Concept of effective density: Key to gravity depth determinations for sedimentary basins

Geophysics ◽  
1989 ◽  
Vol 54 (11) ◽  
pp. 1474-1482 ◽  
Author(s):  
Vadim A. Litinsky
Keyword(s):  
Gravity Anomaly ◽  
Gravity Data ◽  
Effective Density ◽  
Density Contrast ◽  
Fast Method ◽  
Residual Gravity ◽  
Depth Dependence ◽  
Depth Function ◽  
Infinite Slab ◽  
Tucson Basin

The gravity anomaly observed over a layered sedimentary basin is close to that calculated over a basin with the same configuration and depth but filled by homogeneous sediments with density equal to the effective (weighted average) density of the real layered basin. Therefore, it is possible to calculate the effective density contrast of a basin from the residual gravity anomaly over it, if the depth of the basin is known at least at one point. Assuming a mathematical function for density‐depth dependence, an expression for the gravity‐depth dependence using the infinite slab (Bouguer) formula can be developed. This expression makes it possible to calculate the depth of a basin or an isopach map from gravity data. An exponential function and a new hyperbolic density‐depth function and their gravity‐depth functions are analyzed and implemented for determining the depths of basins from gravity data for the San Jacinto graben, California, and the Tucson basin, southern Arizona. The hyperbolic functions are more reliable and realistic than exponential ones. The effective density contrast of sediments filling the Tucson basin and density‐depth dependence were calculated using the infinite‐slab formula from residual gravity data over the deepest borehole, which entered pre‐Eocene bedrock at a depth of 3.66 km. The hyperbolic density‐depth dependence for the Tucson basin is also assumed to be effective for all other basins in the Basin and Range Province, southern Arizona and southwestern New Mexico. The density‐depth function is easily converted into a gravity‐depth function, using the infinite‐slab formula. The residual gravity map of each basin in this Province can then be transformed into a map of basin depth contours, using the hyperbolic gravity‐depth function. Comparison of depths of basins calculated from gravity data with available borehole information (19 wells) showed that the proposed simple and fast method has an error in the range of about 13 percent of the actual depth.

scholarly journals A Fast Interpretation Method for Inverse Modeling of Residual Gravity Anomalies Caused by Simple Geometry

2012 ◽  
Vol 2012 ◽  
pp. 1-10 ◽  
Author(s):  
Khalid S. Essa
Keyword(s):  
Gravity Anomaly ◽  
Shape Factor ◽  
Vertical Cylinder ◽  
Gravity Anomalies ◽  
Depth Estimation ◽  
Model Parameters ◽  
Fast Method ◽  
Random Errors ◽  
Residual Gravity ◽  
Amplitude Coefficient

An inversion technique using a fast method is developed to estimate, successively, the depth, the shape factor, and the amplitude coefficient of a buried structure using residual gravity anomalies. By defining the anomaly value at the origin and the anomaly value at different points on the profile, the problem of depth estimation is transformed into a problem of solving a nonlinear equation of the form . Knowing the depth, the shape factor can be estimated and finally the amplitude coefficient can be estimated. This technique is applicable for a class of geometrically simple anomalous bodies, including the semiinfinite vertical cylinder, the infinitely long horizontal cylinder, and the sphere. The efficiency of this technique is demonstrated with gravity anomaly due to a theoretical model, in each case with and without random errors. Finally, the applicability is illustrated using the residual gravity anomaly of Mobrun ore body, situated near Noranda, QC, Canada. The interpreted depth and the other model parameters are in good agreement with the known actual values.

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.

scholarly journals Integrated Subsurface Analysis of Thickness and Density for Liquefaction Hazard: Case Study of South Cilacap Region, Indonesia.

2021 ◽  
Vol 6 (1) ◽  
pp. 58-66
Author(s):  
Maulana Rizki Aditama ◽  
Huzaely Latief Sunan ◽  
FX Anjar Tri Laksono ◽  
Gumilar Ramadhan ◽  
Sachrul Iswahyudi ◽  
...  
Keyword(s):  
Gravity Anomaly ◽  
Gravity Data ◽  
Pass Filter ◽  
Low Pass Filter ◽  
Near Surface ◽  
Residual Gravity ◽  
Liquefaction Hazard ◽  
Residual Gravity Anomaly ◽  
Resistivity Model ◽  
2D Resistivity

The thickness of the liquefable layer can be the factor inducing liquefaction hazard, apart from seismicity. Several studies have been conducted to predict the possibility of the liquefable layer based on the filed sampling. However, a detailed investigation of the subsurface interpretation has not been defined, in particular the thickness estimation of the liquefable layer. This study is carried out in south Cilacap area where potential liquefaction is exists due to the earthquake history data and near surface condition. The aim of this study is to investigate the physical properties and thickness distribution using GGMplus gravity data and resistivity data. This research is conducted by spectrum analysis of gravity model and 2D resistivity model . This study’s main results is by performing the residual gravity anomaly with the associated SRTM/DEM data to define the subsurface physical distribution and structural orientation of the area. Residual gravity anomaly is also separated through the low pass filter in order to have robust interpretation. The residual anomaly indicates that the area has identical structural pattern with geological and SRTM map. The results show a pattern of high gravity index in the northeast area of ​​the study having range of 70 – 115 MGal gravity index, associated with the volcanic breccia, and a low gravity profile with less than 65 in the southwest, associated with the alluvial and water table dominated distribution. The thickness of Alluvial is determined by resistivity model with H1 at a range of 3 meters and H2 at a range of 4 m. This research is included in the potential liquefaction category with the potential for a large earthquake.

Multivariable Modified Teaching Learning Based Optimization (MM-TLBO) Algorithm for Inverse Modeling of Residual Gravity Anomaly Generated by Simple Geometric Shapes

2020 ◽  
Vol 25 (4) ◽  
pp. 463-476
Author(s):  
Ata Eshaghzadeh ◽  
Alireza Hajian
Keyword(s):  
Gravity Anomaly ◽  
Gravity Data ◽  
Optimal Solution ◽  
Model Parameters ◽  
Major Purpose ◽  
Residual Gravity ◽  
Tlbo Algorithm ◽  
Residual Gravity Anomaly ◽  
Teaching Learning Based Optimization ◽  
Teaching Learning

This paper presents an improved nature-based algorithm, namely multivariable modified teaching learning based optimization (MM-TLBO) algorithm, as in an iterative process can estimates the best values for the model parameters in a multi-objective problem. The algorithm works in two computational phases: the teacher phase and the learner phase. The major purpose of the MM-TLBO algorithm is to improve the value of the learners and thus, improving the value of the model parameters which leads to the optimal solution. The variables of each learner (model) are the radius ( R), depth ( h), shape factor ( q), density contrast ( ρ) and axis location ( x0) parameters. We apply MM-TLBO and TLBO methods for the residual gravity anomalies caused by the buried masses with a simple geometry such as spheres, horizontal and vertical cylinders. The efficiency of these methods are also tested by noise corruption synthetic data, as the acceptable results were obtained. The obtained results indicate the better performance the MM-TLBO algorithm than the TLBO algorithm. We have utilized the MM-TLBO for the interpretation of the six residual gravity anomaly profiles from Iran, USA, Sweden and Senegal. The advantage of the MM-TLBO inversion is that it can estimates the best solutions very fast without falling into local minimum and reaches to a premature convergence. The considered primary population for the synthetic and real gravity data are thirty and fifty models. The results show which this method is able to achieve the optimal responses even if a small population of learners had been considered.

On: “A least‐squares approach to depth determination from gravity data” by O.P. Gupta (GEOPHYSICS, 48, 357–360, March 1983)

Geophysics ◽  
1990 ◽  
Vol 55 (3) ◽  
pp. 376-377 ◽  
Author(s):  
El‐Sayed M. Abdelrahman
Keyword(s):  
Nonlinear Equation ◽  
Least Squares ◽  
Gravity Anomaly ◽  
Gravity Data ◽  
Residual Gravity ◽  
Depth Determination ◽  
Residual Gravity Anomaly ◽  
Least Squares Approach

In the article by Gupta, the problem of depth determination of a buried structure from the residual gravity anomaly has been transformed into a problem of finding the solution of a nonlinear equation of the form f(z) = 0. Gupta begins his formulation of the problem with equation (1) from Mettleton (1942) Eq. (1) [Formula: see text]

New gravity data and 3-D density model constraints on the Ivrea Geophysical Body (Western Alps)

2020 ◽  
Vol 222 (3) ◽  
pp. 1977-1991 ◽  
Author(s):  
M Scarponi ◽  
G Hetényi ◽  
T Berthet ◽  
L Baron ◽  
P Manzotti ◽  
...  
Keyword(s):  
Gravity Anomaly ◽  
Sea Level ◽  
Seismic Velocity ◽  
Gravity Data ◽  
Western Alps ◽  
Density Model ◽  
Density Contrast ◽  
Surface Rock ◽  
Spatial Coverage ◽  
Density Map

SUMMARY We provide a high-resolution image of the Ivrea Geophysical Body (IGB) in the Western Alps with new gravity data and 3-D density modelling, integrated with surface geological observations and laboratory analyses of rock properties. The IGB is a sliver of Adriatic lower lithosphere that is located at shallow depths along the inner arc of the Western Alps, and associated with dense rocks that are exposed in the Ivrea-Verbano Zone (IVZ). The IGB is known for its high seismic velocity anomaly at shallow crustal depths and a pronounced positive gravity anomaly. Here, we investigate the IGB at a finer spatial scale, merging geophysical and geological observations. We compile existing gravity data and we add 207 new relative gravity measurements, approaching an optimal spatial coverage of 1 data point per 4–9 km2 across the IVZ. A compilation of tectonic maps and rock laboratory analyses together with a mineral properties database is used to produce a novel surface rock-density map of the IVZ. The density map is incorporated into the gravity anomaly computation routine, from which we defined the Niggli gravity anomaly. This accounts for Bouguer Plate and terrain correction, both considering the in situ surface rock densities, deviating from the 2670 kg m–3 value commonly used in such computations. We then develop a 3-D single-interface crustal density model, which represents the density distribution of the IGB, including the above Niggli-correction. We retrieve an optimal fit to the observations by using a 400 kg m–3 density contrast across the model interface, which reaches as shallow as 1 km depth below sea level. The model sensitivity tests suggest that the ∼300–500 kg m–3 density contrast range is still plausible, and consequently locates the shallowest parts of the interface at 0 km and at 2 km depth below sea level, for the lowest and the highest density contrast, respectively. The former model requires a sharp density discontinuity, the latter may feature a vertical transition of densities on the order of few kilometres. Compared with previous studies, the model geometry reaches shallower depths and suggests that the width of the anomaly is larger, ∼20 km in west–east direction and steeply E–SE dipping. Regarding the possible rock types composing the IGB, both regional geology and standard background crustal structure considerations are taken into account. These exclude both felsic rocks and high-pressure metamorphic rocks as suitable candidates, and point towards ultramafic or mantle peridotite type rocks composing the bulk of the IGB.

Moho depth and sediment thickness estimation beneath the Red Sea derived from satellite and terrestrial gravity data

Geophysics ◽  
2013 ◽  
Vol 78 (5) ◽  
pp. G89-G101 ◽  
Author(s):  
Ahmed Salem ◽  
Chris Green ◽  
Simon Campbell ◽  
J. Derek Fairhead ◽  
Lorenzo Cascone ◽  
...  
Keyword(s):  
Red Sea ◽  
Gravity Data ◽  
Bouguer Anomaly ◽  
Density Contrast ◽  
Moho Depth ◽  
Pass Filter ◽  
Low Pass Filter ◽  
Terrestrial Gravity ◽  
Residual Gravity ◽  
Northern Red Sea

We sought to map the depth and density contrast of the Mohorovičić discontinuity (Moho) across the Red Sea area and to model sedimentary thickness from gravity data. The gravity data that are used are a combination of satellite and terrestrial gravity data processed into a Bouguer anomaly grid. A 200-km low-pass filter was used to separate this grid into regional and residual gravity grids. We inverted the regional gravity grid to a Moho depth map based on a density contrast map that is constrained by published seismic results. The interpreted Moho is shallowest ([Formula: see text]) along the axis of the central Red Sea, [Formula: see text] along the axis to the south, and [Formula: see text] in the northern Red Sea. The depth increased to [Formula: see text] at the coast and 35–40 km in the adjacent continents. The residual gravity data provided insights into the tectonic fabric along the whole rift and provided a good correlation with magnetic lineaments where these are available. We used the complete Bouguer anomaly to model sedimentary thicknesses constrained by wells and the interpreted Moho. The modeling results are consistent with the presence of large-scale, ridge parallel tilted fault blocks forming subbasins with a maximum depth of about 6–7 km. Our models suggest that the northern Red Sea has an asymmetric basement surface with the western side deeper than the eastern side. The results indicate the presence of oceanic crust in the central and southern parts of the Red Sea, but not in the north. The very thin crust and interpreted oceanic crustal density in the central Red Sea suggest that this area is fully oceanic—although possibly quite young.

scholarly journals RESIDUAL GRAVITY ANOMALY OF BRAZILIAN MARAJÓ BASIN USING CRUSTAL MODELING: IDENTIFYING STRUCTURAL AND TECTONIC FEATURES

2019 ◽  
Vol 37 (2) ◽  
Author(s):  
Gilberto Carneiro dos Santos Junior ◽  
Cristiano Mendel Martins ◽  
Nelson Ribeiro-Filho
Keyword(s):  
Gravity Anomaly ◽  
Gravity Data ◽  
Sedimentary Basins ◽  
Data Interpretation ◽  
Earth’S Crust ◽  
Residual Gravity ◽  
North Brazil ◽  
Residual Gravity Anomaly ◽  
Earth's Crust ◽  
Tectonic Features

ABSTRACT. Dealing with gravity data at complex geological environments is a hard task because regional and residual anomalies are unknown. Due to the fact former techniques do not apply geologic information for separating gravity data, interpretation could lead to common mistakes. In order to allow a better interpretation at sedimentary basins, we applied a different approach for separating regional and residual anomalies for gravity data: the crustal modeling procedure. This approach consists on discretizing the Earth’s crust in prismatic cells and calculating the predicted signal due to Earth’s crust. We set horizontal dimensions of each prism, while the top and bottom are defined by Earth’s topography and depth of crust-mantle boundary, usually called Moho. Additionally, when the predicted signal is calculated, the residual anomaly is obtained from simple subtraction. We applied our methodology at Marajó basin (North, Brazil), where previous geological studies identified a system of faults and grabens, also known as Marajó graben system. Moreover, our results are well compared with previous interpretation through the seismic method, exemplifying the approach’s quality and efficiency. We believe, therefore, that the crustal modeling approach should be considered for studying any Brazilian sedimentary basin and other interesting areas.Keywords: crustal modeling; residual gravity anomaly; Marajó basin; Marajó graben system. RESUMO. Interpretar dados gravimétricos em ambientes geológicos de grande complexidade é uma tarefa difícil de ser realizada, visto que anomalias regionais e residuais são desconhecidas. Devido ao fato de que conhecidas técnicas de separação regional-residual não consideram informações geológicas, a interpretação final pode fornecer resultados equivocados. A fim de permitir uma melhor interpretação nas bacias sedimentares, aplicamos uma diferente abordagem para separação regional-residual: a modelagem crustal. Esta abordagem consiste em discretizar a crosta terrestre em células prismáticas e calcular o sinal regional predito. Definimos as dimensões horizontais de cada prisma, enquanto o topo e a base são definidos pela topografia e profundidade da interface crosta-manto, respectivamente. Após o cálculo do sinal predito, a anomalia residual é calculada via subtração. Aplicamos nossa metodologia na bacia do Marajó (região Norte, Brasil), onde estudos geológicos identificaram um sistema de falhas e grábens, definido por sistema de gráben do Marajó. Nossos resultados apresentam boa correspondência quando comparados com interpretações realizadas via método sísmico, o que exemplifica a qualidade e eficiência da nossa proposta. Acreditamos, portanto, que esta abordagem de modelagem crustal deve ser considerada para o estudo de qualquer bacia sedimentar brasileira e de outras regiões de interesse.Palavras-chave: modelagem crustal; anomalia gravimétrica residual; bacia do Marajó; sistema de gráben do Marajó.  

A least‐squares minimization approach to shape determination from gravity data

Geophysics ◽  
1995 ◽  
Vol 60 (2) ◽  
pp. 589-590 ◽  
Author(s):  
El‐Sayed M. Abdelrahman ◽  
Sharafeldin M. Sharafeldin
Keyword(s):  
Least Squares ◽  
Gravity Anomaly ◽  
Shape Factor ◽  
Gravity Data ◽  
Correlation Factor ◽  
Walsh Transform ◽  
Residual Gravity ◽  
Residual Gravity Anomaly ◽  
Minimization Approach ◽  
Least Squares Minimization

The gravity anomaly expression produced by most geologic structures can be represented by a continuous function of both shape (shape‐factor) and depth‐related variables with an amplitude coefficient related to mass (Abdelrahman and El‐Araby, 1993). Few methods have been developed to determine the shape of the buried geologic structure from residual gravity anomaly profiles. These methods include a Walsh transform approach (Shaw and Agarwal, 1990) and the employment of a correlation factor between successive least‐squares residuals (Abdelrahman and El‐Araby 1993). In the present note, a least‐squares minimization approach to shape‐factor determination from a residual gravity anomaly profile is presented. The problem of the shape‐factor determination is transformed into the problem of finding a solution of a nonlinear equation of the form f(q) = 0.

Nonlinear inversion of isostatic residual gravity data from Montage Basin, northern Gulf of California

Geophysics ◽  
2017 ◽  
Vol 82 (3) ◽  
pp. G45-G55 ◽  
Author(s):  
Juan García-Abdeslem
Keyword(s):  
Inverse Problem ◽  
Gravity Anomaly ◽  
Gulf Of California ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Nonlinear Inversion ◽  
Residual Gravity ◽  
Sedimentary Sequences ◽  
Residual Gravity Anomaly ◽  
Northwestern Mexico

The flexural isostatic response to surface loads is used to estimate the crustal thickness in northwestern Mexico and Southwestern USA. This estimate is used to compute an isostatic regional gravity, which, subtracted from Bouguer gravity anomalies, led to the isostatic residual gravity anomaly at Montage Basin. This basin is located between the southern portion of the Mexicali Valley and the northern Gulf of California, it roughly has an extension of [Formula: see text] wide, and it shows a gravity minimum reaching approximately [Formula: see text]. Montage Basin is within the extensional province of the Gulf of California, where rifting is currently an ongoing geologic process, and deep exploratory wells drilled by Petróleos Mexicanos have shown that the basin accommodates thick sedimentary sequences greater than 5 km. The interpretation of the isostatic residual gravity anomaly is considered as a nonlinear inverse problem, constrained using density as a function of depth derived from Gardner’s equation applied to dual time [Formula: see text]-logs, assuming isostatic equilibrium and considering the basin as a subsurface load that is compensated at depth by a mass of unknown shape and density. The outcome of the inverse problem suggests that Montage Basin accommodates as much as 7.5 km thick sedimentary sequences and a compensating mass at a minimum depth of 13 km.

Sign in / Sign up

Export Citation Format

Share Document