Gravity Stations Research Articles

Apparent calibration shift of the Scintrex CG-5 gravimeter caused by reading-dependent scale factor and instrumental drift

Calibration measurements of the Scintrex CG-5 gravimeter were conducted using absolute gravity stations of the Japan Gravity Standardization Net, to constrain the scale factor and its temporal changes. The calibration data were obtained from a total gravity interval of 1.4 Gal through six campaigns, conducted for over 5 years between years 2012 and 2017. The scale factors varied by 1500 ppm in a range from 0.9991 to 1.0006, according to station combinations of the six campaigns. The scale factor depends primarily on the gravity reading ranges: for similar gravity reading ranges, no significant differences in the estimated scale factors were recognised, even though station combinations and observed times are different. Therefore, the gravity readings can be corrected by introducing a gravity reading-dependent scale factor. Furthermore, even though the scale factor essentially depends on gravity readings and not on time, temporal changes were observed during repeated calibration measurements at the same station combinations. A long-term instrumental drift of the CG-5 gravimeter could explain this phenomenon. In conclusion, the calibration shifts recognised during repeated measurements were apparently caused by: (1) the scale factor dependence on the gravity reading ranges and (2) the shift of the gravity reading ranges due to the instrumental drift.

Laser geodetic satellites: a high-accuracy scientific tool

Satellite Laser Ranging (SLR) began in the mid-1960s on satellites of opportunity with retro-reflectors intended as a part of intercomparison tests of satellite tracking techniques. Shortly thereafter, data from these satellites began to work their way into geodetic solutions and dedicated geodesy experiments. By early 1970s when future requirements for centimeter accuracy were envisioned, planning began for dedicated, spherical retro-reflector geodetic satellites. Built with high mass-to-area ratios, these satellites would have important applications in gravity field modeling, station geolocation and fiducial reference systems, Earth rotation, and fundamental physics. Early geodetic satellites were Starlette, launched in 1975 by Centre National d’Etudes Spatiales (CNES), and LAGEOS in 1976 by the National Aeronautics and Space Administration (NASA). Recent geodetic satellites include LARES, launched in 2012, and LARES-2 under development, both by the Italian Space Agency (ASI). Today, a complex of these ‘geodetic satellites’ from low to high altitude Earth orbits supports many space geodesy requirements. This paper will discuss the evolution of the geodetic satellites from the early days, through current programs and out to future needs as we approach our goal for millimeter accuracy.

A preliminary geophysical analysis of gravity time-variation in Daejeon

A gravity time series slightly longer than a month acquired by using a Burris gravimeter at Daejeon, Korea has been analyzed. After removal of linear drift of the gravimeter, the observed data was found to closely match with the body tide prediction except; (i) small phase differences of each main tidal constituents and (ii) existence of long period component in the observed data. The r.m.s. observed gravity tide variation was 62.4 μGal. The overall g-factor was found as 1.165. The values of g-factor for the four main tidal constituents; M{nt2}, S2, O1, K1, were found as 1.166, 1.172, 1.168, and 1.142 respectively. The r.m.s. value of ocean tidal loading effect on gravity at Daejeon was 0.95 μGal during the time span. Both the rainfall and the atmospheric pressure change have induced gravity variations of amplitude of about 10 to 1 μGal. The long period component, which is clearly found in the observed spectrum, while not existent in the prediction, is possibly caused by the local ground water variation. It is desirable to have a few number of permanent gravity stations operating in Korean peninsula.

Structural interpretation of Mae Suai Basin, Chiang Rai Province, based on gravity data analysis and modelling

The Mae Suai Basin, an intermountain basin in northern Thailand, became an area of interest in 2014 following the M6.1 earthquake that reactivated the ENE-WSW trending Mae Loa Fault. This fault is associated with the Cenozoic rifting. Terrestrial gravity modelling is a suitable method to visualize subsurface geometry and understand its structural control related to the recent earthquakes. Six hundred twenty-seven terrestrial gravity stations with a spacing of ∼500 m were collected; standard gravity correction methods were applied with a density reduction of 2.67 g/cm3 to produce the residual Complete Bouguer anomaly (CBA). The residual CBA map reveals a NNE-SSW striking basin, showing gravity lows are located within the basin. The gravity highs cover regions of Triassic granite intrusions to the west and Silurian-Devonian metasedimentary and Carboniferous sedimentary basements to the east. Structural edge detections and basin depth estimates indicate the main fault lineaments lie ENE-SWS and NNE-SSW striking along the eastern and northern margins respectively. These faults may act as splay faults of the active sinistral Mae Loa fault. The gravity models suggest that the Mae Suai Basin is an asymmetrical half-graben with a maximum depth of ∼770 m and a range of basin sediments from 1.9 to 2.3 g/cm3. The depocentre is located near the eastern boundary faults. The structural patterns present the rifting has formed within an extensional transfer zone that relates to a releasing bend fault in NNE-SSW trend, linked by the sinistral Mae Loa Fault in NE-SW trend. The E-W maximum of extension in the transfer zone is formed under the activation of the major Cenozoic strike-slip faults in Northern Thailand.

Tectonics mapping using geophysical data around Nuweiba area, Gulf of Aqaba, Sinai, Egypt

ABSTRACTThe main target is to delineate the structural features and tectonic hazard for the study area. Three geophysical tools that are used to configure the tectonic setting in the study are gravity, magnetic and seismic activity. Gravity measurements were acquired to represent about 201 gravity stations covered the studied area. Different gravity corrections such as drift, tide, latitude, free-air, Bouguer and Terrain corrections were performed using the commercial software Oasis Montaj version 8.3. The interpretation of gravity data indicates that different trends of structures, for example, NW-SE, N–S and NE–SW trends where most of these fault elements are active faults according to seismic events which were recorded on their locations. Also, 201 land magnetic stations were measured at same sites of gravity stations. The basement depth derived from the magnetic data has value between 744 m and 4122 m.

GRAVITY AND MAGNETIC INTERPRETATION USING GEOPHYSICAL DATA AROUND NUWEIBA AREA, GULF OF AQABA, SINAI, EGYPT.

Delineating the structural features and tectonic hazard for the area under study represent the main target of the present study. Three geophysical tools are used to configurated the tectonic setting in the study are such as gravity, magnetic and seismic activity. Gravity measurements were acquired to represent about 201 gravity stations covered the studied area. Different gravity corrections such as drift, tide, latitude, free-air, Bouguer and Terrain corrections were performed using the commercial software Oasis Montaj version 8.3. The interpretation of gravity data indicates that different trends of structures for example; NW-SE, N–S and NE–SW trends where most of these fault elements are active faults according to seismic events which recorded on their locations. Also, 201 land magnetic stations were measured at same sites of gravity stations. The basement depth derived from the magnetic data has value between 744 m to about 4122 m.

Borehole gravity in Horizontal Wells.

Gravity can now be measured practically anywhere a drill hole can go. This extends our ability to perform 3D and 4D gravity mapping of the sub-surface with applications in the energy and mineral sectors. New tools have been developed to perform these measurements. The tools address issues of size, measurement accuracy, stability and ruggedness. The data handling has also changed to address the 3 dimensional nature of the measurements. Whereas previous borehole gravity measurements in near vertical wells could ignore latitude corrections and use simpler terrain correction, surveys in highly deviated wells involve calculating and using 3 dimensional coordinates for each gravity station in the correction process. This all needs to be done preserving the survey accuracy requirements that are often only a few microGals. Tool location is extremely important, more in the vertical direction due to the vertical free air and Bouguer gradients. Tool location control tends to be more difficult in more horizontal holes. A new high sampling resolution CCL tool has proven very useful for repeat location of the tool for drift calculations and time lapse measurements. For oil and gas, most applications in highly deviated or horizontal wells are expected to be of a time lapse mode such as monitoring of traditional and SAGD type steam floods. For mining the tool significantly extends the volume of investigation for deep exploration and development holes.

Água Bonita pull apart basin and its relationship to Transbrasiliano Lineament

The Transbrasiliano Lineament (TBL), a NE-SW trending strike slip system at least 2700 km long, is one of the main structures of the South American Platform. This lineament, along with other structures, influenced greatly the installation and depositional history of the Paraná and Parnaíba sedimentary basins. The Água Bonita Basin (ABB) occurs at an intermediate position between both basins. This work aims to provide information on the origin and evolution of the Água Bonita Basin, considered a Silurian-Devonian basin, as well as its relationship with the Transbrasiliano Lineament.Geological, aero and ground geophysics data were used to determine a structural geological model for the ABB located in the Brasilia Fold Belt in the Tocantins tectonic province. The airborne geophysical magnetic data used in the study were acquired in the 1970s by the existing 147 ground gravity stations in addition to the 498 new ground gravity and geodetic stations that were added to the existing ones.The analysis of the outcrops allowed identifying the sedimentary facies associations providing new information on the depositional environments. The compilation of existing data, satellite images analysis, geological maps and shaded relief provided an integrated model for the study area.A geological model was proposed for the ABB based on the interpretation, integration, depth analysis and 2D gravity forward modeling according to a theoretical model for an extensional duplex in a strike slip system with flower structures. We observed several segmented gravity anomalies, which were interpreted as steps/faults. This interpretation is consistent with a pull apart basin. According to the depth data estimated by this model, the maximum thickness of the Água Bonita Formation is 1.6 km and 2.4 km for the precursor rift. The basin has two dextral faults on each border, whose main fault of the TBL strike slip system and the master fault controlling the ABB are observed on the east border of the rift phase. Lineaments perpendicular to the west edge, in addition to those crossing the ABB in the N65E, delineate peculiar magnetic, gravity and geological signatures in the northern part of the basin. The basin is located in the weak region of the TBL and its main eastern fault is dipping sub-vertically to the west. This fault modeled up to the 8 km depth using the gravity data, was also observed in the Tilt Angle of the Total Horizontal Gradient (TAHG) matched filter grid result at approximately 8 km, however this may not be this fault maximum depth.Three main sedimentary-tectonic stages were observed, (i) a Pre-Devonian precursor rift present in the Água Bonita Basin and Jaibaras Group; (ii) the sag stage developed during the Devonian, when the Paleozoic basins of Paraná, Parnaíba and Água Bonita Basin were connected evidenced by the sedimentary facies and similar thicknesses; and (iii) the Pos-devonian rocks that are probably not preserved in the Água Bonita Basin.

Geodetic Changes Associated with Crustal Deformation on the Andean Backarc of San Juan, Argentina

In this work we have analyzed the results of temporal geodetic variations associated with tectonic deformation on the Andean backarc in southern San Juan province, Argentina. This area is affected by the subduction of the Juan Fernandez Ridge and has the greatest seismic hazard in Argentina. Since 2000, we have monitored the variations of gravity annually with high precision (g4D) in the active deformation belt, around the city of San Juan (31°S to 32°S, 68°W to 69°W). Temporal gradients of g4D have been associated with active structures and interpreted as subsurface mass redistributions as a result of accumulation and release of stress. These data, in addition to the topographic deformations measured by the Global Positioning System (GPS) and the identification of structures in the crystalline basement from the geophysical models, allow us interpret mechanisms of deformation in the contact between the Precordillera and Pampean Ranges in the foreland of San Juan. In this area, after more than a decade of low activity, in 2012 the Tulum Fault System (TFS) was reactivated, generating crustal events of moderate magnitude including an earthquake of Mw 5.0 (January 18, 2012). According to our interpretation, the fault system accumulated elastic deformation for more than 10 years expressed as height increases at gravity stations (decrease of g4D) until the year 2011. The January 18, 2012 earthquake released part of the energy at the same time as a coseismic increase of g4D. This should be reflected in regional tectonic subsidence from the point of view of conventional gravimetric models. However, GPS reoccupation of the benchmarks located in this fault system indicates a generalized increase in altitude of the fault system in the last 15 years. TFS behavior could be assumed as topographic flexural displacements induced by a system of blind faults. We analyzed the time series of a permanent Global Navigation Satellite System (GNSS) station located to the north of the TFS. The GRLS station (31.61oS; 68.32oW) presents slow variation in the vertical and north components. The variation began on December 20, 2011 reaching its maximum amplitude on January 18, 2012. The geodetic variations coincide with the period of seismic reactivation.

MicroGravity mapping for King Abdulaziz University campus: imaging subsurface

A gravity survey was undertaken to cover the King Abdulaziz University’s campus (~ 8 km2) as a pilot project funded from Deanship of Scientific Research (DSR). In the current study, 408 gravity stations were collected using the Scintrex CG5 gravimeter and a DGPS system, covering the whole university’ campus. The aim of this survey is to produce the first microgravity map for the King Abdulaziz University campus and image the subsurface conditions which help in the future constructions. Gravity surveys are carried out for 16 days, nights, and weekends when the campus has less traffic noise. Data were processed using conventional gravity data reduction technique, and then microgravity map for the campus was generated. Data were subject to geophysical filters such as edged detection (e.g., horizontal gradient) and depth estimation (e.g., Euler deconvolution, and 3D inversion) to image the subsurface conditions. Results indicate that bedrock relief surface is located at about 150 m. Some cracks/fractures were traced/detected underneath the surface in various directions. The results also indicate that the university campus is located on high-density rock zones, giving more stability to buildings.

HIGH-RESOLUTION RESIDUAL TERRAIN MODEL AND TERRAIN CORRECTIONS FOR GRAVITY FIELD MODELLING AND GEOID COMPUTATION IN NIGER REPUBLIC

In this study, we computed and presented grid maps of high-resolution terrain corrections and residual terrain model (RTM) as short-wavelengths of the gravity field and the geoid in Niger. We constructed RTM elevations from mean elevation surfaces corresponding to ~100 km and ~9 km of spatial scales and 3 arc-seconds SRTM data. The computations are performed at gravity stations and 1.5 arc-minute regular grid, out to 10 and 200 km for inner and outer zones respectively with the standard density of 2670 kg/m-3. The study area is characterized by low values of terrain effects. The indirect effects are lower than 10 cm for ~9 km and reach 1.8 m for ~100 km. In Niger, 98.44% of indirect effects are lower than 1 cm and 98.2% of direct effect are lower than 5 mgal for ~9 km. For ~100 km, 85.87% of indirect effects are lower than 10 cm and 89.77% of direct effects are lower than 5 mgal for ~100 km, and 98.77% of terrain corrections are lower than 1 mgal. We found out that height discrepancies between gravity stations and SRTM influences the precision of terrain effects. The results are value for applications in geodesy and geophysics that require accurate interpretations.

New Geodetic and Gravimetric Maps to Infer Geodynamics of Antarctica with Insights on Victoria Land

In order to make inferences on the geodynamics of Antarctica, geodetic and gravimetric maps derived from past and new observations can be used. This paper provides new insights into the geodynamics of Antarctica by integrating data at regional and continental scales. In particular, signatures of geodynamic activity at a regional extent have been investigated in Victoria Land (VL, Antarctica) by means of Global Navigation Satellite System (GNSS) permanent station observations, data from the VLNDEF (Victoria Land Network for Deformation control) discontinuous network, and gravity station measurements. At the continental scale, episodic GNSS observations on VLNDEF sites collected for 20 years, together with continuous data from the International GNSS Service (IGS) and Polar Earth Observing Network (POLENET) sites, were processed, and the Euler pole position assessed with the angular velocity of the Antarctic plate. Both the Bouguer and the free-air gravity anomaly maps were obtained by integrating the available open-access geophysics dataset, and a compilation of 180 gravity measurements collected in the VL within the Italian National Program for Antarctic Research (PNRA) activities. As a result, new evidence has been detected at regional and continental scale. The main absolute motion of VL is towards SE (Ve 9.9 ± 0.26 mm/yr, Vn −11.9 ± 0.27 mm/yr) with a pattern similar to the transforms of the Tasman and Balleny fracture zones produced as consequence of Southern Ocean spreading. Residual velocities of the GNSS stations located in VL confirm the active role of the two main tectonic lineaments of the region, the Rennick–Aviator and the Lillie–Tucker faults with right-lateral sense of shear. The resulting VL gravity anomalies show a NW region characterized by small sized Bouguer anomaly with high uplift rates associated and a SE region with low values of Bouguer anomaly and general subsidence phenomena. The East and West Antarctica are characterized by a different thickness of the Earth’s crust, and the relative velocities obtained by the observed GNSS data confirm that movements between the two regions are negligible. In East Antarctica, the roots of the main subglacial highlands, Gamburtsev Mts and Dronning Maud Land, are present. The Northern Victoria Land (NVL) is characterized by more scattered anomalies. These confirm the differences between the Glacial Isostatic Adjustment (GIA) modeled and observed uplift rates that could be related to deep-seated, regional scale structures.

Joint interpretation of gravity and resistivity data from the Northern Kenya volcanic rift zone: Structural and geothermal significance

The Northern Kenya Rift has been the least studied sector of the Kenyan part of the East African Rift due to its remote location. Geothermal exploration conducted in the last ten years has greatly improved the geophysical constraints on the geology of this area. Recently, about 515 gravity stations have been surveyed around the Korosi, Paka and Silali volcanoes and analysed in conjuction with resistivity data from over 300 MT stations in order to jointly interpret the density and resistivity distribution of the Northern Kenya rift. Our models of the gravity data are in good agreement with previous MT and TEM resistivity studies of the individual volcanoes. The new Bouguer gravity map is characterized by a 10 to 15 km wide gravity high of 8 mGal amplitude striking NNE along the inner rift corresponding to resistivity > 50 Ωm below 2 km depth. Gravity lows due to structures shallower than 2 km depth at the Paka and Korosi volcanoes have been interpreted as low density bodies within their edifices, likely to consist of either unaltered near-surface pyroclastics or deeper tuffs altered at 60 to 180°C to hydrothermal smectite clay. Magnetotelluric (MT) resistivity models were used to further constrain the 2D gravity models. The high resistivity, low density near-surface rocks on the flanks of the volcanoes are interpreted to represent unaltered pyroclastics above the water table, whereas low resistivity, low density bodies underneath the Paka and Korosi volcanoes indicate low density tuffs, hydrothermally altered to hydrated smectite clay. Below 2 km depth the deeper high density zone beneath the volcanic inner rift is likely to be a combination of higher temperature, low porosity alteration associated with geothermal reservoirs and/or denser rocks related to intrusions. As expected, the greater proportion of dense lavas associated with fissure swarms mapped at the surface coincide with both relatively high gravity and MT resistivity south of Paka volcano. However, farther north between Paka and Silali volcanoes, the trends in higher gravity and resistivity lie west of the currently active fissure zone. This apparently inconsistent trend in the gravity and resistivity has been interpreted as lavas buried below recent tuffs and clastics associated with a former alignment of fissure eruptions in the rift about 7 km to the west of the current axis of eruption between Paka and Silali.

Structure and evolution of the Ban Pong Basin, Chiang Mai Province, Thailand

The Ban Pong Basin (BPB) is a small intermontane basin of Neogene age on the western margin of the Chiang Mai Basin in northern Thailand. The BPB was discovered in 2012 because of new quarrying activity. The basin complex formed in the hanging wall of a major Cenozoic low-angle normal fault (LANF) called the Inthanon Detachment, and provides a unique record of the Neogene deformation related to the detachment. The BPB’s preservation is probably due to its location in a synform within the rising Western Ranges metamorphic complex, which forms the Suthep-Inthanon mountain range. Initially, extensional faults in the BPB were oriented ∼N-S but then rotated to NNW-SSE, NW-SE, and ENE-WSW to NE-SW directions, which we interpret as arising due to local stress rotation related to the adjacent rising metamorphic complex. Folds, thrusts, and inverted normal faults both within Neogene sediments of the Mae Rim Formation and adjacent Palaeozoic metasedimentary rocks indicate a phase of compression that affected the basin late in its history (probably latest Miocene-Pliocene based on the regional timing of inversion). Complete Bouguer anomaly interpretations of 235 reduced gravity stations reveal two en echelon sub-basins, bounded by gravity highs related to shallow-lying Palaeozoic rocks and underlying high-grade metamorphic basement rocks. Two gravity-density cross-section models indicate the basin fill comprises up to 250 m of weakly lithified syn-rift sediments. The absence of a classic half graben morphology is probably related to inversion and erosion that significantly modified the extensional basin geometry and original thickness.

Regional Gravity Field Model of Egypt Based on Satellite and Terrestrial Data

This study presents a recent combined regional gravity field model over Egypt, developed by integrating satellite and terrestrial data via applying the remove-compute-restore (RCR) principle and the least-squares collocation (LSC) procedure. A high-resolution digital terrain model was exploited for the computation of the terrain and residual terrain corrections. Hereby, all the signals that can be modelled or deterministically computed are considered known and then removed in order to reduce the order of magnitude of the input gravity data prior to applying the LSC. Several GOCE-only and combined global geopotential models (GGMs) have been thoroughly investigated with respect to the EGM2008, in which the space-wise (SPW) solution, namely the SPW-R5 model, demonstrated the best performance. For the development of the combined model, the SPW-R5 GGM has been integrated with both the EGM2008 GGM and the terrestrial data retrieved from 56,250 gravity stations of the Getech data, acquired in the framework of the African Gravity Project. The combined regional gravity model was compared to the state-of-art XGM2016 global gravity model. The standard deviation of the differences is 18.0 mGal in terms of Bouguer anomalies. The combined regional model fits well with the terrestrial gravity data along the chosen North–South oriented profile through the Nile Delta region. The improvements of the developed combined regional model over the XGM2016 are due to the use of a more extensive terrestrial dataset. In conclusion, our model is more suitable than solely using the ground data or GGMs for regional density modelling over Egypt. As an example, the comparison of using a global or regionally defined gravity model with the forward gravity modelling based on Saleh (Acta Geodaetica et Geophysica Hungarica 47(4):402–429, 2012) density model is performed.

Inverse Modeling of Gravity Data with Two Layers Density in Sedimentary Basin Structure

Study of sedimentary basin structure and basement variation in the subsurface using gravity methods are very common in early stage of hydrocarbon exploration. Most of inverse modeling are designed with several series of juxtaposed vertical prism in the subsurface. In this study, we utilize two layers of vertical prism below each gravity stations to represent homogeneous density of sedimentary layer and basement in the subsurface. In order to reach best miss-fit data we used iterative calculation utilizing differences between observed and calculated data. Performance of iterative calculation will attempt to find solution of interface density in the subsurface. In this paper, we use two synthetic data of symmetrical sedimentary basin model for inversion performance test. The performance test will evaluate the effect of initial model, exploration well data, topographic data, and noisy data effect. Based on synthetic data test, the performance of iterative method provide small order of miss-fit data with residual solution model of noisy data in the range -44 to 35 meters. Real data application identified three shallow sedimentary layer and two deeper sedimentary layer in the study area.

Developing an autonomous unmanned aerial system to estimate field terrain corrections for gravity measurements

We have developed a method employing an autonomous unmanned aerial system (UAS) to provide a more robust measure of the field terrain correction (FTC) for gravity measurements than is offered by traditional methods. The resolution of digital terrain that is typically available for the United States (10 or 30 m) is too low to adequately estimate the FTC in steep terrain. The FTC corresponds to the innermost zone around the gravity station (e.g., extending to a 68 m radius for the Hayford-Bowie zones A and B) and is most often estimated in the field with the aid of templates and charts that approximate sectors of the terrain as uniform slopes. These techniques can incur significant error if they are not performed by experienced practitioners. In our approach, we dispatch a UAS to collect camera images around the gravity station, which we use to construct a digital elevation model (DEM) of the area with the structure from motion (SfM) method. The resulting DEMs allow us to precisely calculate the FTC. We have developed software to automate most of the procedure, including the autonomous flight and image capturing by the UAS. We have experimented with a variety of flight paths at several sites spanning a range of terrain conditions to determine the most efficient flight characteristics for this application. Selecting field sites with existing light detection and ranging data has enabled us to characterize errors in the DEMs derived from SfM and to assess the trade-offs between flight time, processing time, and accuracy of the resulting FTCs. Our methodology is fast (flight time: 3–4 minutes) and robust, primarily because the UAS flight is automated. It can be used to calculate the FTC of a variety of terrain conditions and delivers results that are much more precise than existing methods that do not make use of high-resolution digital terrain data.

Structure of the Marathon basin (NE Attica, Greece) based on gravity measurements

This work presents the results of a high resolution gravity survey conducted in the area of Marathon Basin to asses the structure of the basement. A total of 120 gravity stations were measured. Standard data reduction procedures were followed, albeit based on modern, high precision techniques. The inversion of the Bouguer anomaly map provided a rather detailed image of the basement topography completing the information available from surface geological and morphological surveys. The major structural features of the Marathon Basin appear to be controlled by a system of NE-SW neotectonic faults causing stepwise NW-SE deepening of the basement to a maximum detectable depth of~450m. These faults, together with a system of NW-SE major, and a host of minor faulting features, lend to the Basin characteristics of a Riedel structure formed by a predominantly N-S extensional field effecting clockwise block rotation. This mode of deformation may also have facilitated the formation of local anticlinal/ synclinal structures, which endow the area with its rather complicated morphology. Finally, the multiple intersecting faults appear to form a permeable network, presumably responsible for the salination of the local aquifer system; the NE-SW major normal faults may the most significant contributor to this effect.

Using a 3D gravity inversion technique to image the subsurface density structure in the Lunayyir volcanic field, Saudi Arabia

An earthquake of magnitude (M = 5.4) occurred on 19 May 2009 in the Lunayyir Volcanic Field (LVF), Saudi Arabia. The LVF is covered by recent basaltic rocks. The earthquake caused damages to the surrounding areas and resulted in surficial rapture trending in NNW-SSE direction with about 8-km length. Due to concern in future damaging earthquakes or volcanic activities, the government evacuated the residents for over 3 months.Seismologists from the Saudi Geological Survey (SGS) studied the recorded seismicity data around the damaged area to locate the epicenter of the (M = 5.4) earthquake. After extensive studies, they suggested that the main cause of the M = 5.4 event was magma dike intrusion at depth of about 5 km. However; the dike did not reach the surface and no eruption took place.Detailed ground and airborne gravity surveys around the earthquake area was carried out in an attempt to understand the reason of the earthquake and imaging the subsurface geology. In that regard, 380 gravity stations were measured in an area of about 600 km2 covering the surficial rupture. The gravity data was analyzed using edge detection tools and a 3D inversion technique.The results indicate that the 8-km surficial rupture extends toward the SE direction into the sub-surface which suggested a future seismic activity along this extension. Additionally, the dike intrusion location has low density values compared with the surrounding area which imply that this dike intrusion was created in the past, cooled, and solidified. Consequently, we assign the M 5.4 earthquake and the associated seismic swarm to simultaneous Red-Sea-Rift-parallel faulting and magmatic intrusion.

Characterization of the depocenters and the basement structure, below the central Chile Andean Forearc: A 3D geophysical modelling in Santiago Basin area

AbstractSince the last century, several geological and geophysical studies have been developed in the Santiago Basin to understand its morphology and tectonic evolution. However, some uncertainties regarding sedimentary fill properties and possible density anomalies below the sediments/basement boundary remain. Considering that this is an area densely populated with more than 6 million inhabitants in a highly active seismotectonic environment, the physical properties of the Santiago Basin are important to study the geological and structural evolution of the Andean forearc and to characterize its seismic response and related seismic hazard. Two and three‐dimensional gravimetric models were developed, based on a database of 797 compiled and 883 newly acquired gravity stations. To produce a well‐constrained basement elevation model, a review of 499 wells and 30 transient electromagnetic soundings were used, which contribute with basement depth or minimum sedimentary thickness information. For the 2‐D modelling, a total of 49 gravimetric profiles were processed considering a homogeneous density contrast and independent regional trends. A strong positive gravity anomaly was observed in the centre of the basin, which complicated the modelling process but was carefully addressed with the available constrains. The resulting basement elevation models show complex basement geometry with, at least, eight recognizable depocenters with maximum sedimentary infill of ~ 500 m. The 3‐D density models show alignments in the basement that correlates well with important intrusive units of the Cenozoic and Mesozoic. Along with interpreted fault zones westwards and eastwards of the basin, the observations suggest a structural control of Santiago basin geometry, where recent deformation associated with the Andean contractional deformation front and old structures developed during the Cenozoic extension are superimposed to the variability of river erosion/deposition processes.