Free-air Anomalies Research Articles

Gravity Anomaly in Kelud, Kasinan-Songgoriti, and Arjuno-Welirang Volcano Hosted Geothermal Area, East Java, Indonesia

This study aims to determine the relationship of heat reservoirs in the Kelud, Kasinan-Songgoriti, and Arjuno-Welirang geothermal systems based on gravity data analysis. Gravity data are obtained from Geodetic Satellite (GEOSAT) and European Remote Sensing-1 (ERS-1) Satellite which have been corrected to free air correction. The result of gravity data analysis is in the form of a complete Bouguer anomaly which represents the gravity anomaly below the surface. The results of the complete Bouguer anomaly value obtained were -15,238 mGal to 86,087 mGal. Based on these results, regional anomalies and residual anomalies will be separated to determine the depth of the two anomalies. 3D modeling was carried out based on the complete Bouguer anomaly data to determine the reservoir relationships in the Kelud, Kasinan-Songgoriti, and Arjuno-Welirang geothermal systems.

Evolution of the Ecuador offshore nonaccretionary-type forearc basin and margin segmentation

Forearc basins have developed along accretionary and nonaccretionary convergent margins. Here, we investigate the evolution of an offshore nonaccretionary-type forearc basin that has developed along the Ecuadorian convergent margin. We used 2D-Multichannel Seismic Reflection profiles to recognize five seismic units overlying the acoustic basement. The units were dated by correlation with wells and regional stratigraphy. They recorded the structural evolution of the margin into North, Central-North and Central-South margin segments identified from Gravity Free Air Anomalies. Intersegment transition zones accommodated deformation related to the margin segmentation, contributing to the development of shelf forearc depocenters. An offshore forearc basin (subunit L1a) developed on accreted oceanic terranes during the middle-to-late Eocene. The basin was affected by normal faulting during the Oligocene to early-middle Miocene (subunit L1b). This extension stage initiated during the rifting and oblique subduction of the Farallon plate beneath the Ecuador margin. The basin underwent an early-middle Miocene uplift outlined by unconformity U1 potentially associated with the subduction of the young Nazca plate, in a less oblique plate convergence environment. During the early-Pliocene, the Central-South margin segment individualized by uplift during the formation of unconformity U2. The Manta anticline, which developed in the intersegment transition zone, accommodated the segmentation. This event is associated with the arrival of the Carnegie ridge in the subduction. The North segment individualized by uplift during the Pliocene early-Pleistocene formation of the Pedernales structural high, which may coincide with a reactivation of the Canandé fault. Synchronously, unit L3 sediment recorded the subsidence of the Central-North segment, and strike-slip tectonics in the area of the Jama-Bahía depocenter. A mosaic of middle-to-late Pleistocene unit L5 depocenters that alternate with uplifted zones along the shelf likely recorded the subduction of the topographically irregular Carnegie ridge. The study provides a remarkable example of the evolution of a non-accretionary-type forearc basin.

INTERPRETASI SISTEM PANAS BUMI SUWAWA BERDASARKAN DATA GAYA BERAT

The research of gravity on Suwawa Sub-District geothermal is done for the purposes to determine fault structure using Second Vertical Derivative (SVD) technique, create a 2D subsurface model and 3D tentative model using regional anomaly data, and interpreting Suwawa geothermal system. Data processing is done in the research include: drift correction, terrain correction, free air correction, complete Bouguer anomaly, spectral analysis, SVD analysis, 2D modeling and 3D inversion modeling and tentative model. The research results showed that the research area has low Bouguer anomaly with a range of 75.8 to 79.5 mGal values in the West and Southeast, while high anomaly with a range of 90.9 to 111.2 mGal values in the Northern and Southern, there is correlation of fault based on SVD analysis with geological fault that indicate the presence of Libungo hot springs, the inversion results indicate the presence of low density (ρ = 1.8 g/cc) which is an alluvial rocks and high density (ρ = 2.9 g/cc) which is Andesite Lava rocks, 3D tentative modeling indicate the presence of reservoir is at a depth of 2 km from the ground surface. Based on the model created, Cap Rock is located on Andesite Lava rocks with ρ = 2.9 g/cc at a depth of 1200 m and Heat Source located at a depth of 2000 m.

ANALYSIS AND INTERPRETATION OF REGIONAL-SCALED GRAVITY MEASUREMENTS IN THE CENTRAL EQUATORIAL ATLANTIC REGION OF AFRICA

Regional-scaled gravity data was interpreted for the Central Equatorial Atlantic African region, a region comprising of both oceanic and continental areas, with a view to understanding more about its complex geological settings and the processes of tectonic evolution. The methods adopted involved the generation of topography/bathymetry, free-air and Bouguer anomaly grids from topography, free-air anomaly and Bouguer anomaly datasets; regional-residual gravity field separation, lineament mapping from the Horizontal Derivative and 2-D modelling of the sub-surface. Within the study area, the elevation ranged from -4,200 to 3,840 m. The free-air anomaly map showed a combination of elongated and circular positive free-air anomalies and elongated, negative free-air anomalies whose values varied between -40 mGal and 70 mGal. The Bouguer anomaly values ranged from -121 to +229 mGal. The Bouguer anomaly map also showed both elongated and circular positive and negative Bouguer anomalies. The dominant orientations of the elongated Bouguer anomalies were the NE-SW, the E-W and the ESE-WNW orientations. Gravity modelling within the oceanic region re- vealed the presence of a basaltic cover whose thickness varied between 1,330 m to 7,700 m in the oceanic and continental margin regions. Associated with the occurrence of this basaltic cover were kinks interpreted as eruption fissures from which the basaltic cover was ejected. It was discovered that the African West Coast Atlantic margin consisted of a trench around the continental margin region adjoined just to the north by some high density basalt, and also that the West African Coastline is heavily laced with magmatism and must have therefore been an active magmatic region in the past. Based on the interpreted data, the study concluded that the Equatorial Atlantic Ocean might have opened on a frame- work of rifts which extended into the inland regions to form the present day sedimentary basins and from the Bouguer anomaly profile along latitude 10oN that it is possible that both the Minna batholith and the Jos Plateau exploited the Proterozoic basement weaknesses for their emplacement.

Depth Estimation of Geothermal Heat Structures by Euler Deconvolution of Gravity Data at Eburru Area, Kenya

Gravity survey was done at the Eburru area to estimate the source depth locations and delineate the fault boundaries using 3D Euler deconvolution. Gravity data was collected using CG-5 gravimeter. Gravity data reductions were done by applying drift correction, latitude correction, free air correction, Bouguer correction and terrain correction to the observed raw data to obtain complete bouguer anomaly (CBA). The CBA data was transferred to Oasis montaj software for Euler deconvolution processing. The 3D Euler deconvolution was carried out to determine and estimate the depth of the density bodies. Euler deconvolution locates the gravity anomaly source and estimates its depth from the gravity observation level. Euler deconvolution was preferred to other filtering methods in this study as solutions are only determined over identified analytic signal peaks, the window size varies according to anomaly size and the final solution involves only a few more precise depth estimates. The Euler deconvolution was performed using structural indices of 0.5, 1.0 and 2.0. Results from this analysis indicated that the CBA values in this study area range from gravity values of -272 mGal to -286 mGal and residual Bouguer anomaly amplitude range between -3 mGal and 3.4 mGal. The 0.5, 1.0 and 2.0 structural indices generated five solutions at depth range of 433 m - 2269 m, 801 m - 1433 m and 1170 m - 2246 m respectively occurring almost at the same locations on gravity highs. The deep structures were observed to occur in the northern part of the study area, and interpreted to be dense intruding masses likely to be trapped by the overlying cap rock at these depths. These could be geothermal heat sources that can be exploited to generate geothermal energy.

EVALUATION AND TAILORING OF GLOBAL GEOPOTENTIAL MODELS IN THE DETERMINATION OF GRAVITY FIELD IN SERBIA

<jats:p>Within this paper, we evaluated the quality of three Global Geopotential Models entitled: EGM96,EGM2008, and GOCO05c. The models were evaluated by using 1001 terrestrial discrete values ofheight anomalies determined by Global Navigation Satellite Systems and normal heights, which weconsidered to be true values within this research. In addition to the quality evaluation, we tailoredthe models by using more than 80000 free air anomalies. The results obtained from the evaluationand tailoring indicate that by using the GOCO05c it is possible to determine a set of anomaly heightsacross Serbia, which are in agreement with terrestrial values with an average value of -7 cm, thestandard deviation of ±9 cm and with the range of 44 cm.</jats:p>

Crustal thickness beneath Atlas region from gravity, topographic, sediment and seismic data

The Atlas region in northwest Africa is characterized by the Quaternary volcanism and elevated topography with past complex tectonic mitigation between the African and European plates. Geodynamics of this atypical region has left indubitably imprints in crustal architectonics, mainly regarding the crustal thickness as well as the crustal density structure. The knowledge of crustal thickness variations is of a significant interest, since it provides a crucial constraint to geodynamic and geophysical modelling of this region. In this study, we use gravity, topographic, bathymetric and sediment data together with results of seismic surveys to image the Moho topography beneath the Atlas region. The Bouguer gravity anomalies used for a gravimetric Moho recovery are obtained from the free-air gravity anomalies after subtracting the gravitational contributions of topography, bathymetry and sediments. The regional gravimetric Moho inversion constrained on seismic data is carried out by applying a regularized inversion technique based on Gauss-Newton's formulation of improved Bott's method, while adopting Earth's spherical approximation. The numerical result reveals relatively significant Moho depth variations in the Moroccan Atlas, with minima of approximately 24 km along continental margins of the Mediterranean Sea and maxima exceeding 51 km beneath the Rif Cordillera. The Moho depth beneath the West African Craton varies from 32 km in its southern margin to 45 km beneath the Middle Atlas. The Tell Atlas is characterized by the shallow Moho depth of approximately 22 km and further deepening to 42 km towards the northern edge of the Aures Mountains. Our findings indicate a limited tectonic shortening of the High Atlas with the crustal thickness mostly within 36–42 km. Topographic discrepancies between the Rif Cordillera and the Atlas Mountains suggest that the hypothesis of isostatic compensation cannot be fully established.

Extricating dynamic topography from subsidence patterns: Examples from Eastern North America's passive margin

Global sea-level (eustatic) histories generated by backstripping stratigraphy are predicated upon the lithosphere having a well understood tectonic history. However, sub-plate processes play a role in governing lithospheric vertical motions with timescales and amplitudes akin to eustasy, which are difficult to predict. We examine how stratigraphic and geophysical observations combined with simple isostatic models can be used to disentangle histories of sub-plate support and eustasy. We focus on the passive margin of Eastern North America, where a generally accepted history of eustasy has been estimated. Negative long wavelength free-air gravity anomalies, residual ocean-age depth estimates, fast upper mantle shear wave velocities, and geodynamic models suggest that Cenozoic evolution of this passive margin has been influenced by upper mantle drawdown. We build on existing analyses to backstrip sixteen wells, which, combined with seismic data, constrain timing and extent of Cenozoic subsidence. Results indicate up to ∼1000 m of water-loaded subsidence between ∼20–0 Ma centered on the Baltimore Canyon Trough. Seismic data from the trough shows Neogene aggrading clinoforms. There is little evidence for faulting or stratigraphic growth, which indicates that Neogene lithospheric strain rates were low. Amplitude and spatial extent of Neogene subsidence are difficult to explain by glacio-eustasy or glacio-isostatic adjustment. Instead, sub-plate support calculated from conversion of shear wave velocities to temperature and isostatic calculations indicate that upper mantle drawdown was responsible for subsidence of the margin. Because mantle convection is vigorous such observations are expected throughout the stratigraphic archive.

THE CONNECTION OF SEISMICITY WITH EXCESS MASSES OF THE GEOLOGICAL ENVIRONMENT BY THE EXAMPLE OF CENTRAL ASIA

The facts and examples of seismicity connection with localized excess masses of the earth's crust, located both on the earth's surface in the form of different elevations of different density rocks, and to a lesser extent at depths under the day surface in the form of limited volumes of rocks with high density are described. These relationships are shown both globally and regionally for Central Asia using geodetic and gravimetric data in the form of detailed maps of digital elevation models and free-air gravity anomalies obtained from high-precision satellite imagery.

Assessment of the latest generation GOCE-based global gravity field models using height and free-air gravity anomalies over South Africa

The latest Gravity field and steady-state Ocean Circulation Explorer (GOCE) global gravity field models (GGMs) are evaluated over South Africa using 118 GPS/levelling, 507 gravity and 88,968 SAGEOID10 data points. It is observed that GOCE data can improve quasi-geoid model in the long-to-medium wavelength components over South Africa. GOCE-based GGMs perform better than EGM2008 in recovering height anomalies up to 240°. Related results are obtained for free-air gravity anomalies, but the improvement over EGM2008 by GOCE data is only up to 180°. TIM5 (up to 240°) is selected as a candidate GOCE model and combined with EGM2008 (from 241 up to 2190°). This model (TIM5/EGM2008) is evaluated by observed data (GPS/levelling and gravity) and the latest hybrid quasi geoid model over South Africa (SAGEOID10). The solutions converge at 1200 and 1800° for height and free-air gravity anomalies, respectively, when TIM5/EGM2008 is used. Results show that TIM5/EGM2008 can effectively recover height and free-air gravity anomalies over South Africa. The standard deviations of the differences between observed (118 GPS/levelling and 507 gravity points) and TIM5/EGM2008 implied height and free-air gravity anomalies are ± 23.7 cm and ± 9.0 mGal, respectively. In addition, the standard deviation of the differences between SAGEOID10 and TIM5/EGM2008 implied height anomalies (at 88,968 grid points) is ± 21.7 cm. The results for height anomalies from TIM5/EGM2008 are comparable with the latest gravimetric quasi-geoid model over South Africa.

Analisis Continuous Wavelet Transform (CWT) Anomali Residual Medan Gravitasi Di Situs Geologi Karangsambung Kebumen Jawa Tengah

Gravity method is a passive geophysical method that provides information on the distribution of rock density below the surface. The gravity method has a weakness at the level of ambiguity in determining the depth of the anomaly. This study aims to determine the depth of the anomaly using Continuous Wavelet Transform (CWT) analysis to overcome the value of ambiguity, so that the results obtained have a high degree of accuracy. The research method is data survey / acquisition and data analysis. This research was conducted in Karangsambung Kebumen, Central Java with the acquisition of gravitational data as many as 56 measurement points. The results of data acquisition then analyzed included reading to mGal, tool height correction, drift, tides, latitude, free air correction, Bouguer correction, and field correction. The results of this correction obtained Complete Bouguer Anomalies (ABL) values which were then reduced to flat fields and regional-residual anomaly filters. The next step is CWT analysis by making incisions on residual anomaly maps. The results showed that the source of the anomaly was between ± 39.2 - 122.9 meters.

Geological Structure Analysis of Satellit Gravity Data in Oil and Gas Prospect Area of West Aceh-Indonesia

Estimation of the subsurface geological structures in oil and gas prospect area of West Aceh has been done by utilizing gravity field anomaly of satellite gravity data. This research aim to analyze satellite gravity data in order to get geological features which is include deep and shallow structure or fault around oil and gas prospect area in West Aceh. The satellite gravity data is originally provided as Free Air Anomaly and should be corrected to get Complete Bouguer Anomaly (CBA). Furthermore, CBA was transformed into a horizontal plane and corrected from regional anomalous effects to obtain residual anomaly, horizontal and vertical derivative. From CBA, the gravity anomaly show good correlation with geological boundaries on different rock formation and the anomaly is decrease from NE-SW. Residual anomaly also gives same information with CBA but this anomaly focus on shallow structure. Furthermore, horizontal derivative and vertical derivative also show good correlation with geological structure or fault but in some areas the anomaly related with deep structure cannot be seen on the surface or geological map. Despite the result cannot correlate directly with oil and gas prospect area, satellite gravity can be used to identify gravity anomaly and also fault that related with hydrocarbon anomaly area

Comparison of Three Gravimetric-Geometric Geoid Models for Best Local Geoid Model of Benin City, Nigeria

The conversion of geometric as well as ellipsoidal heights from GNSS observations to practical heights for engineering constructions has necessitated the determination of the local geoid model of areas. Benin City is a developing area which requires a local geoid model for conversion of geometric heights to orthometric heights for physical developments in the area. This paper is on the best local geoid model of Benin City, Nigeria by comparing three gravimetric-geometric geoid models of the study area. GNSS and gravimetric observations were carried out on 49 points to respectively obtain their coordinates and absolute gravity values. The theoretical gravity values of the points were computed on the Clarke 1880 ellipsoid, subtracted from the absolute gravity values and corrected for the air (free air) to obtain the free air gravity anomalies of the points. The computed free air gravity anomalies were applied to compute the geoid heights of the points using the integration of the modified Stokes integral. Three geometric geoid surfaces (plane, second degree and third degree surfaces) were fitted to the computed gravimetric geoid heights using the least squares technique to obtain the gravimetric-geometric geoid models of the study area. The RMSE of the three gravimetric-geometric geoid models were computed to determine their (the models) accuracy. The three gravimetric-geometric geoid models were compared using their accuracy to obtain the most suitable geoid model of the study area. The results of the comparison showed that the third degree gravimetric-geometric geoid model is most suitable for application in the study area. It is recommended that ellipsoidal heights obtained from GNSS observation within Benin City, Nigeria should be converted to orthometric heights using the third degree geoid model.

Gravity satellite data analysis for subsurface modelling in Mount Merapi-Merbabu, Java, Indonesia

Mount Merapi and Mount Merbabu are active volcanoes that lies in Java Island. Java island is part of Indonesia region. This island was subduction product of Eurasian and Indo-Australian plates, caused the island consist of many volcanoes. A regional gravity study was carried out over Mt. Merapi-Merbabu by TOPEX/Poseidon satellite data. The data was corrected by free air correction and become free air anomaly. Then, that anomaly was corrected by Bouguer and Terrain corrections, become Complete-Bouguer Anomaly. This study present subsurface density model beneath Mt. Merapi and Merbabu to identify the magma chamber.

Geodynamic Complexes and Structures of Transbaikalia: Record in Gravity Data

Abstract —The Transbaikalian region comprises several known geologic structures: the Mesozoic Mongolia-Okhotsk orogen, the Cenozoic Baikal rift system, and the world largest Angara–Vitim granitic batholith. They all formed upon heterogeneous Neoproterozoic– Early Paleozoic continental-margin complexes of the Siberian craton. The region is subject to the influence of mantle plumes, which induced Mesozoic–Cenozoic volcanism and controlled structural and lithological changes in the crust in the early history. Transbaikalia, which has been a scene of multiple tectonic events, is a model area for geophysical (in particular, gravity) surveys for various geological and geodynamic applications. As a novel approach, we interpret geological and geodynamic data from the region with reference to the pattern of free-air and Bouguer gravity anomalies revealed by satellite altimetry. Bouguer anomalies highlight large structures in the lithospheric mantle which were produced in the Cenozoic mainly by the activity of mantle plumes. Basaltic lava fields were confirmed to be almost coeval with mantle anomalies and to record the presence of the plume head at the crustal base. However, the origin of the Late Paleozoic Angara–Vitim granitic batholith was only tentatively attributed to the plume activity, based on gravity data. Mesozoic metamorphic core complexes (MCC) and basins that formed during the evolution of the Mongolia–Okhotsk orogen show up clearly on the map of free-air anomalies. Most of the MCC revealed in Transbaikalia coincide with oval gravity highs and border negative elongate features corresponding to Mesozoic basins. The zone of Cenozoic tectonism stands out in the pattern of free-air anomalies as maximum gravity contrasts, with the values changing from –110 to –120 mGal in basins to + 90 or +100 mGal in ranges. This zone encompasses rift basins filled with Cenozoic or, locally, Mesozoic sediments, which jointly form a domino-like system of rhomb-shaped structures typical of the Baikal rift system and, in general, of Cenozoic Central Asia resulted from the far-field effect of the India–Eurasia collision.

Postglacial Uplift: Record in the Gravity Field and in Neogene–Quaternary Structures

Abstract The history of Quaternary glaciation and postglacial uplift in Fennoscandia is considered in relation to the surface topography, gravity, and number of glacial deposits and is compared to the respective processes in North America. The surface topography and the gravity field are correlated over the Fennoscandian region as a whole and for two reference areas of South Norway and the Kola Peninsula. The gravity field is composed as free-air and Bouguer gravity anomalies using modern global models based on satellite data. The impact of glaciation may be responsible for zoned patterns of both topography and gravity. The glaciation centers of Norway are marked by uplifts reaching 2470 and 1500 m and by circular Bouguer gravity lows of ≤ –200 mGal, which correspond to residual crust thickening. The gravity patterns of the Kola Peninsula and Norway consist of circular and polygonal anomalies caused by both postglacial rebound and rock density variations. The general uplift and related extension of the crust led to the formation of fractures of different sizes, from 100–200 km long and 1–2 km deep fjords in Norway to 1–2 km long and 30–20 m deep local fractures on the Kola Peninsula. The gravity field of central North America is characterized by a generalized map of free-air anomalies within the limits of glaciation and a map of free-air anomalies compiled from the global database. Glaciation and its records in the surface topography and gravity patterns are described in more detail for Wisconsin State (USA), where extension fractures are similar to those on the Kola Peninsula. The models explaining the correlation between postglacial uplift and gravity variations can be further updated using seismic and tectonic data, as it was shown previously for the case of Finland.

Insights on the Japanese Subduction Megathrust Properties From Depth and Lateral Variability of Observed Ground Motions

AbstractTwo ground motion prediction equation models for subduction zones have been tested using a public ground motion database of the KiK‐net records obtained by automated processing protocols (Dawood et al., 2016, https://doi.org/10.1193/071214EQS106). The database contains records of more than 700 interface earthquakes that occurred on the Japan subduction between 1998 and 2012. The Zhao et al. (2006, https://doi.org/10.1785/0120050122) ground motion prediction equation was shown to be the best suited model for the region. It was then used as backbone to analyze the variability of ground motion records. The residuals between observed and predicted ground motions have been analyzed to study the spatial variation of the earthquakes' ground motion frequency content on the Japan megathrust. This analysis revealed a depth dependency of generated ground motions consistent with the downdip segmentation proposed for subduction interfaces (Lay et al., 2012, https://doi.org/10.1029/2011JB009133), a regional ground motion dependency that may be related with lateral variations of the mechanical properties of the subduction interface and a high‐frequency radiations drop in the earthquake sequence that preceded the Tohoku‐Oki earthquake Mw 9.0. The regional ground motion dependency suggests the existence of different domains along trench of the Japan subduction megathrust that control the ground motions and the wave radiation patterns of interface earthquakes. The location of their boundaries is consistent with the extension of the rupture of the 2011 Tohoku‐Oki earthquake, with pre‐Tohoku interseismic coupling, and with the free air gravity anomalies.

A three-dimensional density model of north Andaman subduction zone

We present a three-dimensional (3D) density model of northern Andaman subduction zone (10°–15°N) constructed through joint modelling of satellite free-air gravity and geoid anomalies with constraints from previous studies. This novel modelling approach utilizes geoid and gravity anomalies simultaneously to better resolve the structure of inadequately known part of the subduction zone. The density model suggests:•A contrasting situation of subducting Indian plate, mapped up to a depth of ∼180 km in the southern part, and almost missing in the north of the study region.•Dip of the Benioff zone and the width of the accretionary prism decreases from south to north.•Nature of the overriding plate varies from oceanic to semi-continental towards the north.•Moho depth is relatively higher under the seamounts and 90°E ridge.The change in the subduction geometry is probably due to the subducting aseismic ridge, difference in the sediment influx and variation in the convergence rates. The absence of the subducting slab in the north is probably caused due to tearing between northern Andaman in the south and Burmese Arc in the north.

Qualitative appraisal of high resolution satellite derived free air gravity anomalies over the Maldive Ridge and adjoining ocean basins, western Indian Ocean

High resolution satellite derived Free Air Gravity (FAG) anomalies over the Maldive Ridge segment of the Chagos-Laccadive Ridge and adjoining areas of the Arabian Basin and Central Indian Basin was analyzed to understand their structure. Indian Ocean Geoidal Low corrected FAG depict the N-S trending relative highs associated with the Maldive Ridge, the lows associated with the Vishnu Fracture Zone, and the seafloor fabrics associated with spreading Central Indian Ridge in the Arabian Basin. Transformation operations for enhancing the short, intermediate and long wavelength anomalies were carried out. The northward continuation of the Vishnu Fracture Zone, the eastward extension of the fracture zones of the Arabian Basin into the Deep Sea Channel (DSC) region, between the Maldive Ridge and the Chagos Bank, and the signatures related to north-south trending coral reefs have been deciphered from the enhancement of short wavelength anomalies. The long wavelength anomalies show N-S highs along the axis of the Maldive Ridge and the DSC, which on comparison with previous studies, is inferred to be the signature related with the underplated material from the passage of the Indian plate over the Reunion Plume. Depth to top of two interfaces over the Maldive Ridge and the DSC were computed using the power spectral method by assuming a window of 110 km dimension and 50 km overlap. Depths were thus calculated for 44 blocks along the ridge axis. The average depth to the top of the shallow interface was found to be 5.5 km while that of the deeper was 11.0 km. These depths and interfaces were confirmed by undertaking 2D forward modelling along a 300 km long N-S profile along the western segment of the Maldive Ridge. Integrating with the available previous geophysical studies, it is inferred that the shallow interface represents the depth to the top of the acoustic basement underlying 1–1.5 km thick lava flow unit while the deeper is associated with the interface between the crust and initial Moho before the underplating took place (presently top of the underplated material). From the present study, it has been inferred that the Deep Sea Channel between the Maldive Ridge and Chagos Bank can be purely oceanic in nature.

Gravity anomaly to identify Walanae fault using second vertical derivative method

<p class="Abstract">Research has been conducted to identify the Walanae Fault, coordinates 4–6 S and 118-120 E using anomalous gravity data. This research uses data measurement of Topography and the Free Air Anomaly from the TOPEX/Poseidon satellite. Then the authors processed to obtain the bouguer anomalies and made modeling by using the Surfer 10. The authors used the Second Vertical Derivative (SVD) with filter Elkins of Moving Average then analyze the graph of the SVD. The results shows the value of the residual anomaly in the north of fault is 25.21 mGal, in the middle occur range 17.67 mGal to 24.98 mGal and 30,376 mGal in the south of fault. The authors indicates the existence of a difference between the gravity between the Walanae Fault with surrounding geologic. From these results also show that Walanae Fault has a reverse fault mechanism in the northern part and the normal fault mechanism in the middle to the south, the authors conclude that the Walanae Fault is divided into two segments, that is the northern and the southern segment.</p>