scholarly journals Constraining the geometry at depth of La Maladeta and Andorra-Mont Louis granites (Central Pyrenees) through gravity modelling

2020 ◽  
Author(s):  
Conxi Ayala ◽  
Pilar Clariana ◽  
Ruth Soto ◽  
Joan Martí ◽  
Aina Margalef ◽  
...  
Keyword(s):  
Gravity Anomaly ◽  
Cross Sections ◽  
Gravity Data ◽  
Gravity Modelling ◽  
The North ◽  
Geological Surveys ◽  
Residual Gravity Anomaly ◽  
Paleozoic Rocks ◽  
Subsurface Geometry ◽  
Central Pyrenees

<p>In the Central Pyrenees, where density contrast between the Paleozoic rocks and the intruded granitic bodies is measurable, geological cross-sections constrained with gravity data help to unravel the subsurface geometry of the granites.</p><p>With this goal in mind, during 2018 and 2019 several gravimetric surveys were carried out in the Central Pyrenees to improve the existent spatial resolution of the gravity data from the databases of the Spanish and Catalan Geological Surveys, especially in La Maladeta and Andorra Mont-Louis granites’ area. After the gravity reductions, we obtained the Bouguer gravity anomaly from which we calculated the residual gravity anomaly by subtracting a third degree polynomial which represents the regional anomaly in agreement with the geometry of the crust in this region.</p><p>The gravimetric response over La Maladeta and Andorra Mont-Louis granites is markedly dissimilar pointing out differences in the composition and geometry at depth of the two granites. La Maladeta granite shows a gravimetric zonation with small variations in its amplitude from one zone to the next, consistent with small lateral changes in its composition, predominantly granodioritic. By contrast, the Andorra Mont-Louis pluton is characterized by a relative minimum suggesting a more granitic composition.</p><p>With respect to the inferred geometry at depth, the results obtained from gravity modelling show that the La Maladeta granite displays a laccolithic shape with its basal contact deeping to the North whereas the Andorra Mont-Louis granite has a more batholitic shape. Although the emplacement age of both granites is similar (Late Carboniferous – Early Permian), their different geometry at depth suggests that either (1) their emplacement mechanisms were different or (2) the subsequent Alpine orogeny affected both granites in different ways better preserving the original geometry of the Andorra Mont-Louis granite.</p>

scholarly journals Basement and cover architecture in the Central Pyrenees constrained by gravity data

2021 ◽  
Author(s):  
P. Clariana ◽  
R. Soto ◽  
C. Ayala ◽  
A. M. Casas-Sainz ◽  
T. Román-Berdiel ◽  
...  
Keyword(s):  
Cross Sections ◽  
Gravity Data ◽  
Leading Edge ◽  
Gravity Modelling ◽  
Thrust Sheet ◽  
Density Analysis ◽  
Southern Half ◽  
Anomaly Map ◽  
Structural Architecture ◽  
Central Pyrenees

AbstractA new gravity survey (1164 gravity stations and 180 samples for density analysis) combined with two new geological cross sections has been carried out in a sector of the Central Pyrenees in order to improve the characterization of basement and cover architecture. From North to South, the study area comprises the southern half of the Axial Zone and the northernmost part of the South-Pyrenean Zone. New gravity data were combined with previous existing databases to obtain the Bouguer and residual anomaly maps of the study area. The two cross sections, oriented NNE–SSW, were built from field data and previous surficial and subsurface data and cross the La Maladeta plutonic complex. The residual anomaly map shows values ranging from −18 to 16 mGal and anomalies mainly oriented N120E. The two 2.5D modelled cross sections show similar observed gravity curves coinciding with similar interpreted structural architecture. Data show a gravity high oriented N120E coinciding with the Orri basement thrust sheet and an important gravity depression, with the same orientation, coinciding with the leading edge at depth of the Rialp basement thrust sheet and interpreted as linked to a large subsurface accumulation of Triassic evaporites. The volume at depth of the La Maladeta and Arties granites has been constrained through gravity modelling. This work highlights that the combination of structural geology and gravity modelling can help to determine the structural architecture of an orogen and localize accumulations of evaporites at depth.

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 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]

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.  

scholarly journals PENELITIAN GAYABERAT DAN GEOMAGNIT KEPULAUAN ARU, CEKUNGAN WOKAM

2016 ◽  
Vol 12 (1) ◽  
pp. 1
Author(s):  
Tatang Patmawidjaya ◽  
Subagio Subagio
Keyword(s):  
Gravity Anomaly ◽  
Normal Fault ◽  
High Gravity ◽  
The South ◽  
Shallow Subsurface ◽  
The North ◽  
Residual Gravity Anomaly ◽  
Geomagnetic Anomaly ◽  
Geomagnetic Models ◽  
Geomagnetic Anomalies

Paparan Arafura di bagian selatan Kepulauan Aru merupakan depresi Lempeng Indo-Australia yang ditunjukkan oleh anomali gayaberat dan geomagnet tinggi. Sedangkan depresi di bagian utara yang tertahan Palung Aru diperlihatkan oleh anomali gayaberat tinggi dan anomali geomagnet rendah. Anomali ini diduga sebagai penebalan batuan metamorf yang mengalami pangangkatan sebagai alas cekungan Wokam. Analisis data gayaberat dan geomagnet mengindikasikan penurunan cekungan ini ke arah utara. Penurunan anomali gayaberat ke arah Pulau Wokam yang berarah baratdaya-timurlaut di selatan dan baratlaut-tenggara di daerah utara ditafsirkan sebagai struktur sesar. Hal ini dicirikan oleh arah sungai atau selat sebagai pemisah pulau-pulau di Kepulauan Aru. Anomali gayaberat residual mengindikasikan cekungan dan punggungan berarah baratdaya-timurlaut yang membentuk antiklin dan sinklin. Sesar dan lipatan ini menerus sebagai struktur geologi dangkal bawah permukaan. Berdasarkan pemodelan gayaberat dan geomagnet dapat dikatakan bahwa Cekungan Wokam cenderung menurun ke arah utara akibat sesar normal. Kata Kunci : gayaberat, geomagnet, pemodelan geologi, Cekungan Wokam Arafura Shelf in the southern part of the Aru Islands is a depression of the Indo-Australian crust at the south that indicated by high gravity and geomagnetic anomalies. While the northern depression blocked by the Aru Trough indicated by high gravity anomaly and low geomagnetic anomaly. These anomalies presumed as a thickening of the metamorphic rocks due to uplifting and acting as the basement of Wokam Basin. Gravity and geomagnetic analyses indicate a northward subsidence of the basin. The decreasing of the southwest–northeast gravity anomaly in the south and the northwest-southeast in the north, interpreted as a fault. It this characterized by the river direction or strait as islands separation in Aru Archipilagoes. Residual gravity anomaly indicates a southwest-northeast basin and ridge form anticline and syncline. These faults and folds are continuous as the shallow subsurface geological structures. On the base gravity and geomagnetic models, it can be concluded that Wokam Basin tends to subside northward as the result of a normal fault. Keywords: gravity, geomagnetic, geological models, Wokam Basin

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.

Gravity modelling of the lherzolite body at Lers (French Pyrenees); some regional implications

1985 ◽  
Vol 122 (1) ◽  
pp. 51-56 ◽  
Author(s):  
Helen J. Anderson
Keyword(s):  
High Temperature ◽  
Seismic Data ◽  
Gravity Anomalies ◽  
Sole Source ◽  
Gravity Survey ◽  
Gravity Modelling ◽  
Maximum Volume ◽  
The North ◽  
High Temperature Metamorphism ◽  
Central Pyrenees

AbstractLherzolites outcrop throughout the North Pyrenean Zone of the Pyrenees and are everywhere associated with metamorphosed carbonates. It has been suggested that heat from the cooling of the lherzolites was responsible for the high temperature metamorphism of the carbonates. A gravity survey reported here shows that the volume of the lherzolite body at Lers is approximately 0.8 km3. The maximum volume of carbonates that such a body could metamorphose is 3.2 km3. This latter value is so much less than the volume of carbonates inferred from field mapping that the lherzolite body cannot have been the sole source of heat for metamorphism of the carbonates.It has been suggested from seismic data that there is a step in the Moho beneath the North Pyrenean Fault in the central Pyrenees. Gravity anomalies reported here show that either the step is less than 10 km high or that the density contrast is very low at the base of the crust in the Pyrenees.

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