Gravity Coverage Research Articles

Computation and Analysis of Geopotential Number in São Paulo, Brazil

In recent decades, important steps have been taken to implement the physical concepts of Geodesy in practice, con- cerning height systems. Despite the difficulties involving gravity field modeling, with the establishment of conventions, standards, and computation strategies, the realization of the International Height Reference System (IHRS) is well underway. For a global system, there are constraints for some countries, especially for those with sparse gravity data, mountain regions, and vast areas. In terms of methodology, the computation can be performed directly using the Global Geopotential Models (GGM), recovering existing geoid models, or determining pointwise the gravity potential using integral formulas. In general, the regional gravity modeling is given by numerical integration or least-squares collocation and more recently adopting the spherical radial basis functions. The first approach allows determining the earth’s gravity component at a specific point and adjusting the integral formula according to the gravity coverage. Since so far there is no common sense about the best methodology, computation strategies are been analyzed. In this con- text, the paper aims to contribute to IHRF, computing the geopotential number in the scope of IHRF, using numerical integration to solve the Geodetic Boundary Value Problem and an existing recent quasi-geoid model in four stations in São Paulo state, Brazil. The first approach was performed considering two cases: a radius of 210 km and 110 km of gravimetric data coverage and the Global Geopotential Model GOCO05S truncated at 100 and 200, respectively. The results between solutions have shown a maximum difference of 94 cm, and a minimum difference of 10 cm.

Using High–Low Flight Gravity Data to Improve Geoid Model Precision: A Case Study in U.S. Virgin Islands (July 2021)

The Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project collected airborne gravity data over Puerto Rico (PR) and the U.S. Virgin Islands (VI). There was one large N-S/E-W survey at about 11-km altitude running from N17 to N21 latitude and E291 to E297 longitude. Considering the very limited surface gravity coverage on VI, only about 500 points, additional flights were conducted at lower altitudes (1.5–1.8 km). By adding these additional low altitude aerial gravity data, the geoid model precision is improved from 6.55 cm with no airborne data to 2.55 cm with both the high and low altitude data. Using only the high altitude flights achieves only 4.99-cm geoid model precision. The ~500 surface gravity data points in the target area do not improve the model precision significantly, which justifies the rationale of carrying out these lower altitude flights. Noticing the nearly three times geoid model precision improvements from the low flight data, we suggest to fly in low altitudes in future airborne gravimetric survey for geoid modeling in the areas that have limited surface gravity coverage and/or small topographic features like islands that are not well-sampled in high-altitude, wider-spaced data.

Towards an International Height Reference System: insights from the Colorado geoid experiment using AUSGeoid computation methods

We apply the AUSGeoid data processing and computation methodologies to data provided for the International Height Reference System (IHRS) Colorado experiment as part of the International Association of Geodesy Joint Working Groups 0.1.2 and 2.2.2. This experiment is undertaken to test a range of different geoid computation methods from international research groups with a view to standardising these methods to form a set of conventions that can be established as an IHRS. The IHRS can realise an International Height Reference Frame to be used to study physical changes on and within the Earth. The Colorado experiment study site is much more mountainous (maximum height 4401 m) than the mostly flat Australian continent (maximum height 2228 m), and the available data over Colorado are different from Australian data (e.g. much more extensive airborne gravity coverage). Hence, we have tested and applied several modifications to the AUSGeoid approach, which had been tailored to the Australian situation. This includes different methods for the computation of terrain corrections, the gridding of terrestrial gravity data, the treatment of long-wavelength errors in the gravity anomaly grid and the combination of terrestrial and airborne data. A new method that has not previously been tested is the application of a spherical harmonic high-pass filter to residual anomalies. The results indicate that the AUSGeoid methods can successfully be used to compute a high accuracy geoid in challenging mountainous conditions. Modifications to the AUSGeoid approach lead to root-mean-square differences between geoid models up to ~ 0.028 m and agreement with GNSS-levelling data to ~ 0.044 m, but the benefits of these modifications cannot be rigorously assessed due to the limitation of the GNSS-levelling accuracy over the computation area.

Challenges in the use of potential field data for studies of remote, deep-water frontier basins

Given the sparsity of seismic data covering remote offshore frontier basins, Geoscience Australia?s studies of these areas rely heavily on gravity and magnetic data. These data complement 2D seismic reflection data by allowing interpretations to be extrapolated away from the generally widely-spaced seismic lines. However, interpreting potential-field data in frontier areas is challenging. Continuous gravity coverage is only available from satellite-altimetry-derived data, but the resolution of these data is often not sufficient for detailed geological interpretation and the data are subject to error in areas of shallow water or close to the coastline. Shipborne data provide higher-resolution data along track, but the sparsity of ship-tracks means that line levelling is difficult. Modelling gravity data to test seismic interpretations of sediment thickness is further complicated by poor constraints on the depth to the Moho. Efforts to understand frontier basins could be improved with better knowledge of the Moho (e.g. from seismic refraction data) and by better ship-track coverage. Extensive acquisition of airborne gravity and magnetic data over Australia?s margins would also provide better coverage and provide a seamless link between offshore and onshore areas.

Airborne gravimetry takes off in the Western Australia ‘Generation 2’ reconnaissance gravity mapping project

In 1974, the Australian Bureau of Mineral Resources, Geology and Geophysics completed a 15-year systematic reconnaissance gravity survey of Australia with stations spaced at 11 km. The 1976 Gravity Map of Australia was a seminal product; half a century later, the data still provide the only coverage for substantial parts of the continent.In 2005, the Geological Survey of Western Australia, supported by Geoscience Australia, commenced a program of regional ground gravity surveys with 2.5 km station spacing, a sixteen-fold improvement of resolution over the ‘first generation’ BMR data. In 2013, GSWA declared its aim of completing ‘second-generation’ reconnaissance gravity coverage of WA by 2020.In 2016, with 45% of the State yet to be surveyed in the north and east, and ground access issues slowing progress and making uniform coverage increasingly difficult, GSWA and GA undertook the first government-commissioned regional aerogravity survey in Australia, using the Sander Geophysics AIRGrav system. The 38,000 line-km survey covering 84,000 km2 in the East Kimberley region was flown at 2.5 km line-spacing for compatible spatial resolution with GSWA’s regional ground surveys.We compare airborne with ground gravimetry in the context of the East Kimberley project and conclude that, for reconnaissance surveys: aerogravity costs now approach those of ground surveys; spatial resolution is equivalent; data precision is not a critical factor; and airborne and ground data can be merged seamlessly for interpretation.Consequently, new aerogravity surveys were commissioned over 264,000 km2 of northern WA in the Tanami, northeast Canning and Kidson regions.

A GOCE only gravity model GOSG01S and the validation of GOCE related satellite gravity models

We compile the GOCE-only satellite model GOSG01S complete to spherical harmonic degree of 220 using Satellite Gravity Gradiometry (SGG) data and the Satellite-to-Satellite Tracking (SST) observations along the GOCE orbit based on applying a least-squares analysis. The diagonal components (Vxx, Vyy, Vzz) of the gravitational gradient tensor are used to form the system of observation equations with the band-pass ARMA filter. The point-wise acceleration observations (ax, ay, az) along the orbit are used to form the system of observation equations up to the maximum spherical harmonic degree/order 130. The analysis of spectral accuracy characteristics of the newly derived gravitational model GOSG01S and the existing models GOTIM04S, GODIR04S, GOSPW04S and JYY_GOCE02S based on their comparison with the ultra-high degree model EIGEN-6C2 reveals a significant consistency at the spectral window approximately between 80 and 190 due to the same period SGG data used to compile these models. The GOCE related satellite gravity models GOSG01S, GOTIM05S, GODIR05S, GOTIM04S, GODIR04S, GOSPW04S, JYY_GOCE02S, EIGEN-6C2 and EGM2008 are also validated by using GPS-leveling data in China and USA. According to the truncation at degree 200, the statistic results show that all GGMs have very similar differences at GPS-leveling points in USA, and all GOCE related gravity models have better performance than EGM2008 in China. This suggests that all these models provide much more information on the gravity field than EGM2008 in areas with low terrestrial gravity coverage. And STDs of height anomaly differences in China for the selected truncation degrees show that GOCE has improved the accuracy of the global models beyond degree 90 and the accuracies of the models improve from 24 cm to 16 cm. STDs of geoid height differences in USA show that GOSG01S model has best consistency comparing with GPS-leveling data for the frequency band of the degree between 20 and 160.

A Flight Test of the Strapdown Airborne Gravimeter SGA-WZ in Greenland

An airborne gravimeter is one of the most important tools for gravity data collection over large areas with mGal accuracy and a spatial resolution of several kilometers. In August 2012, a flight test was carried out to determine the feasibility and to assess the accuracy of the new Chinese SGA-WZ strapdown airborne gravimeter in Greenland, in an area with good gravity coverage from earlier marine and airborne surveys. An overview of this new system SGA-WZ is given, including system design, sensor performance and data processing. The processing of the SGA-WZ includes a 160 s length finite impulse response filter, corresponding to a spatial resolution of 6 km. For the primary repeated line, a mean r.m.s. deviation of the differences was less than 1.5 mGal, with the error estimate confirmed from ground truth data. This implies that the SGA-WZ could meet standard geophysical survey requirements at the 1 mGal level.

Comparison of surface and shipborne gravity data with satellite-altimeter gravity data in Hudson Bay

Several free satellite-altimetry models in the public domain may be used to study the Earth's gravity field. In this study, we compare the Bouguer anomaly map of the Hudson Bay region calculated from the satellite-derived V18 model of D. Sandwell (Scripps Institution of Oceanography) and W. Smith (National Oceanic and Atmospheric Administration) with one compiled from measurements (GSC data set) made on the ice shelf and by ship. Noise exists in both data sets and takes the form of linear anomalies along ship tracks on the GSC map and of random high-frequency anomalies on the map derived from satellite altimetry. Spectral analysis of both maps indicates a good correspondence for anomalies wider than 15 km. In central Hudson Bay, the mean difference between the two maps is −1.6 mGal indicating only a small offset between the two data sets. Standard deviation of the difference is 2.35 mGal. The correspondence between anomaly shapes on profiles indicates that satellite-derived gravimetry is certainly good enough for regional studies. Our analysis confirms the usefulness of satellite-derived gravity in areas with incomplete gravity coverage such as the Hudson Strait and Foxe Basin north of Hudson Bay.

Telfer Region FALCON Airborne Survey, Western Australia

The Telfer region in the Paterson Province consists of Proterozoic metasediments of the Yeneena Group, of lower-upper Greenschist facies. Sediment lithologies vary from siltstones to quartzites and carbonates. These are intruded by intrusive suites ranging from felsic to mafic, plutonic to hyperbyssal. Known world-class assets include the Telfer Gold operation, and the O?Callaghan?s Tungsten resource (both 100% Newcrest owned). The sedimentary and intrusive lithologies have considerable density contrast. These factors, combined with steeply dipping on-edge stratigraphy make the Telfer region ideal for gravity surveying. Previous gravity coverage in the region has been a combination of small prospect scale surveys, and 1980s vintage 2 x 4 km regional helicopter surveys with barometric levelling control. In order to get systematic gravity coverage of good resolution, a FALCON survey of 250m line spacing, 60m flight height, was flown in 2010. The data show a number of blind felsic plutons, some intersected in drilling, some interpreted ? especially associated with the O?Callaghan?s tungsten resource, and the Trotman?s Stockwork tungsten prospect. The data also outline much more clearly the NE over SW thrust faults in the region, and the late north-south transfer faults, the most well known of which is the Graben Fault through Telfer Dome. Three dimensional gravity inversions performed over the O?Callaghan?s and Trotman?s Stockwork tungsten areas in Gocad/VPMG software have defined extra felsic intrusive targets for tungsten and gold mineralisation. Further diamond drilling will increase inversion constraints.

Airborne gravity tests in the Italian area to improve the geoid model of Italy

ABSTRACTAirborne gravimetry is an important method for measuring gravity over large unsurveyed areas. This technology has been widely applied in Canada, Antarctica and Greenland to map the gravity fields of these regions and in recent years, in the oil industry. In 2005, two tests in the Italian area were performed by ENI in cooperation with the Politecnico di Milano and the Danish National Space Center. To the knowledge of the authors, these were the first experiments of this kind in Italy and were performed over the Ionian coasts of Calabria and the Maiella Mountains. The Calabria test field is characterized by strong gravity variations due to the geophysical and topographic structure of the area. The ground gravity coverage is also quite dense. It was thus possible to compare airborne gravity with the ground observed values in order to check the precision of the airborne gravimetry. The second campaign was performed in an unsurveyed area centred on the Maiella Mountains, thus filling the data gap of this zone. Comparisons with existing ground data were also carried out in this area.After smoothing, the collected data have an accuracy of 2–3 mgal, as derived by cross‐over analysis. Moreover, the collocation method applied to compare and merge ground‐based and airborne data proved to be efficient and reliable. The standard deviation of the discrepancies between airborne data and collocation upward continued gravity is, in both cases, less than 8 mgal. In the Maiella test, the gravity field obtained by merging airborne and ground data using collocation also provides a more detailed description of the high‐frequency pattern of the geopotential field in this area.

New South Australian Geophysical Data – Pace Northern G2 Gravity Survey

A combined heli-borne ground gravity acquisition program of approximately 17,000 gravity stations was acquired over the Northern G2 region within South Australia, as part of The Plan for Accelerating Exploration (PACE) program. The resolution of the gravity data was dramatically enhanced from the existing gravity coverage with a station spacing of approximately 7 kilometres to a 1.5 kilometre grid within the PACE survey. There have been several discoveries in the south of this region, leading to this area becoming a focus for data acquisition. Acquisition of geophysical data, especially gravity on a regional scale is a key strategy of the PACE program. It is expected that the new gravity data will provide exploration companies with many geophysical targets ready to drill.Technical Area: Geophysics in Government Surveys

Interpretation of the Petroleum Potential of the Darling Basin, a Process of Integration and Iteration

With an area of over 100.000 Km2 and sedimentary thicknesses of over 8000 m. the Darling Basin is the largest onshore basin in New South Wales and one of the largest in Australia. In the entire basin only 20 wells have been drilled and approximately 1800 km of multifold seismic recorded. This quantity of seismic is grossly inadequate for a basin of this size and in addition acquisition has been concentrated over regional structural highs. These highs are usually poor data areas due to structural complexity and coupling problems in acquisition related to the high proportion of outcrop. However the available seismic data has confirmed the presence of large structures and the NSW Department of Mineral Resources is now undertaking a major study of the basin?s petroleum potential. The Darling Basin does have fair to good gravity and magnetic coverage and this has provided a solid basis for the current studies of the basin. Interpretations of the potential field data have been made with a view to interpreting the depth and structure of ?basement? and also to interpreting intra-sedimentary structures. The current evaluation of the basin is an iterative process. Recent potential field interpretations have utilised all the available information including seismic, well and surface geology. The Department of Mineral Resources is carrying out seismic programs based on these interpretations of the potential field and other available data. These seismic results are being used to update and modify, if necessary, the earlier interpretations. The seismic surveys as a by-product, have also indicated the existence of more extensive and better outcrop data than had previously been expected and the interpretation, or in some cases reinterpretation, of this outcrop data, is also being incorporated in the current study.

Late Hercynian leucogranites modelling as deduced from new gravity data : the example of the Millevaches massif (Massif Central, France)

Abstract The Millevaches granitic complex, located in the northern part of the French Massif Central, is elongated in a N-S direction, perpendicular to the main E-W trend of the Hercynian belt. It is affected on its limits and in its core by several ductile shear zones that have necessarily played a great role in the emplacement and exhumation of the massif. Based on gravity modelling and recent field observations, this study intends to highlight the massif structure at depth and discuss its mode of emplacement and relations with the surrounding terrains. The new gravity and density measurements on the north-east part of the Millevaches massif improve the gravity coverage of the northern Limousin. Using these new data we model the deep structure of the Millevaches plateau. The density measurements made on the different types of granites of the massif, and on the surrounding terrains improve the interpretation of the Bouguer anomaly. Analysis and inversion of the residual Bouguer anomaly in the area show that the Millevaches massif is 2 to 4 km-thick, from north to south and from west to east, locally rooting down to about 6 km deep in its eastern and southern terminations. These two zones coincide with porphyritic plutons and, because of the complex composite structure of the massif, cannot be definitively interpreted as feeding zones. In the field, the N-S-oriented Pradines vertical fault affects the core of the massif on 4 to 5 km width. Microstructural observations evidence that the faulting is contemporaneous of the granites emplacement. We suggest that this tectonic lineament could have triggered the migration of the magma, although it is not related to a clear gravity anomaly. AMS measurements in the north-central part of the Millevaches massif suggest that the magnetic foliation and lineation display a general sub-horizontal pattern. Moreover, on the western border of the Millevaches massif, the Argentat deep seismic profile shows sub-horizontal layering of gneisses and micaschists and evidences normal faulting offset of this layering along Argentat fault. This agrees fairly well with the gravity results, suggesting that (i) the Millevaches massif would be at a high structural level in the crust, (ii) the exhumation of the massif would have been favoured along the Argentat normal fault. As a whole, the massif can be described as a laccolith, 2 to 4 km-thick, emplaced as a “magmatic lens” into the sub-horizontally foliated gneisses and micaschists.

On geoid determination from airborne gravity

Recent advances in the performance of scalar airborne gravimetry, routinely yielding gravity data with an accuracy as high as 1 mGal with a minimum spatial resolution (full wavelength) of 10 km, allow for the use of airborne gravity in precise geoid computations. Theoretical issues related to geoid determination from discrete samples of band-limited airborne gravity and practical geoid computations based on high-frequency synthetic data and actual observations are discussed. The mathematical model for geoid determination from band-limited airborne gravity which is introduced is based on Helmert's reduction of gravity/co-geoid and the application of the discretized integral formula developed specifically for airborne gravity data. Computational formulae are derived for the following specifications of airborne gravity data: almost-white observation noise, a band-limited frequency content of processed observations, and a smooth regular surface on which gravity observations are collected. Two major applications of airborne gravity data for geoid determination are discussed. Airborne gravimetry can be used over areas with a sparse or no ground gravity coverage to provide the medium- to high-frequency components of the Earth's gravity field. These data can be used in combination with global gravity models for the computation of the gravimetric geoid. Airborne gravimetry can also be used to fill in gaps in existing ground gravity coverage (mainly in inaccessible areas). Both approaches are tested with actual airborne data observed using an inertially referenced airborne gravimeter at the Alexandria test range near Ottawa, Canada. Observed airborne gravity disturbances, combined with either ground or global gravity data, are used for the determination of the gravimetric geoid in the two applications outlined above. The combined solutions are compared to the latest official Canadian gravimetric geoid and to available GPS/levelling data in the area. Numerical results show that airborne data can be used for geoid determination with centimetre-level accuracy (medium- and high-frequency information) over areas with negligible topographical effects on gravity and the geoid.

Merging of Airborne Gravity and Gravity Derived from Satellite Altimetry: Test Cases Along the Coast of Greenland

The National Survey and Cadastre - Denmark (KMS) has for several years produced gravity anomaly maps over the oceans derived from satellite altimetry. During the last four years, KMS has also conducted airborne gravity surveys along the coast of Greenland dedicated to complement the existing onshore gravity coverage and fill in new data in the very-near coastal area, where altimetry data may contain gross errors. The airborne surveys extend from the coastline to approximately 100 km offshore, along 6000 km of coastline. An adequate merging of these different data sources is important for the use of gravity data especially, when computing geoid models in coastal regions.

Gravity survey of the pantanal wetland: data acquisition and processing

A gravity survey was conducted between 1988 and 1993, mostly during the dry season (july to october) in the Pantanal Wetland and surrounding highlands, aiming to obtain a first regional gravity coverage. The Pantanal Wetland is an important geomorphological unit extending from -54º to -59.º, east-west, and from 14º to 23º , north-south, in SW Brazil. The topography ranges from 80 m to 200 m above sea level within the plain. A total of 3,662 gravity stations were collected. Because of the difficulty in accessing the flood plain, apart from conventional ground survey, fluvial and air routes were used in the field data acquisition. The data acquisition procedure is described in detail and a discussion on the final gravity anomaly accuracy is presented. From the total number of stations, 7 stations belong to the Fundamental Brazilian Gravity Network, 1,886 gravity stations were obtained on bench marks, 1,505 stations were set up together with barometric levelling or the altitude was estimated from topographic maps in places with smooth topography (horizontal gradient < 0.5 m/km), 237 stations were collected along the main rivers and 33 stations using aircraft.

Regional gravity and regolith geochemistry as an integrated tool for mineral exploration

The Geological Survey of Western Australia has carried out helicopter-supported regional regolith sampling and gravity capture, based on a 4×4 km sampling grid. Started initially as a regolith-sampling program in 1994, gravity capture was added in 1998 with little increase in cost or time. Each survey targets a 1:250 000 scale map sheet in less extensively explored parts of Western Australia, which invariably have poor gravity coverage. The gravity data provide valuable information on regional-scale structures and help in the interpretation of regolith chemistry. In conjunction with regional aeromagnetic surveys, the gravity data are also of use in regional mapping. In an area of Proterozoic and Archaean rocks in central Western Australia, areas of potential mineralization have been identified by combining gravity data with regolith chemistry and bedrock mapping to indicate where regional faults and shear zones have probably acted as conduits for mineralizing fluids. The simultaneous capture of gravity data and regolith samples in the same field program is an effective approach to regional mineral exploration.

Synthetic Spectra of H Balmer and He i Absorption Lines. II. Evolutionary Synthesis Models for Starburst and Poststarburst Galaxies

We present evolutionary stellar population synthesis models to predict the spectrum of a single-metallicity stellar population, with a spectral sampling of 0.3 A in five spectral regions between 3700 and 5000 A. The models, which are optimized for galaxies with active star formation, synthesize the profiles of the hydrogen Balmer series (Hβ, Hγ, Hδ, H8, H9, H10, H11, H12, and H13) and the neutral helium absorption lines (He I λ4922, He I λ4471, He I λ4388, He I λ4144, He I λ4121, He I λ4026, He I λ4009, and He I λ3819) for a burst with an age ranging from 106 to 109 yr, and different assumptions about the stellar initial mass function (IMF). Continuous star formation models lasting for 1 Gyr are also presented. The input stellar library includes non-LTE absorption profiles for stars hotter than 25,000 K and LTE profiles for lower temperatures. The temperature and gravity coverage is 4000 K ≤ Teff ≤ 50,000 K and 0.0 ≤ log g ≤ 5.0, respectively. The metallicity is solar. It is found that the Balmer and He I line profiles are sensitive to the age, except during the first 4 Myr of the evolution, when the equivalent widths of these lines are constant. At these early stages of the evolution, the profiles of the lines are also sensitive to the IMF. However, strong H Balmer and He I lines are predicted even when the low-mass cutoff of the IMF is as high as 10 M☉. The equivalent widths of the Balmer lines range from 2 to 16 A and those of the He I lines from 0.2 to 1.2 A. During the nebular phase (cluster younger than about 10 Myr), Hβ ranges from 2 to 5 A and He I λ4471 ranges between 0.5 and 1.2 A. The strength of the lines is maximum when the cluster is a few hundred (for the Balmer lines) and a few tens (for the He I lines) of Myr old. In the continuous star formation scenario, the strength of the Balmer and He I lines increases monotonically with time until 500 and 100 Myr, respectively. However, the lines are weaker than in the burst models owing to the dilution of the Balmer and He I lines by the contribution from very massive stars. The high spectral resolution of the profiles is useful to reproduce the absorption wings observed in regions of recent star formation and to estimate the effect of the underlying absorption on the nebular emission lines. The strength of the nebular emission Balmer and He I lines compared with the stellar absorption components indicates that Hδ and the higher order terms of the Balmer series and He I are dominated by the stellar absorption component if an instantaneous burst is older than 5 Myr. Some of the He I lines (e.g., He I λ3819 and He I λ4922) are more favorable than others (e.g., He I λ4471) for the detection of stellar features in the presence of nebular emission. We estimate that the correction to the He I λ4471 nebular emission line due to the stellar absorption is between 5% and 25%, if the nebular emission has equivalent width between 10 and 2.5 A (corresponding to a burst age between 1 and 3 Myr). The models can be used to date starburst and poststarburst galaxies until 1 Gyr. They have been tested on data for clusters in the LMC, the super-star cluster B in the starburst galaxy NGC 1569, the nucleus of the dwarf elliptical NGC 205 and a luminous E+A galaxy. The full data set is available for retrieval at our websites or on request from the authors.

Tectonic pattern of South America inferred from tidal gravity anomalies

The knowledge of several physical parameters that characterize a tectonic unit is important to evaluate its dynamic evolution and, therefore, its role in the continental evolution. Several methodologies have been used to measure the elastic properties of tectonic units using: seismic waves, the level of coherence between the wavelength pattern of the topography and gravity fields, borehole deformation rate, mineral fabric, etc. In this study, the Earth's dynamic elastic response to gravity tides is applied for measuring the lithosphere effective elastic thickness. This methodology proved to be a good alternative to the conventional coherence method. It was applied over the continent, taking into account the poor gravity coverage in South America and the restricted area of some tectonic units. Results are presented as lateral distribution of the effective elastic thickness in South America.

The gravity field over the Ungava Bay region from satellite altimetry and new land-based data: implications for the geology of the area

Gravity data were obtained along two transects on the southern coast of Ungava Bay, which provide continuous gravity coverage between Leaf Bay and George River. The transects and the derived gravity profiles extend from the Superior craton to the Rae Province across the New Quebec Orogen (NQO). Interpretation of the transect along the southwestern coast of Ungava Bay suggests crustal thickening beneath the NQO and crustal thinning beneath the Kuujjuaq Terrane, east of the NQO. Two alternative interpretations are proposed for the transect along the southeastern coast of the bay. The first model shows crustal thickening beneath the George River Shear Zone (GRSZ) and two shallow bodies correlated with the northern extensions of the GRSZ and the De Pas batholith. The second model shows constant crustal thickness and bodies more deeply rooted than in the first model. The gravity models are consistent with the easterly dipping reflections imaged along a Lithoprobe seismic line crossing Ungava Bay and suggest westward thrusting of the Rae Province over the NQO. Because no gravity data have been collected in Ungava Bay, satellite altimetry data have been used as a means to fill the gap in data collected at sea. The satellite-derived gravity data and standard Bouguer gravity data were combined in a composite map for the Ungava Bay region. The new land-based gravity measurements were used to verify and calibrate the satellite data and to ensure that offshore gravity anomalies merge with those determined by the land surveys in a reasonable fashion. Three parallel east-west gravity profiles were extracted: across Ungava Bay (59.9°N), on the southern shore of the bay (58.5°N), and onshore ~200 km south of Ungava Bay (57.1°N). The gravity signature of some major structures, such as the GRSZ, can be identified on each profile.