Mapping crustal thickness using marine gravity data: Methods and uncertainties

Crustal thickness is a critical parameter for understanding the processes of continental rifting and breakup and the evolution of petroleum systems within passive margins. However, direct measurements of crustal thickness are sparse and expensive, highlighting the need for methodologies using gravity anomaly data, jointly with other geophysical data, to estimate crustal thickness. We evaluated alternative gravity inversion methodologies to map crustal thickness variations at rifted continental margins and adjacent oceanic basins, and we tested our methodology in the South China Sea (SCS). Different strategies were investigated to estimate and remove the gravity effect of density variations of sediments and the temperature and pressure variations of the lithospheric mantle from the observed free air gravity anomaly data. Sediment density was calculated using a relationship between sediment thickness, porosity, and density. We found that this method is essential for crustal thickness inversion in the presence of a thick sedimentary cover by comparing the Moho depths obtained from gravity inversion and seismic interpretation in the Yinggehai Basin where sediments are up to 13 km thick; the inversion accuracy depended on the parameters of the exponential equation between porosity and the buried depth. We modeled the lithospheric mantle temperature field based on oceanic crustal age, continental crustal stretching factors, and other boundary conditions. We tested three different methods to calculate the thermal expansion coefficient, which is either held constant or is a linear/polynomial function of temperature, for applying a thermal correction and found that the inversion results were relatively insensitive to alternative methods. We compared inversion results with two recent deep seismic profiles that image the rifted continental edge at the northern margin of the SCS and the continental Liyue Bank (Reed Bank) at the southern margin, and we found that the inversion accuracy was improved considerably by removing sediment, thermal, and pressure gravity effects.

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Architecture and dynamics of the magmatic system feeding the 2018 offshore Mayotte eruption from satellite gravity data

10.5194/egusphere-egu21-3623 ◽

2021 ◽

Author(s):

Hélène Le Mével ◽

Craig A. Miller ◽

Yan Zhan

Keyword(s):

Gravity Data ◽

System Structure ◽

Constant Thickness ◽

Gravity Inversion ◽

Model Parameters ◽

Low Density ◽

Submarine Eruption ◽

Magmatic System ◽

Magma Flow ◽

Marine Gravity

In May 2018, a submarine eruption started offshore Mayotte (Comoros archipelago, Indian Ocean), and was first detected as a series of earthquake swarms. Since then, at least 6.4 km3 of lava has erupted from a newly mapped volcanic edifice (MAYOBS campaigns), about 50 km east of Mayotte island. Since the onset of the eruption, GNSS stations on the island have recorded subsidence (up to 17 cm) and eastward displacement (up to 23 cm). We combine marine gravity data derived from satellite altimetry with finite element models to examine the magmatic system structure and its dynamics. First, we calculate the Mantle Bouguer Anomaly (MBA) by taking into account the gravitational effect of the bathymetry and the Moho interfaces, assuming a crust of constant thickness of 17.5 km and correction densities of 2.8 g/cm3 and 3.3 g/cm3 for the crust and mantle, respectively. We then invert the MBA to determine the anomalous density structures within the lithosphere, using the mixed Lp-norm inversion and Gauss-Newton optimization implemented in the SimPEG framework. The gravity inversion reveals two zones of low density, east of Mayotte island. The first is located NE of Petite Terre island between ~15 and 35 km depth, and the second is located further east, south of La Jumelle seamounts and extends from ~25 to 35 km depth. We interpret these low density regions as regions of partial melt stored in the lithosphere and estimate the volume of stored magma. Finally, we use the newly imaged low density bodies to constrain the magma reservoir geometry and simulate magma flow from this reservoir to the eruptive vent in a 3D, time-dependent, numerical model. The model parameters are adjusted by minimizing the misfit between the modeled surface displacement and that measured at the 6 GPS sites, between May 2018 and 2020. The deformation modeling reveals the temporal evolution of the magma flux during the eruption, and the resulting stress distribution in the crust explains the patterns of recorded seismicity. Together with the existing seismic and geodetic studies, the gravity data analysis and FEM models bring new constraints on the architecture of the magma plumbing system and the magmatic processes behind the largest submarine eruption ever documented.

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Crustal Thickness of Chinese Mainland Inversed by GOCE Satellite Gravity Data

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.568-570.288 ◽

2014 ◽

Vol 568-570 ◽

pp. 288-291

Author(s):

Hai Jun Xu ◽

Hu Rong Duan ◽

Jian Ye Zhou

Keyword(s):

Comparative Analysis ◽

Gravity Anomaly ◽

Gravity Data ◽

Crustal Thickness ◽

Geoid Height ◽

Chinese Mainland ◽

Satellite Gravity ◽

Inversion Result ◽

Goce Satellite

GOCE satellite gravity data is often used to compute gravity anomaly and geoid height. In the paper, GOCE gravity data is used to inverse the crustal thickness of Chinese mainland (E70°~130°, N20°~50°) in this paper. In order to test the reliability of the result, the computing result is compared with previous studies. The comparative analysis shows that the inversion result by GOCE gravity data has higher resolution and has good consistence with the previous studies.

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Method Based on Wavelet and Empirical Mode Decomposition for Extracting the Gravity Signal

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.668-669.1076 ◽

2014 ◽

Vol 668-669 ◽

pp. 1076-1080

Author(s):

Li Ye Zhao

Keyword(s):

Gravity Anomaly ◽

Empirical Mode Decomposition ◽

Gravity Data ◽

Measurement Data ◽

Low Frequency ◽

Wave Form ◽

Similar Frequency ◽

Mode Decomposition ◽

Marine Gravity ◽

Gravity Signal

The measurement data of the marine gravity contains a lot of noise, the low frequency part of which have a similar frequency with the gravity signal. It’s difficult to inhibit the noise of the measurement data and extract the gravity signal by classical algorithms. Therefore, in order to effectively eliminate the noise of the measurement gravity data and improve the accuracy of the extracted signal, based on algorithms of wavelet and Empirical Mode Decomposition (EMD), a novel method to extract the sea gravity anomaly signal is proposed. Firstly, the measurement gravity signal is decomposed into detail signals and approximate signals. Secondly, the algorithm EMD is used to extract the low-frequency part of the decomposed signals, and the estimation of the gravity anomaly is reconstructed by inverse wavelet transform. The de-noising experiment has been emulated based on the measurement gravity data. Results of theoretical analysis and emulation experiments indicate that the proposed method can effectively eliminate the noise of the measurement gravity and recovery the wave form of gravity signal, the accuracy of the signal can be approximately increased 40% than classical algorithms.

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Inverse and forward gravity modeling for revealing the crustal structure of Volga-Uralian subcraton

10.5194/egusphere-egu21-7815 ◽

2021 ◽

Author(s):

Igor Ognev ◽

Jörg Ebbing ◽

Peter Haas

Keyword(s):

Lower Crust ◽

Sedimentary Cover ◽

Central Area ◽

Crustal Thickness ◽

Gravity Modeling ◽

Gravity Inversion ◽

Thickness Variation ◽

Rift System ◽

Ural Mountains ◽

Crustal Model

A new crustal model of the Volga-Uralian subcraton was built. The compilation of the model was subdivided in two steps: (1) inverse gravity modeling followed by (2) thorough forward gravity modeling.For inverse gravity modeling GOCE gravity gradients were used. The effect of the Earth sphericity was taken into account by using tesseroids. Density contrasts between crust and mantle were varied laterally according to the tectonic units present in the region.&#160; The model is constrained by the available seismic data including receiver function studies, and deep reflection and refraction profiles.The Moho discontinuity obtained during the gravity inversion was consequently modified, and complemented by the sedimentary cover, upper crust, lower crust, and lithospheric mantle layers in the process of forward gravity modeling. Obtained model showed crustal thickness variation from 34 to more than 55 km in some areas. The thinnest crust with the thickness below 40 km appeared on the Pericaspian basin with the thickest sedimentary column. A relatively thin crust was found along the central Russia rift system, while the thickest crust is located underneath Ural Mountains as well as in the center of the Volga-Uralian subcraton. In both areas the crustal thickness exceeds 50 km. At the same time, the gravity misfit of ca. 95 mGal between the measured Bouguer gravity anomaly and forward calculated gravity field was revealed in the central area of the Volga-Uralian subcraton. This misfit was interpreted and modeled as high-density lower crust which can possibly represent an underplated material.In the end, the new crustal model of Volga-Uralian subcraton respects the gravity and seismic constraints, and reflects the main geological features of the region. This model will be used for further geothermal analysis of the area.

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3D Mafic Topography of the Transition Zone between the North-Western Boundary of the Congo Craton and the Kribi-Campo Sedimentary Basin from Gravity Inversion

International Journal of Geophysics ◽

10.1155/2019/7982562 ◽

2019 ◽

Vol 2019 ◽

pp. 1-15 ◽

Cited By ~ 1

Author(s):

Sévérin Nguiya ◽

Willy Lemotio ◽

Philippe Njandjock Nouck ◽

Marcelin M. Pemi ◽

Alain-Pierre K. Tokam ◽

...

Keyword(s):

Transition Zone ◽

Sedimentary Basin ◽

Gravity Data ◽

Gravity Anomalies ◽

Gravity Inversion ◽

Western Boundary ◽

Northern Margin ◽

Velocity Models ◽

The North ◽

North Western

The structure of the transition zone between the north-western boundary of the Congo Craton and the Kribi-Campo sedimentary basin is still a matter of scientific debate. In this study, the existing gravity data are interpreted in order to better understand the geodynamics of the area. Qualitatively, results show that the major gravity highs are associated with long-wavelength shallow sources of the coastal sedimentary basin, while large negative anomalies trending E-W correlate to low dense intrusive bodies found along the northern limit of the Congo Craton. For the delineation of the causative sources, the gravity anomalies have been inverted based on the Parker-Oldenburg iterative process. As inputs, we used a reference depth of 20 km obtained by spectral analysis and successively, the density contrasts 0.19 g/cm3 and 0.24 g/cm3, deduced from available 1D shear wave velocity models. The results reveal an irregular topography of the mafic interface characterized by a sequence of horst and graben structures with mafic depths varying between 15.6 km and 23.4 km. The shallower depths (15.6-17 km) are associated with the uprising of the mafic interface towards the upper crust. This intrusion may have been initiated during the extension of the Archean Ntem crust resulting in a thinning of the continental crust beneath the coastal sedimentary basin. The subsidence of the mafic interface beneath the craton is materialized by 2 similar graben structures located beneath both Matomb and Ebolowa at a maximum depth of 23.4 km. The intermediate depths (18-22 km) are correlated to the suture zone along the Pouma-Bipindi area. The location of some landslides across the area matches within the northern margin of the Congo Craton and suggests that this margin may also impact on their occurrence. This work provides new insights into the geodynamics, regional tectonics, and basin geometry.

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Evaluation of the BGM-3 sea gravity meter system onboard R/V Conrad

Geophysics ◽

10.1190/1.1442196 ◽

1986 ◽

Vol 51 (7) ◽

pp. 1480-1493 ◽

Cited By ~ 50

Author(s):

Robin E. Bell ◽

A. B. Watts

Keyword(s):

Gravity Anomaly ◽

Gravity Data ◽

Gravity Anomalies ◽

Test Range ◽

Free Air ◽

Marine Gravity ◽

Abrupt Changes ◽

Spring Type ◽

Free Air Gravity ◽

Free Air Anomaly

The first Bell Aerospace BGM-3 Marine Gravity Meter System available for academic use was installed on R/V Robert D. Conrad in February, 1984. The BGM-3 system consists of a forced feedback accelerometer mounted on a gyrostabilized platform. Its sensor (requiring no cross‐coupling correction) is a significant improvement over existing beam and spring‐type sea gravimeters such as the GSS-2. A gravity survey over the Wallops Island test range together with the results of subsequent cruises allow evaluation of the precision, accuracy, and capabilities of the new system. Over the test range, the BGM-3 data were compared directly to data obtained by a GSS-2 meter onboard R/V Conrad. The rms discrepancy between free‐air gravity anomaly values at intersecting ship tracks of R/V Conrad was ±0.38 mGal for BGM-3 compared to ±1.60 mGal for the GSS-2. Moreover, BGM-3’s platform recovered from abrupt changes in ship’s heading more rapidly than did the platform of GSS-2. The principal factor limiting the accuracy of sea gravity data is navigation. Over the test range, where navigation was by Loran C and transit satellite, a two‐step filtering of the ship’s velocity and position was required to obtain an optimal Eötvös correction. A spectral analysis of 1 minute values of the Eötvös correction and the reduced free‐air gravity anomaly determined the filter characteristics. To minimize the coherence between the Eötvös and free‐air anomaly, it was necessary to prefilter the ship’s position and velocity. Using this procedure, reduced free‐air gravity anomalies with wavelengths as small as a few kilometers can be resolved.

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Gravity observations on Santorini island (Greece): Historical and recent campaigns

Contributions to Geophysics and Geodesy ◽

10.31577/congeo.2021.51.1.1 ◽

2021 ◽

Vol 51 (1) ◽

pp. 1-24

Author(s):

Melissinos PARASKEVAS ◽

Demitris PARADISSIS ◽

Konstantinos RAPTAKIS ◽

Paraskevi NOMIKOU ◽

Emilie HOOFT ◽

...

Keyword(s):

Gravity Anomaly ◽

Bronze Age ◽

Gravity Data ◽

Absolute Gravity ◽

Bouguer Gravity ◽

Near Surface ◽

Bouguer Gravity Anomaly ◽

Minoan Eruption ◽

Marine Gravity ◽

Gravity Measurements

Santorini is located in the central part of the Hellenic Volcanic Arc (South Aegean Sea) and is well known for the Late-Bronze-Age “Minoan” eruption that may have been responsible for the decline of the great Minoan civilization on the island of Crete. To use gravity to probe the internal structure of the volcano and to determine whether there are temporal variations in gravity due to near surface changes, we construct two gravity maps. Dionysos Satellite Observatory (DSO) of the National Technical University of Athens (NTUA) carried out terrestrial gravity measurements in December 2012 and in September 2014 at selected locations on Thera, Nea Kameni, Palea Kameni, Therasia, Aspronisi and Christiana islands. Absolute gravity values were calculated using raw gravity data at every station for all datasets. The results were compared with gravity measurements performed in July 1976 by DSO/NTUA and absolute gravity values derived from the Hellenic Military Geographical Service (HMGS) and other sources. Marine gravity data that were collected during the PROTEUS project in November and December 2015 fill between the land gravity datasets. An appropriate Digital Elevation Model (DEM) with topographic and bathymetric data was also produced. Finally, based on the two combined datasets (one for 2012–2014 and one for the 1970s), Free air and complete Bouguer gravity anomaly maps were produced following the appropriate data corrections and reductions. The pattern of complete Bouguer gravity anomaly maps was consistent with seismological results within the caldera. Finally from the comparison of the measurements made at the same place, we found that, within the caldera, the inner process of the volcano is ongoing both before, and after, the unrest period of 2011–2012.

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