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

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.

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

Journal of Geoscience Engineering Environment and Technology ◽

10.25299/jgeet.2021.6.1.5892 ◽

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.

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Geological interpretation of offshore Central Sumatra basin using topex satellite gravity data

IOP Conference Series Earth and Environmental Science ◽

10.1088/1755-1315/944/1/012034 ◽

2021 ◽

Vol 944 (1) ◽

pp. 012034

Author(s):

I Setiadi ◽

J Widodo ◽

T B Nainggolan

Keyword(s):

Spectral Analysis ◽

Gravity Data ◽

Mass Density ◽

Bouguer Anomaly ◽

Hydrocarbon Exploration ◽

Band Pass Filter ◽

Satellite Gravity ◽

Pass Filter ◽

Band Pass ◽

Central Sumatra

Abstract Topex is a geodetic satellite to map earth surface topography with very high precision. Two types of data can be obtained from Topex satellite, namely topographic and free-air gravity field data. Then, it is processed to produce Bouguer anomaly which will be used to interpret the subsurface geology of a specific study area. The purpose of this study was to delineate sedimentary basin and basement configurations. The methods used in this research are spectral analysis, band-pass filter and 2D forward modeling. The spectral analysis results show the average thickness of the sedimentary rocks is 2.1 km. Sub-basin patterns based on the band-pass filter are 7 sedimentary sub-basins and the structural patterns found in this area comprise basement height, graben and fault. The 2D modeling results show that the bedrock in the eastern part of the Central Sumatra basin is granitic with a mass density value of 2.67 gr/cc and the layer above the bedrock is interpreted as a sedimentary rock with a mass density value of 2.35 gr/cc. Analysis of the gravity data shows significant results as initial information to delineate sedimentary sub-basin and regional structure to enhance information to the next stage of hydrocarbon exploration.

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Airborne gravimetry: An investigation of filtering

Geophysics ◽

10.1190/1.1444530 ◽

1999 ◽

Vol 64 (1) ◽

pp. 61-69 ◽

Cited By ~ 36

Author(s):

Vicki A. Childers ◽

Robin E. Bell ◽

John M. Brozena

Keyword(s):

Gravity Data ◽

Substantial Improvement ◽

Airborne Gravity ◽

Airborne Gravimetry ◽

Pass Filter ◽

Low Pass Filter ◽

West Antarctic ◽

Low Pass ◽

Filter Approach ◽

Gravity Signal

Low‐pass filtering in airborne gravimetry data processing plays a fundamental role in determining the spectral content and amplitude of the free‐air anomaly. Traditional filters used in airborne gravimetry, the 6 × 20-s resistor‐capacitor (RC) filter and the 300-s Gaussian filter, heavily attenuate the waveband of the gravity signal. As we strive to reduce the overall error budget to the sub-mGal level, an important step is to evaluate the choice and design of the low‐pass filter employed in airborne gravimetry to optimize gravity anomaly recovery and noise attenuation. This study evaluates low‐pass filtering options and presents a survey‐specific frequency domain filter that employs the fast Fourier transform (FFT) for airborne gravity data. This study recommends a new approach to low‐pass filtering airborne data. For a given survey, the filter is designed to maximize the target gravity signal based upon survey parameters and the character of measurement noise. This survey‐specific low‐pass filter approach is applied to two aerogravimetry surveys: one conducted in West Antarctica and the other in the eastern Pacific off the California coast. A reflight comparison with the West Antarctic survey shows that anomaly amplitudes are increased while slightly improving the rms fit between the reflown survey lines when an appropriately designed FFT filter is employed instead of the traditionally used filters. A comparison of the East Pacific survey with high‐resolution shipboard gravity data indicates anomaly amplitude improvements of up to 20 mGal and a 49% improvement of the rms fit from 3.99 mGal to 2.04 mGal with the appropriately designed FFT filter. These results demonstrate that substantial improvement in anomaly amplitude and wavelength can be attained by tailoring the filter to the survey.

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

Geophysics ◽

10.1190/1.1444585 ◽

1999 ◽

Vol 64 (3) ◽

pp. 754-764 ◽

Cited By ~ 40

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.

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Evaluation of GGMs Based on the Terrestrial Gravity Data of Gravity Disturbance and Moho Depthin Afar, Ethiopia

Artificial Satellites ◽

10.2478/arsa-2021-0007 ◽

2021 ◽

Vol 56 (3) ◽

pp. 78-100

Author(s):

Eyasu Alemu

Keyword(s):

Gravity Data ◽

Gravity Models ◽

Moho Depth ◽

Equal Distribution ◽

Terrestrial Gravity ◽

Global Gravity ◽

Gravity Disturbances ◽

Combined Models ◽

Global Gravity Models ◽

Best Fit

Abstract To estimate Moho depth, geoid, gravity anomaly, and other geopotential functionals, gravity data is needed. But, gravity survey was not collected in equal distribution in Ethiopia, as the data forming part of the survey were mainly collected on accessible roads. To determine accurate Moho depth using Global Gravity Models (GGMs) for the study area, evaluation of GGMs is needed based on the available terrestrial gravity data. Moho depth lies between 28 km and 32 km in Afar. Gravity disturbances (GDs) were calculated for the terrestrial gravity data and the recent GGMs for the study area. The model-based GDs were compared with the corresponding GD obtained from the terrestrial gravity data and their differences in terms of statistical comparison parameters for determining the best fit GGM at a local scale in Afar. The largest standard deviation (SD) (36.10 mGal) and root mean square error (RMSE) (39.00 mGal) for residual GD and the lowest correlation with the terrestrial gravity (0.61 mGal) were obtained by the satellite-only model (GO_CONS_GCF_2_DIR_R6). The next largest SD (21.27 mGal) and RMSE (25.65 mGal) for residual GD were obtained by the combined gravity model (XGM2019e_2159), which indicates that it is not the best fit model for the study area as compared with the other two GGMs. In general, the result showed that the combined models are more useful tools for modeling the gravity field in Afar than the satellite-only GGMs. But, the study clearly revealed that for the study area, the best model in comparison with the others is the EGM2008, while the second best model is the EIGEN6C4.

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

Geophysics ◽

10.1190/1.1442611 ◽

1989 ◽

Vol 54 (11) ◽

pp. 1474-1482 ◽

Cited By ~ 59

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.

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Gravity Studies of Earthquake-Related Structures Near Massena, New York

Seismological Research Letters ◽

10.1785/gssrl.50.1.13 ◽

1979 ◽

Vol 50 (1) ◽

pp. 13-24

Author(s):

Reinhard K. Frohlich ◽

Robert L. Albert ◽

Frank A. Revetta

Keyword(s):

New York ◽

Sea Level ◽

Gravity Data ◽

Bouguer Anomaly ◽

Gravity Anomalies ◽

River Valley ◽

Density Contrast ◽

Positive Anomaly ◽

Eastern United States ◽

St Lawrence River

Abstract The causes of the seismicity of the St. Lawrence River Valley are not well understood. As is the case for the entire east coast of North America, epicentral zones often occur in regions where no correlation exists between seismicity and mapped geologic structures. Several explanations have been proposed for such a phenomenon: a) earthquakes occur along unmapped surface faults; b) earthquakes occur along subsurface faults showing no surface expression; or c) the earthquakes are not related to existing faults. Conventional analytical techniques, such as upward and downward continuation, were applied to gravity data from the St. Lawrence River Valley in an attempt to delineate possible seismic–related structures. The analysis of the gravity data indicates that the anomalies trend in a north-northeast direction similar to the structural trends of the Precambrian rocks. The major feature of the Simple Bouguer anomaly map is an extensive positive gravity anomaly centered at Massena, New York. Profiles across the Bouguer gravity anomalies and the up-and downward continued gravity anomalies were reproduced with a two–dimensional modeling technique. Among the various non-unique anomaly-producing structures tested we prefer a model suggesting that the positive anomaly near Massena is derived from two bodies with different density contrasts. The first is a wedge (8 km deep by 35 km wide) located 6 km below sea level with a density contrast of +0.11 gm/cm3 and the second is a smaller body (2 km deep by 6 km wide) located 3.3 km below sea level with a density contrast of +0.2 gm/cm3. The large wedge may represent a sequence of interlayered metasediments and metavolcanics related to the Grenville sequence. The smaller body may represent a mafic intrusive. Several authors have suggested that high gradients of gravity (toward positive) produced by mafic intrusives are associated with earthquakes in the eastern United States. The possible existence of a mafic intrusive near Massena, New York, and its proximity to epicentral zones suggest a similar association for earthquakes in the study area.

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