Simple Bouguer Anomalies Research Articles

Identification of the Manna Segment Sumatran Fault Using GGMplus Gravity Anomaly Data with the Second Vertical Derivative (SVD) Method

This study uses the gravity method to assess the characteristics of the Manna segment fault, a part of the Sumatran fault. This research aims to contribute to the existing knowledge by providing additional insights into the fault's characteristics, which is essential for future disaster mitigation efforts. The Manna segment fault is geographically situated between coordinates 4.4⁰S - 3.9⁰ S and 102.7⁰ E – 103.2⁰ E, making this area prone to earthquake hazards. The gravity anomaly data, including free air and topography, were collected from 62,501 data points. The Simple Bouguer Anomaly (SBA) values obtained ranged from 25 mGal to 95 mGal. By utilizing the Second Vertical Derivative (SVD) method and dividing the study area into three regions with multiple sections, the analysis revealed that the minimum and maximum SVD values were relatively similar. This suggests that the Manna fault can be classified as a strike-slip fault, with SVD values ranging from Elkins-20 mGal to 6 mGal. The study identified branches of the main fault in regions 2 and 3 based on the SVD map. Overall, this study enhances our understanding of the Manna segment fault, providing valuable insights into its characteristics and contributing to future disaster mitigation efforts.

The 2022 Mw 6.1 Pasaman Barat, Indonesia Earthquake, Confirmed the Existence of the Talamau Segment Fault Based on Teleseismic and Satellite Gravity Data

A Mw 6.1 earthquake on 25 February 2022, at around 8:39 a.m. local time, struck Pasaman Barat Regency, West Sumatra, Indonesia, and was felt in Singapore and Malaysia. The hypocenter of this earthquake was 12 km deep and preceded by an Mw 4.9 foreshock a few minutes earlier. The earthquakes originated on a blind fault and triggered a landslide at Mount Talamau. Herein, the slip distribution and asperities along the plane fault during the earthquake were examined by teleseismic inversion and the fault location was identified by Global Gravity Model plus (GGMPlus) satellite gravity data. The slip distribution was calculated from the source parameters (strike: 136°; dip: 70°; rake: 174°) by inversion techniques based on teleseismic data. Based on the slip distribution, the earthquake was generated by stress from the Sianok fault that spread to the north and reached the uncertain fault segment in the Talamau area. In addition, the results of the First Horizontal Derivative and Second Vertical Derivative from the GGMplus data revealed a straight Simple Bouguer Anomaly pattern, confirming the existence of the uncertain Talamau fault as part of the Great Sumatra Fault. This work shows the potential application of the combination of teleseismic and gravity observation for delineating the fault structure that caused the 2022 Mw 6.1 Pasaman earthquake, which can also be applied to other locations of similar geological backgrounds.

Gravity Anomaly Evaluation of the Geothermal Field Distribution Around the Seismo-Volcanic Area of Toba Lake

Toba Lake is one of the largest caldera lakes in the world, formed by an enormous eruption about 74,000 years ago. The emergence of hot spring manifestations around Lake Toba is closely related to volcanic activity, which is also controlled by regional geological structures. There are two areas in Lake Toba: Simbolon and Pusuk Buhit, with several geothermal manifestations, such as fumaroles, hot springs, and steaming ground. With the large caldera area, there should be other locations with geothermal potential. Therefore, we analyzed the regional gravity anomaly to find specific patterns that might indicate a possible geothermal field. First, we obtained the data of Free Air Anomaly (FAA) with a resolution of 1.85 km from altimetry satellites and acceleration of gravity anomaly data of 0.1 mGal at 1 minute per grid with an altitude accuracy of 1 meter. In order to acquire the Simple Bouguer Anomaly, the data were corrected. Then, we analyzed the results based on derivative analysis. The result shows that the location has simple Bouguer anomaly values between - 26.2 mGal-36.0 mGal with an initial density of 1.74 gr/cc. Furthermore, we speculated that indicated regional patterns of structures and sub-surface properties might be associated with that prospective geothermal.

Colorado geoid modeling at the US National Geodetic Survey

The geoid computation method currently used at the US National Geodetic Survey is based on the analytical downward continuation solution of the Molodensky geodetic boundary value problem. The quasigeoid is computed first in 1′ × 1′ grids for the whole area, and then, the geoid is obtained by adding the geoid–quasigeoid separation term based on the simple Bouguer anomalies. A few variations in data combination and topographic/gravity reductions are tested, and three models are selected for the Colorado experiment. Historical GPS/leveling data are used as a guide in the computations. The standard deviation of geoid differences between the historical GPS/leveling data and geoid models is 5.2, 5.0 and 4.9 cm for the models of the initial run, the first and second iteration, respectively. After the models were submitted to the Colorado experiment, they were validated using the GSVS17 (Geoid Slope Validation Survey 2017) GPS/leveling data. The standard deviation of the geoid differences on 223 GSVS17 marks is 3.2, 2.6 and 2.3 cm for the three runs, respectively. In addition to the (quasi)geoid models, the geopotential numbers at 223 GSVS17 marks are also computed and submitted to the Colorado geoid computation experiment and it is evaluated in Sanchez et al. (2020 this issue).

ANALISIS DATA GAYA BERAT DI DAERAH BENDAN DUWUR SEMARANG

This study was about the gravity in the area of ​​Bendan Duwur, Gajah Mungkur, Semarang. The method used in this research was gravity. The following steps in this activity were: calculating the value of Reader Gravity (gobs), Free Air Correction (FAC), Free Air Anomaly (FAA), Bouguer Correction (BC), Simple Bouguer Anomaly (SBA), Terrain Correction (TC), and Complete Bouguer Anomaly (CBA). The results of this research were listed as the Duwur bendan area, which can be seen from the shape of the points contained in the CBA, which is the smallest mGal value, 2.78 and the largest was 14.14. This can be assessed as an anomaly value, blue was the lowest value while pink the highest value, and yellow was the medium category. Based on the results of gravity measurements in the Bendan Duwur area, Semarang, the smallest mGal value was recognized by the TC compared to the gobs, FAC, FAA, and BC values. TC was producing 0.24 and the highest value was 1.13. The largest correction value was the FAC, the lowest value of mGal was 16.14, while the highest value of mGal was 26.33. Adapters for the Bendan Duwur soil structure based on CBA including the underground category generated were directly related to tectonic activity, but were more related to geology that requires special sedimentation and anthropogenic activities such as water needed.

Groundwater Change Detection by Gravity Measurement on Northern Coast of Java: A Case Study in Semarang City of Central Java of Indonesia

Northern part of Semarang city is affected by tidal flood due to land subsidence. Land subsidence is triggered by soil compaction and massive groundwater extraction. One of famous methods in analysing groundwater is gravity method. This research aim to present groundwater change in Semarang city based on gravity measurement and to analyze problems related to equipment and groundwater behaviour. Gravimeter Scintrex CG-5 was applied for gravity measurement. Geographic coordinates were measured using GNSS receiver namely TopCon HiperII. This research occupied SRTM90 Plus and regional geology map at 1:100.000 scales. Groundwater change was detected by differentiating simple Bouguer anomaly of 2014 and 2016. Data in gravity unit were then converted by coefficient of 0.012 mgal per meter. Simple Bouguer anomaly in 2014 ranged from -3.423 mgal to 19.874 mgal, while 2016 data ranged from -3.427 mgal to 19.835 mgal. Discrepancy of gravity anomaly ranged from -0.326 mgal to 0.209 mgal. The data confirmed that groundwater change ranged from -27.160 m to 17.408 m. Groundwater depletion occurred commonly on alluvial plain in northern area, while recharging could be found in some locations in southern part. This research concluded that gravity measurement had potential to be applied for groundwater detection in coastal areas.

Response of Gravity, Magnetic, and Geoelectrical Resistivity Methods on Ngeni Southern Blitar Mineralization Zone

The research with entitle response of gravity, magnetic, and geoelectrical resistivity methods on Ngeni Southern Blitar mineralization zone has been done. This study aims to find the response of several geophysical methods of gravity, magnetic, and geoelectrical resistivity in an integrated manner. Gravity data acquisition was acquired 224 data which covers the whole region of Blitar district by using Gravity Meter La Coste & Romberg Model “G”, and magnetic data acquisition were acquired 195 data which covers the southern Blitar district only by using Proton Precession Magnetometer G-856. Meanwhile geoelectrical resistivity data only done in Ngeni village which is the location of phyropilite mining with the composition content of Fe, Si, Ca, S, Cu, and Mn by using ABEM Terrameter SAS 300C. Gravity data processing was performed to obtain the Bouguer anomaly value, which included unit conversion, tidal correction, drift correction, correction of tie point, base station correction, free air correction, and Bouguer correction. Magnetic data processing has been done by some corrections i.e daily, drift, and IGRF(International Geomagnetic Refference Field) to obtain the total magnetic anomaly. From gravity data processing has been obtained the simple Bouguer anomaly value in range from -10mGal until 115mGal. From this data processing has been obtained the total magnetic anomaly value in range from -650nT until 800nT. Meanwhile from geoelectrical resistivity 3.03Ωm until 11249.91 Ωm. There is a correlation between gravity anomaly, magnetic anomaly, and geoelectrical resistivity anomaly that are associated with deep anomaly, middle anomaly, and shallow anomaly.

Geophysical perspective on the structural interference zone along the Neoproterozoic Brasília and Ribeira fold belts in West Gondwana

ABSTRACT: The Brasília and Ribeira fold belts have been established in south-southwestern São Francisco Craton during the Brasiliano-Pan African orogeny (0.9-0.5 Ga - Tonian to Cambrian), and played an important role in West Gondwana continent assembly. The region is given by a complex regional fold and thrust belt superposed by shearing during the orogeny late times, with superposing stress fields forming a structural interference zone. These thrust sheets encompasses assemblies from lower- to upper-crust from different major tectonic blocks (Paranapanema, São Francisco), and newly created metamorphic rocks. Re-evaluation of ground gravity datasets in a geologically constrained approach including seismology (CRUST1 model) and magnetic data (EMAG2 model) unveiled details on the deep- crust settings, and the overall geometry of the structural interference zone. The Simple Bouguer Anomaly map shows heterogeneous density distribution in the area, highlighting the presence of high-density, high metamorphic grade rocks along the Alterosa suture zone in the Socorro-Guaxupé Nappe, lying amid a series of metasedimentary thrust scales in a regional nappe system with important verticalization along regional shear zones. Forward gravity modeling favors interpretations of structural interference up North into Guaxupé Nappe. Comparison to geotectonic models shows similarities with modern accretionary belts, renewing the discussion.

Gravity-Based Structural Modelling of Awataib Area, River Nile, State, Central Sudan

In this paper the authors are discussing the structural setting of Awataib valley area through Gravity-based variation measurements, which have been obtained by Simple Bouguer Anomaly formula that uses the relative difference in the vertical component of the earth gravitational field between Base station and other observational points. The study objective at using Regional gravity survey is to reveal the subsurface structural geology of Awataib valley and adjacent areas. Land Sat image aided by collected field data were used in this study. Surfer software version 8.02 used to produce contoured-base map of gravity data set. Grav Modeler Program in the Geological Research Authority of Sudan (G. R. A. S.) has been just used to perform 2-D Density-Depth modeling. Free-hand graphical technique has been used to perform geological sections from derived gravity models. The NW-SE lineament has been considered as fault coincides with valley of Awataib, thus faults map has been suggested from the calibration of Bouguer anomaly steep gradients and field observations. Some of the geology faults are considered to be a rejuvenation of pre-Nubian fractures; so far, this study suggests an extension of Umm Marahik strike slip fault with general trend of NE-SW ending at Awataib valley. The depth of sediments less than 450 meters and the average effective thickness is about 275 meters which’s making up basinal features that probably considered as water-bearing formations ready to be drilled.

Seismicity patterns along the Ecuadorian subduction zone: new constraints from earthquake location in a 3-D a priori velocity model

To improve earthquake location, we create a 3-D a priori P-wave velocity model (3-DVM) that approximates the large velocity variations of the Ecuadorian subduction system. The 3-DVM is constructed from the integration of geophysical and geological data that depend on the structural geometry and velocity properties of the crust and the upper mantle. In addition, specific station selection is carried out to compensate for the high station density on the Andean Chain. 3-D synthetic experiments are then designed to evaluate the network capacity to recover the event position using only P arrivals and the MAXI technique. Three synthetic earthquake location experiments are proposed: (1) noise-free and (2) noisy arrivals used in the 3-DVM, and (3) noise-free arrivals used in a 1-DVM. Synthetic results indicate that, under the best conditions (exact arrival data set and 3-DVM), the spatiotemporal configuration of the Ecuadorian network can accurately locate 70 per cent of events in the frontal part of the subduction zone (average azimuthal gap is 289° ± 44°). Noisy P arrivals (up to ± 0.3 s) can accurately located 50 per cent of earthquakes. Processing earthquake location within a 1-DVM almost never allows accurate hypocentre position for offshore earthquakes (15 per cent), which highlights the role of using a 3-DVM in subduction zone. For the application to real data, the seismicity distribution from the 3-D-MAXI catalogue is also compared to the determinations obtained in a 1-D-layered VM. In addition to good-quality location uncertainties, the clustering and the depth distribution confirm the 3-D-MAXI catalogue reliability. The pattern of the seismicity distribution (a 13 yr record during the inter-seismic period of the seismic cycle) is compared to the pattern of rupture zone and asperity of the Mw = 7.9 1942 and the Mw = 7.7 1958 events (the Mw = 8.8 1906 asperity patch is not defined). We observe that the nucleation of 1942, 1958 and 1906 events coincides with areas of positive Simple Bouguer anomalies and areas where marine terraces are still preserved on the coastal morphology. From north to south: (1) the 1958 rupture zone is almost aseismic and is attributed to a zone of high coupling; (2) south of the Galera alignment (perpendicular to the trench), the 1942 rupture zone presents moderate seismicity, deeper on the seismogenic interplate zone, and abutting on the Jama cluster (to the south). This cluster is facing the Cabo Pasado cap and positive Bouguer anomalies on the overriding margin. We suspect that this cluster reflects a zone of local asperity (partial coupling). South of the Jama cluster, the spherical aseismic zone in the Bahia area is interpreted as having a low seismic coupling (steady creep motion or slow slip events). We suspect that the site that generated the three M > 7 events (1896, 1956 and 1998) correspond to a small patch of strong coupling. To the south, in the Manta-Puerto Lopez zone, the seismicity is mainly organized in earthquake swarms (1998, 2002, 2005). Although slow slip events have been observed in the area (Vallée et al. submitted), we infer from the coastline shape, the marine terraces and the high positive Bouguer anomalies that the seismicity here might reveal a significant amount of seismic coupling.

Ortalama gravite anomalilerinin enterpolasyonunda basit ve tamamlanmış Bouguer yaklaşımının karşılaştırılması

Comparison between simple and complete Bouguer approaches in interpolation of Mean Gravity Anomalies The determination of the geoid, which is a real shape of the Earth, with an accuracy of 1 cm is one of the most important aim of the today’s geodetic community. The gravimetric geoid modeling is the most preferred technique in order to reach to this target. Nevertheless, the gravity values measured on the physical surface of the Earth can not be directly included to this process. First of all, surface gravity values should be reduced to gravity anomalies and, during this step they should not lose their topographic features of the Earth's surface where the data is collected. On the other hand, in order to generate gravity anomalies on a regular grid, it is expected that dependency of gravity anomalies (to be used as reference data in interpolation) to local topography should be minimum. While free-air anomalies are the basic data source for determining the geoid, Bouguer anomalies describing the smoother Earth’s shape, are convenient to the interpolation of gravity anomalies. Therefore, the dependence of Bouguer gravity anomalies, which is a main data in the interpolation procedure, should be reduced to minimum. Although the simple Bouguer anomalies are preferred in practice, it is known that they contain more or less the negative effects of irregular topography. In this study, the differences between simple and complete Bouguer anomalies are presented in a test area. It is seen that the differences between free-air anomalies derived from both approximations are numerically increased up to 16 mGal, correlated with the topography. This result indicates that complete Bouguer anomalies should be used in interpolation process in regions posing irregular topography, such as Turkey.

EXPERIMENTS WITH TWO DIFFERENT APPROACHES TO GRIDDING TERRESTRIAL GRAVITY ANOMALIES AND THEIR EFFECT ON REGIONAL GEOID COMPUTATION

This paper compares the gridding of two types of terrestrial gravity anomaly prior to the computation of regional gravimetric geoid models over Australia. The aim is to investigate the effects of high-frequency components (by way of the terrain correction) on the resulting grid of mean gravity anomalies, and hence the geoid. The gravity anomaly types investigated comprise simple Bouguer anomalies and refined Bouguer anomalies, both computed using a constant topographic mass density. Irrespective of which anomaly type is used for gridding, the relevant additional correction terms are applied to yield an approximation of the mean Helmert anomaly. Regional gravimetric geoid models are then computed over Australia and compared with one another and with GPS-levelling points on the Australian Height Datum. This shows that the application of terrain corrections before or after gravity gridding has only a relatively small effect on the computed geoid in Australia.

EXPERIMENTS WITH TWO DIFFERENT APPROACHES TO GRIDDING TERRESTRIAL GRAVITY ANOMALIES AND THEIR EFFECT ON REGIONAL GEOID COMPUTATION

This paper compares the gridding of two types of terrestrial gravity anomaly prior to the computation of regional gravimetric geoid models over Australia. The aim is to investigate the effects of high-frequency components (by way of the terrain correction) on the resulting grid of mean gravity anomalies, and hence the geoid. The gravity anomaly types investigated comprise simple Bouguer anomalies and refined Bouguer anomalies, both computed using a constant topographic mass density. Irrespective of which anomaly type is used for gridding, the relevant additional correction terms are applied to yield an approximation of the mean Helmert anomaly. Regional gravimetric geoid models are then computed over Australia and compared with one another and with GPS-levelling points on the Australian Height Datum. This shows that the application of terrain corrections before or after gravity gridding has only a relatively small effect on the computed geoid in Australia.

Interpretation of Subsurface Structure Using Gravity Data from Region of Appalachian Ultradeep Hole: ABSTRACT

More than 5000 new gravity measurements have been made over an area of 7500 km/sup 2/ in the region of the proposed ultradeep core hole in the southern Appalachians (ADCOH). Data were obtained mostly at surveyed elevations at approximately 0.5-km intervals along highways, and county and US Forest Service roads in the study area. The data have been reduced to simple Bouguer anomaly values using a standard crustal density of 2.67 g/cc and the International Gravity Formula of 1967. Simple Bouguer anomaly values range from about -10 to -80 mgal within the study area. Profiles crossing the outcrop of the Brevard fault zone (BFZ) at different locations reveal a characteristic gravity signature, with a decrease in gravity of more than 1.5 mgal. A profile along the route of ADCOH seismic line 3, located along US Highway 64 west of Hayesville, North Carolina, crosses a gravity high with an amplitude of more than 50 mgal. A contour map of simple Bouguer anomaly values reveals a linear gravity high of about 4 mgal, trending about N10/sup 0/E, that crosses ADCOH seismic line 1 northwest of Westminster, South Carolina, and seismic line 3 at its intersection with the BFZ. This anomaly correlates withmore » a family of basement faults having more than 0.5 km of throw, seen on both seismic lines 1 and 3, and is evidence that these faults are oblique to both the trend of surface structure and the direction of the seismic profiles. Low gravity values are observed near the Shooting Creek window and Tallulah Falls dome, indicating that these structures may be cored by low-density, possibly sedimentary rocks.« less

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

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.

Application of the Gravity Survey Method to Watershed Hydrologya

ABSTRACTA gravity survey covering 250 square miles and including the Walnut Gulch watershed at Tombstone, Arizona has been conducted. The watershed is one of the experimental areas of the Southwest Watershed Research Center, Agricultural Research Service in Tucson, Arizona.The gravity survey method was selected as the geophysical method that would give a regional picture of the subsurface geology before making extensive surveys by the seismic. refraction method. Basically, the gravity survey method detects and measures variations in the earth's gravitational force. These variations are associated with changes in rock and alluvium density near the surface. Many geologic structures of interest in watershed ground‐water hydrology cause disturbances in the normal density distribution which give rise to anomalies.The geology of the watershed is typical of Basin and Range province and contains deep basin fill deposits surrounded on the east, south, and southwest by igneous intrusives, volcanics, and sedimentary rocks. Concealed border faults may have a decided effect on the hydrology of the watershed.Four base stations were established and 360 gravity stations occupied by a Worden Educator* gravimeter loaned from the Geophysics Laboratory of the University of Arizona. Bench marks and large scale topographic maps served as control. Raw field data were reduced to the simple Bouguer anomaly values through the use of a computer program.The gravity survey reveals a 16‐mgal. gravity low over the east‐central portion of the watershed. This low is interpreted as indicating that about 3200 feet of low density alluvium underlies the area at this point. Gravity highs are associated with mountain ranges and igneous plugs where the more dense rocks occur.

Interpolation and integration of the normal free air anomalies

If we want to compute the height anomalies and the deflections of the vertical for a point where there are height differences exceeding one kilometer in the neighborhood, a very dense net of gravity values must be observed, even when the topographic corrections are used. In smoother regions, the simple Bouguer anomalies with a moderate spacing and with the estimation of the mean heights give reliable results without further reductions.

Gravity observations at sea in USS Corsair

In 1952, gravity observations at sea were made in USS Corsair (SS 435) with the Vening Meinesz pendulum apparatus at 31 positions from New London, Connecticut, to Nassau, Bahama Islands. The principal facts for these observations, including free‐air and simple Bouguer anomalies, are presented in this paper. Isostatic corrections are being computed by the Mapping and Charting Research Laboratory, Ohio State University, under the direction of W. A. Heiskanen.

Gravity observations at sea in USS Conger, Cruise III

Principal facts, including free‐air and simple Bouguer anomalies, are presented for 60 gravity observations at sea. A Vening Meinesz pendulum apparatus was used during the gravity measuring cruise of USS Conger (SS477), Cruise III.Excluding anomalies associated'with the Puerto Rico Trench and the continental margin, free‐air anomalies observed on a line between San Juan, P. R., and New London, Conn., are −10 to −50 mgal. If the standard oceanic crustal section of Worzel and Shurbet is adopted as a reference, these anomalies can be explained by variations of the sediment or, the crustal rock thicknesses of not more than 1 km from normal oceanic thicknesses, or both.

Gravity observations at sea in USS Diablo

Gravity observations at sea were made in 1953 with the Vening meinesz pendulum apparatus at 46 positions near Puerto Rico during a cruise of USS Diablo (SS 479). Geological interpretations of free-air gravity anomalies for these observations have been presented byWorzel, andEwing13. The principal facts, including free-air and simple Bouguer anomalies, are presented in this paper. Isostatic corrections are being computed by the Mapping and Charting Research Laboratory, Ohio State University, under the direction ofProfW. A. Heiskanen.