975, 1966, 1976 and 1977, respectively. The observations have recently been published in one volume.

Gravity and crustal movements: The canadian experience

Repeated high precision gravity measurement have already played an important role in the detection of crustal deformation in Canada and elsewhere, but even more useful results can be expected through more widespread use of gravity in combination with other techniques. The crucial element in the process is the development of a good physical model on which the experiment can be based. Otherwise, considerable time and effort can be spent on determining the most appropriate field strategy. New technical developments on the horizon appear to offer enhanced opportunities for gravity studies of crustal processes. The coming availability of the Global Positioning System and transportable absolute gravimeters will open up the possibility of regional studies (i.e., areas of the order of 100 km or perhaps greater) of crustal movements at reasonable cost. Within Africa the development of an African Gravity Standardization Net will be a major first step in any program to provide a better understanding of the neo-tectonic framework of this vast continent.

Oscillations in photon distribution of squeezed states and interference in phase space

The drive for both noise-free message transmission1,2 and high precision gravity wave detection3,4 has stimulated immense effort on a key element, a squeezed state5,6 of the electromagnetic field. Such non-classical states have been investigated theoretically in great detail1–7 and have now been realized experimentally in four laboratories in the United States8–13. However, nowhere in the literature have we been able to find the striking feature of a squeezed state which we report here: an oscillatory distribution in photon number14,15. These oscillations, and the conditions which produce them, came to light in the course of an investigation of sudden transitions16 (the Franck–Condon effect in molecular physics17,18) based on the semi-classical description of a quantum state19 as motion of a representative point in the phase space defined by oscillator coordinate and oscillator momentum.

First results of precise gravity measurements on the “A-B-C” profile

Repeated high-precision gravity measurements on a north-south profile between La Paz and Santiago de Chile and east-west profiles between the coastline and the high Cordilleras are conducted in order to determine vertical motion of the ground. Geophysical considerations and results of the first campaign in October 1984 are presented.

A new method to study the liquid-vapour phase transition using very slow neutrons

We show that the reflection pattern for very slow neutrons impinging on thin films of a fluid is highly sensitive to the variation in the density distribution within the sample. As an example, 4He is used to study the temperature dependence of the density distribution in the immediate vicinity of the critical point with high precision and minimal gravity distortions.

A microgravimetric network in East Central Greece -- an area of potential seismic hazard

Summary. The eastern Mediterranean is a region of complex tectonic processes and associated horizontal and vertical displacements. A high precision gravity network has been established in the Atalanti area of central Greece to monitor temporal gravity changes on an annual or more frequent basis. A total of 68 sites have been measured in 1981 and 1982 with a maximum single instrument standard deviation of 0.08 gravity units after a least squares network adjustment. Analysis of the gravity differences between the two measuring epochs exhibits no change of gravity over the network with a precision of 0.1 1 gravity units. It is proposed that the gravity values given form a stable base for continued observations which will enable the authors to resurvey the region in the event of precursory foreshocks. Observation of the Atlantic network will continue on an annual basis preserving the same observation sequence for reasons of symmetry.

Direct gravimetric detection of magma movements at Mount Etna

Mount Etna is Europe's most active volcano, erupting some 2 × 1010 kg of new material each year1. Its eruptions generally have two distinct forms: continuous explosive activity near the summit with low rates of lava effusion, or massive, and often quieter, lava effusion from its flanks along regional tectonic fissures. The question of where magma is stored before eruption is not yet resolved. Here I present results from high-precision gravity surveys covering two recent eruptions. Gravity changes after the major flank eruption of March 1981 indicate draining of magma from a dyke 14 km long, at an elevation of 0–1 km and striking in a direction close to that of the eruptive fissure. Changes from the explosive sub-terminal eruptions of September 1980 do not indicate draining from any near-surface reservoir. It is proposed that short-term magma storage can occur close to the surface before a flank eruption, but that summit eruptive activity is consistent with a Source at greater depth.

Some results concerning crustal movements in Finland

Land uplift isobases determined by two precise levellings cover the whole country. High-precision gravity measurements on the land uplift gravity line running along an approximate latitude of 63° N were repeated nine times in thirteen years. No secular variation in gravity was observed. Records have been made with a long water-tube tilt meter since 1978. The tidal parameters were determined and the drifts of the instrument were studied.

Gravity and height variations connected with the current rifting episode in Northern Iceland

Following a first gravity survey in 1938, high-precision gravity measurements with LaCoste-Romberg gravimeters have been carried out in northern Iceland, in 1965, 1970/ 1971 and 1975, in order to detect eventual secular gravity variations at that supramarine part of a constructive plate boundary. Since 1975, when an active rifting episode started there, gravity and height variations have been observed in annual intervals. These surveys concentrated on the approx. 100 km long and 5 km wide fissure zone associated with the Krafla central volcano, which is the center of the present activity. The observations, which have been performed at an accuracy level of ± 0.01 mGal, resp. ± 0.03 m, give significant information about the long-term behaviour of the vertical mass shifts, occuring during the rifting process. A steep gravity-variation gradient, observed south of the Krafla caldera between 1965 and 1975, may be interpreted as some kind of precursor of the current process. Since 1975 the rifting event is characterized by shifting activity areas, with gravity decrease and elevation at the flanks, and gravity increase and subsidence at narrow central parts of the Krafla fissure zone, the annual variations reaching ± 0.1 mGal, resp. ∓ 0.5 m and more. Since 1978, the activity has concentrated on the Gjástikki region, north of the Krafla volcano, with decreasing variations in the southern and northern parts of the fissure swarm. The average gravity—height-variation ratio calculated for two profiles south and north of Krafla, over the observation periods between 1975 and 1979, amounts to −0.2 mgal/m, but extreme values are found at recently activated areas.