EFFECTS OF STRUCTURE ON THE GRAVITY FIELD OF WYOMING

Geophysics ◽  
1968 ◽  
Vol 33 (5) ◽  
pp. 781-804 ◽  
Author(s):  
Alexander Malahoff ◽  
Ralph Moberly
Keyword(s):  
Gravity Field ◽  
Gravity Data ◽  
Bouguer Anomaly ◽  
Sedimentary Basins ◽  
Local Variation ◽  
Theory Of Change ◽  
Bouguer Gravity ◽  
Rock Outcrops ◽  
Rock Density ◽  
Rock Types

A density model of the upper crust of Wyoming, computed on structural and petrographic evidence without prior reference to gravity data, when compared with the observed gravity field, demonstrates that: (1) Existing measurements and theory of change of sedimentary rock density with depth of burial are satisfactory for gravity interpretation for local rock types and the slower rates of sedimentation in the area. (2) Gravity stations established over sedimentary basins are generally as satisfactory as those on basement rock outcrops for depicting regional Bouguer gravity trends. (3) The Laramide deformation was mainly germanotype. The regional gradient in Bouguer gravity across Wyoming has increasingly negative values to the southwest. Local variation in the Bouguer anomaly field is related mainly to the low density Cretaceous and Cenozoic sediments of the basins, and to a lesser degree to the Laramide structural blocks. Interpretation of the gravity field indicates that the blocks are bounded by near‐vertical faults extending into the upper mantle, and that the uplifted blocks are undercompensated (i.e., rootless). In order to lead to a reasonable depiction of isostasy for the region, plots of mean Bouguer anomalies versus mean elevation must be made over 2°×2° squares, or larger, because of the variation in structure and composition of the crust.

Effect of the lateral topographic density distribution on interpretational properties of Bouguer gravity maps

2019 ◽  
Author(s):  
Samurdhika Rathnayake ◽  
Robert Tenzer ◽  
Martin Pitoňák ◽  
Pavel Novák
Keyword(s):  
Density Distribution ◽  
Gravity Data ◽  
Average Density ◽  
Rift System ◽  
Bouguer Gravity ◽  
East African Rift System ◽  
Rock Density ◽  
Mountainous Regions ◽  
Rock Types ◽  
Crustal Density

Summary Until recently, the information about the topographic density distribution has been limited to only certain regions and some countries, while missing in the global context. The UNB_TopoDens is the first model that provides the information about a lateral topographic density globally. The analysis of this model also reveals that the average topographic density for the entire continental landmass (excluding polar glaciers) is 2247 kg m−3. This density differs significantly from the value of 2670 kg m−3 that is typically adopted to represent the continental upper crustal density. In this study, we use the UNB_TopoDens density model to inspect how the topographic density variations affect interpretational properties of Bouguer gravity maps. Since this model provides also the information about density uncertainties of individual lithologies (main rock types), we estimate the corresponding errors in the Bouguer gravity data. Despite a new estimate of the average topographic density corresponds to relative changes of ∼16 per cent in values of the topographic gravity correction, these changes do not affect interpretational properties of Bouguer gravity maps. The anomalous topographic density distribution (taken with respect to the average density of 2247 kg m−3), however, modifies the Bouguer gravity pattern. We demonstrate that the gravitational contribution of anomalous topographic density is globally mostly within ± 25 mGal, but much large values are detected in Himalaya, Tibet, central Andes and along the East African Rift System. Our estimates also indicate that errors in the Bouguer gravity data attributed to topographic density uncertainties are mostly less than ± 15 mGal, but in mountainous regions could reach large values exceeding even ± 50 mGal. Unarguably, the UNB_TopoDens model provides an improved information about the global topographic density variations and their uncertainties. Nevertheless, much more in situ measurements of rock density samples together with detailed 3D geological models are still necessary to understand better the actual density distribution within the whole topography, particularly to mention a density change with depth.

scholarly journals Evaluation of Gravity Data Derived from Global Gravity Field Models Using Terrestrial Gravity Data in Enugu State, Nigeria

2018 ◽  
Vol 8 (1) ◽  
pp. 145-153 ◽  
Author(s):  
O.I. Apeh ◽  
E.C. Moka ◽  
V.N. Uzodinma
Keyword(s):  
Gravity Field ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Mean Square ◽  
Bouguer Gravity ◽  
Bouguer Anomalies ◽  
The Earth ◽  
Global Gravity ◽  
Enugu State ◽  
Gravity Field Models

Abstract Spherical harmonic expansion is a commonly applied mathematical representation of the earth’s gravity field. This representation is implied by the potential coeffcients determined by using elements/parameters of the field observed on the surface of the earth and/or in space outside the earth in the spherical harmonic expansion of the field. International Centre for Gravity Earth Models (ICGEM) publishes, from time to time, Global Gravity Field Models (GGMs) that have been developed. These GGMs need evaluation with terrestrial data of different locations to ascertain their accuracy for application in those locations. In this study, Bouguer gravity anomalies derived from a total of eleven (11) recent GGMs, using sixty sample points, were evaluated by means of Root-Mean-Square difference and correlation coeficient. The Root-Mean-Square differences of the computed Bouguer anomalies from ICGEMwebsite compared to their positionally corresponding terrestrial Bouguer anomalies range from 9.530mgal to 37.113mgal. Additionally, the correlation coe_cients of the structure of the signal of the terrestrial and GGM-derived Bouguer anomalies range from 0.480 to 0.879. It was observed that GECO derived Bouguer gravity anomalies have the best signal structure relationship with the terrestrial data than the other ten GGMs. We also discovered that EIGEN-6C4 and GECO derived Bouguer anomalies have enormous potential to be used as supplements to the terrestrial Bouguer anomalies for Enugu State, Nigeria.

ANALYSIS OF GRAVITY DATA NEAR CONTINENTAL MARGINS

1974 ◽  
Vol 14 (1) ◽  
pp. 114
Author(s):  
A. J. Flavelle ◽  
Y. Yoshimura
Keyword(s):  
Continental Margin ◽  
Sea Water ◽  
Gravity Data ◽  
Bouguer Anomaly ◽  
Sedimentary Basins ◽  
Continental Margins ◽  
Upper Crust ◽  
Anomaly Map ◽  
Bouguer Anomaly Map ◽  
Thickness Variations

In general large, thick sedimentary basins are delineated by negative gravity features. The gravity data are usually expressed in the form of Bouguer anomaly contours.Ordinary Bouguer anomaly data fail as a direct indicator of approximate sedimentary thickness in zones on and adjacent to the continental margin. Rapid variations in crustal and ocean thickness cause gravitational variations which are not removed during the computation of Bouguer anomaly values.If crustal thickness variations are known or can be calculated then gravitational corrections can be made which take this factor into account. Similar corrections for variations in sea water attraction can be made. The resultant Bouguer anomaly map, corrected for those variations, will indicate in more definite terms density variations in the material of the upper crust. In particular Bouguer anomaly patterns over continental areas adjacent to the continental slope can be more easily interpreted in terms of sedimentary thickness.

Serial gravity-constrained cross-sections in the Central Pyrenees validating its structural style

2020 ◽  
Author(s):  
Ruth Soto ◽  
Pilar Clariana ◽  
Conxi Ayala ◽  
Antonio M. Casas-Sainz ◽  
Teresa Román-Berdiel ◽  
...  
Keyword(s):  
Cross Sections ◽  
Gravity Data ◽  
Bouguer Anomaly ◽  
Leading Edge ◽  
Gravity Modelling ◽  
Rock Types ◽  
Structural Style ◽  
Thrust Sheets ◽  
Structural Architecture ◽  
Central Pyrenees

<p>Cenozoic contractional deformation in the Central Pyrenees generated several basement thrust sheets involving Paleozoic rocks and decoupled Mesozoic and Cenozoic cover units detached on the main décollement level, the Triassic evaporites. The overall geometry and structural architecture of the chain have already been established based on numerous geological and geophysical data obtained during several decades. This work aims to validate the overall accepted geometry of the Central part of the chain by the construction of six serial cross-sections constrained by gravity data and 2.5D gravity modelling. The study area comprises the southern half of the Axial Zone between La Maladeta and Andorra-Mont Louis granites and its southern leading edge as well as the northernmost part of the South-Pyrenean Zone.</p><p>New gravity data were acquired and combined with previous existing databases to obtain Bouguer anomaly and residual anomaly maps of the study area. Six serial gravity-constrained cross sections have been built using available geological maps, previous published works, new geological and gravity data and 2.5D gravity modelling. Density values for gravity modelling were derived from 231 laboratory measurements of rock samples collected in the field from non-weathered outcrops that include all rock types outcropping in the study area. The residual anomaly map shows a good correlation between basement thrust sheets and gravity highs whereas negative anomalies seem to correspond to (1) Mesozoic basins, (2) Triassic evaporites and (3) Late Variscan igneous bodies. The 2.5D gravity modelling along the six cross sections highlights: (i) strong along-strike variations on the gravity signal due to lateral differences of the surficial and subsurface occurrence of Triassic evaporites, (ii) different geometry at depth of the Late Variscan igneous bodies outcropping in the study area and (iii) geometric lateral variations of the basement thrust sheets and their relationship with the Mesozoic-Cenozoic units.</p>

scholarly journals New Bouguer Gravity Maps of Venezuela: Representation and Analysis of Free-Air and Bouguer Anomalies with Emphasis on Spectral Analyses and Elastic Thickness

2012 ◽  
Vol 2012 ◽  
pp. 1-15 ◽  
Author(s):  
Javier Sanchez-Rojas
Keyword(s):  
Gravity Data ◽  
Bouguer Anomaly ◽  
Geodetic Reference System ◽  
Bouguer Gravity ◽  
Free Air ◽  
Data Compilation ◽  
Regional Tectonic ◽  
Moho Depths ◽  
Free Air Anomaly ◽  
Gravity Map

A new gravity data compilation for Venezuela was processed and homogenized. Gravity was measured in reference to the International Gravity Standardization Net 1971, and the complete Bouguer anomaly was calculated by using the Geodetic Reference System 1980 and 2.67 Mg/m3. A regional gravity map was computed by removing wavelengths higher than 200 km from the Bouguer anomaly. After the anomaly separation, regional and residual Bouguer gravity fields were then critically discussed in term of the regional tectonic features. Results were compared with the previous geological and tectonic information obtained from former studies. Gravity and topography data in the spectral domain were used to examine the elastic thickness and depths of the structures of the causative measured anomaly. According to the power spectrum analysis results of the gravity data, the averaged Moho depths for the massif, plains, and mountainous areas in Venezuela are 42, 35, and 40 km, respectively. The averaged admittance function computed from the topography and Free-Air anomaly profiles across Mérida Andes showed a good fit for a regional compensation model with an effective elastic thickness of 15 km.

Stable inversion of gravity anomalies of sedimentary basins with nonsmooth basement reliefs and arbitrary density contrast variations

Geophysics ◽  
1999 ◽  
Vol 64 (3) ◽  
pp. 754-764 ◽  
Author(s):  
Valéria C. F. Barbosa ◽  
João B. C. Silva ◽  
Walter E. Medeiros
Keyword(s):  
Gravity Anomaly ◽  
Gravity Data ◽  
A Priori ◽  
Bouguer Anomaly ◽  
Gravity Anomalies ◽  
Sedimentary Basins ◽  
Upper And Lower Bounds ◽  
Density Contrast ◽  
Good Precision ◽  
Rift Basins

We present a new, stable method for interpreting the basement relief of a sedimentary basin which delineates sharp discontinuities in the basement relief and incorporates any law known a priori for the spatial variation of the density contrast. The subsurface region containing the basin is discretized into a grid of juxtaposed elementary prisms whose density contrasts are the parameters to be estimated. Any vertical line must intersect the basement relief only once, and the mass deficiency must be concentrated near the earth’s surface, subject to the observed gravity anomaly being fitted within the experimental errors. In addition, upper and lower bounds on the density contrast of each prism are introduced a priori (one of the bounds being zero), and the method assigns to each elementary prism a density contrast which is close to either bound. The basement relief is therefore delineated by the contact between the prisms with null and nonnull estimated density contrasts, the latter occupying the upper part of the discretized region. The method is stabilized by introducing constraints favoring solutions having the attributes (shared by most sedimentary basins) of being an isolated compact source with lateral borders dipping either vertically or toward the basin center and having horizontal dimensions much greater than its largest vertical dimension. Arbitrary laws of spatial variations of the density contrast, if known a priori, may be incorporated into the problem by assigning suitable values to the nonnull bound of each prism. The proposed method differs from previous stable methods by using no smoothness constraint on the interface to be estimated. As a result, it may be applied not only to intracratonic sag basins where the basement relief is essentially smooth but also to rift basins whose basements present discontinuities caused by faults. The method’s utility in mapping such basements was demonstrated in tests using synthetic data produced by simulated rift basins. The method mapped with good precision a sequence of step faults which are close to each other and present small vertical slips, a feature particularly difficult to detect from gravity data only. The method was also able to map isolated discontinuities with large vertical throw. The method was applied to the gravity data from Reco⁁ncavo basin, Brazil. The results showed close agreement with known geological structures of the basin. It also demonstrated the method’s ability to map a sequence of alternating terraces and structural lows that could not be detected just by inspecting the gravity anomaly. To demostrate the method’s flexibility in incorporating any a priori knowledge about the density contrast variation, it was applied to the Bouguer anomaly over the San Jacinto Graben, California. Two different exponential laws for the decrease of density contrast with depth were used, leading to estimated maximum depths between 2.2 and 2.4 km.

An integrated geophysical approach for imaging subbasalt sedimentary basins: Case study of Jam River Basin, India

Geophysics ◽  
2007 ◽  
Vol 72 (6) ◽  
pp. B141-B147 ◽  
Author(s):  
V. Chakravarthi ◽  
G. B. K. Shankar ◽  
D. Muralidharan ◽  
T. Harinarayana ◽  
N. Sundararajan
Keyword(s):  
Gravity Field ◽  
River Basin ◽  
Gravity Data ◽  
Sedimentary Basins ◽  
Gravity Inversion ◽  
Data Points ◽  
3D Gravity ◽  
Regional Gravity ◽  
Godavari Basin

An integrated geophysical strategy comprising deep electrical resistivity and gravity data was devised to image subbasalt sedimentary basins. A 3D gravity inversion was used to determine the basement structure of the Permian sediments underlying the Cretaceous formation of the Jam River Basin in India. The thickness of the Cretaceous formation above the Permian sediments estimated from modeling 60 deep-electric-sounding data points agrees well with drilling information. The gravity effect of mass deficit between the Cretaceous and Permian formations was found using 3D forward modeling and subsequently removed from the Bouguer gravity anomaly along with the regional gravity field. The modified residual gravity field was then subjected to3D inversion to map the variations in depth of the basement beneath the Permian sediments. Inversion of gravity data resulted in two basement ridges, running almost east to west, dividing the basin into three independent depressions. It was found that the Katol and Kondhali faults were active even during post-Cretaceous time and were responsible for the development of the subsurface basement ridges in the basin. The inferred 3D basement configuration of the basin clearly brought out the listric nature of these two faults. Further, the extension of the Godavari Basin into the Deccan syneclise and the fact that the source-rock studies show the presence of hydrocarbons in the Sironcha block in the northern part of the Godavari Basin also shed some light on the hydrocarbon potential of the Jam River Basin.

A gravity survey of the Dundas buried valley west of Copetown, Ontario

1986 ◽  
Vol 23 (1) ◽  
pp. 110-114 ◽  
Author(s):  
John P. Greenhouse ◽  
Mark Monier-Williams
Keyword(s):  
Gravity Data ◽  
Bouguer Anomaly ◽  
Three Dimensional ◽  
Bouguer Gravity ◽  
Three Dimensional Model ◽  
Niagara Escarpment ◽  
Reflection And Refraction ◽  
Bedrock Depth ◽  
Regional Gradient ◽  
Estimated Error

A total of 243 Bouguer gravity readings have been completed over a 9 km2 area covering the terminus of the Dundas buried valley, a prominent reentrant in the Niagara Escarpment. A 1.5 mGal (15 μm s−2) Bouguer anomaly clearly defines an abrupt closure of the bedrock topography west of Copetown, confirming and extending the results of earlier seismic reflection and refraction surveys. An apparent tributary in the contoured gravity, entering the terminus from the south, is unfortunately poorly resolved. It underlies property to which access was denied. The Bouguer gravity data are transformed to a model of the bedrock depth via a three-dimensional model. The mean error between data and model gravity, after correcting the former for a regional gradient, is less than 0.1 mGal (1 μm s−2). The estimated error in the observations themselves is ± 0.11 mGal (1.1 μm s−2). The observed Bouguer gravity is compared with that which would be produced by the present Niagara Gorge were it to underlie the survey area. The magnitude and dimensions of the computed anomalies are remarkably similar.

scholarly journals Geometrical Characterisation of the Mamfe Basin, Cameroon, from the Earth, Gravitational Model (EGM 2008)

2018 ◽  
Vol 7 (1) ◽  
pp. 94
Author(s):  
Anatole Eugene Djieto Lordon ◽  
Mbohlieu YOSSA ◽  
Christopher M Agyingi ◽  
Yves Shandini ◽  
Thierry Stephane Kuisseu
Keyword(s):  
Average Length ◽  
Gravity Data ◽  
Bouguer Anomaly ◽  
Igneous Rocks ◽  
Average Depth ◽  
The Earth ◽  
Gravitational Model ◽  
Petroleum Generation ◽  
Anomaly Map ◽  
Bouguer Anomaly Map

Gravimetric studies using the ETOPO1-corrected high resolution satellite-based EGM2008 gravity data was used to define the surface extent, depth to basement and shape of the Mamfe basin. The Bouguer anomaly map was produced in Surfer 11.0. The Fast Fourier Transformed data was analyzed by spectral analysis to remove the effect of the regional bodies in the study area. The residual anomaly map obtained was compared with the known geology of the study area, and this showed that the gravity highs correspond to the metamorphic and igneous rocks while the gravity lows match with Cretaceous sediments. Three profiles were drawn on the residual anomaly map along which 2D models of the Mamfe basin were drawn. The modeling was completed in Grav2dc v2.06 software which uses the Talwini’s algorithm and the resulting models gave the depth to basement and the shape of the basement along the profiles. After processing and interpretation, it was deduced that the Mamfe basin has an average length and width of 77.6 km and 29.2 km respectively, an average depth to basement of 5 km and an overall U-shape basement. These dimensions (especially the depth) theoretically create the depth and temperature conditions for petroleum generation. 

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