Gravity Fault Subsidence and Beach Ridges Progradation in Quinta-Cassino (RS) Coastal Plain, Brazil

Ground penetrating radar (GPR) surveys have being applied to investigate very near-surface stratification of sedimentary units in coastal plains and to define their depositional conditions. This paper presents, however, low-frequency GPR survey to investigate fault-related depositional systems at greater depths. The Quinta-Cassino area in the Rio Grande do Sul Coastal Plain (RGSCP, Brazil) shows a wide strandplain that is made off by very long, continuous, and linear geomorphic features (beach ridges). This strandplain extends for ~70 km southward. The beach ridges show low-angle truncations against the Quinta escarpment, and also truncations in the strandplain. The traditional approach points that RGSCP was developed by juxtaposition of four lagoons/barrier systems as consequence of sea level changes;previous model assumes that no deformational episode occurred in RGSCP. The geophysical and geological surveys carried out in this area showed the existence of listric fault controlling the beach ridges in the escarpments and hanging-wall blocks. The radargrams could distinguish Pleistocene basement unit anticlockwise rotation, thickening of beach ridges radarfacies close to listric normal faults, and horst structures. These deformational features indicate that the extensional zone of a large-scale gravity-driven structure controlled the mechanical subsidence, the Holocene sedimentation and its stratigraphic and geomorphic features in the Quinta-Cassino area to build up an asymmetric delta. The results point to a new approach in dealing with RGSCP Holocene evolution.

Regional gravity survey and application in oil and gas exploration in China

Ground gravity survey for regional structure unit delineation and oil and gas exploration in China is addressed in this paper with examples. Gravity survey scales, coverage, technical regulations and achievements at the national level are introduced, including data processing and anomaly interpretation techniques. Bouguer anomalies of terrestrial territories of China and classification of anomalous feature zones are also described;they are well correlated with lithotectonical boundaries, fault zones, and unexposed igneous plutons. Recent study results of petroliferous sedimentary basins are presented as well, including concealed boundaries and major structures of large basins. It is concluded that gravity survey is the most effective and economic method in unveiling unexposed and deep-seated structures, targeting and delineating oil and gas-bearing sedimentary basins, and locating main trap structures within prolific basins in early stage of exploration in China. Suggestions for improving exploration of both conventional and unconventional oil and gas reservoirs in China are also given in the paper.

Gravity Survey on the Mizuho Plateau, East Antarctica: SEAL Seismic Exploration in 2001-2002

A gravity survey on Mizuho Plateau was carried out in austral summer of 2001-2002 by the 43rd Japanese Antarctic Research Expedition (JARE-43), as one of the geophysical measurements of deep seismic exploration. Gravity measurements were conducted at 151 points in approximately 1 km interval along the 150 km profile. Considering the gravity effect from the ice-sheet, Free-air and Bouguer anomalies were calculated by using precise GPS locations. Furrowed and basin-like negative Free-air anomalies were observed in middle and northern parts of the profile. Bouguer anomalies were calculated by two bedrock elevation data obtained from seismic refraction and radio-echo sounding. High resolution model for bedrock elevation by radio-echo sounding was applied in determining the crustal thickness.

Gravity Survey along the Mizuho Traverse Routes, East Antarctica: SEAL Seismic Exploration in 1999-2000

A gravity survey was conducted along the Mizuho traverse routes in austral summer of 1999-2000 by the 41st Japanese Antarctic Research Expedition (JARE-41). The main purpose of the survey was to obtain detail gravity anomaly along the traverse routes, where deep seismic exploration was simultaneously conducted in order to obtain a fine crustal structure of the Mizuho Plateau. By using SCINTREX (CG-3M) gravity meter, the survey was carried out at 160 sites with about 1 km interval in a distance of 190 km from S16 to Z20 on the traverse routes. Free-air and Bouguer anomalies were calculated using precise locations by GPS measurements, by taking into account an effect of the thick icesheet. The furrowed negative Free-air anomalies are identified around H192, where middle points of the whole traverse routes. Two bedrock elevation models derived both by seismic refraction analyses and radio-echo sounding might correspond to the bottom and the top of a mixture layer composed from ice-moraine rocks over the bedrock surface. Thickness of the crust estimated by Bouguer anomalies is 0.5 - 1 km larger toward inland at the terminal point of Z20 along the profile.

Characteristics of gravity anomalies and prediction of volcanosedimentary boron deposit distribution in the Xiongba area,Tibet

Volcanosedimentary boron deposits are present within Tertiary lacustrine sediments and volcanic rocks in Xiongba, Tibet. Boron deposits are characterized by low density relative to country rocks; thus, it is possible to locate them by gravity measurements. We conducted a 1：50000 high-precision gravity survey in the Xiongba area, Tibet, and obtained the Bouguer and residual gravity anomalies. We analyzed fault systems and the distribution of sedimentary and volcanic rocks and their relation to the volcanosedimentary boron deposits. The processing of the gravity data revealed local gravity variations and fault structures. We applied preferential downward continuation and wavelet transform to the gravity data, and in conjunction with geological data, we predicted the distribution of volcanosedimentary boron deposits.

Spherical External Gravity Correction

In mountainous area, gravity survey external corrections, including terrain and Bouguer corrections, are the prerequisites for gravity data processing and interpretation. This paper presents a suite of techniques for spherical terrain and slab corrections for a finite spherical shell based on the analysis of conventional external corrections and deduces an equation for spherical terrain correction. The processing of gravity data acquired in the ZJJ area, South China shows that this technique significantly improves the accuracy of terrain corrections, and the quality of the data reaches or approaches the data acquired on plains.

High-precision gravity correction methods for loess plateau areas, a case study

The loess plateau is featured by a sharply undulated and remarkably incised surface. The sharp surface undulation means great variation of loess thickness. Since the loess layer exhibits significant density contrast with the underlying formation, a high frequency gravity anomaly with a certain magnitude will appear. The high frequency gravity anomaly can result in some difficulties for processing and interpreting the gravity data, thereby reducing the usefulness of gravity surveys in loess plateau areas. We carried out a high precision gravity survey in the SHJZ structural zone. During data processing, we applied terrain and loess corrections to effectively remove or suppress the gravity effect resulting from surface undulation and variation of loess thickness. We obtained high precision gravity data which matches well with other geophysical data. The comprehensive interpretation based on the final gravity data help to study local structure integrated with other published geophysical data.

Structural characteristics of the KPR-CBR triple-junction inferred from gravity and magnetic interpretations,Philippine Sea Plate

The intersection of the Kyushu-Palau Ridge(KPR)and the Central Basin Rift(CBR)of the West Philippine Basin(WPB)is a relic of a trench-trench-rift(TTR)type triple-junction,which preserves some pivotal information on the cessation of the seafloor spreading of the WPB,the emplacement and disintegration of the proto-Izu-Bonin-Mariana(IBM)Arc,and the transition from initial rifting to steady-state spreading of the Parece Vela Basin(PVB).However,the structural characteristics of this triple-junction have not been thoroughly understood.In this paper,using the newly acquired multi-beam bathymetric,gravity,and magnetic data obtained by the Qingdao Institute of Marine Geology,China Geological Survey,the authors depict the topographic,gravity,and magnetic characteristics of the triple-junction and adjacent region.Calculations including the upward continuations and total horizontal derivatives of gravity anomaly are also performed to highlight the major structural features and discontinuities.Based on these works,the morphological and structural features and their formation mechanisms are analyzed.The results show that the last episode amagmatic extension along the CBR led to the formation of a deep rift valley,which extends eastward and incised the KPR.The morphological and structural fabrics of the KPR near and to the south of the triple-junction are consistent with those of the western PVB,manifesting as a series of NNE-SSW-and N-S-trending ridges and troughs,which were produced by the extensional faults associated with the initial rifting of the PVB.The superposition of the above two reasons induced the prominent discontinuity of the KPR in deep and shallow crustal structures between 15°N‒15°30′N and 13°30′N‒14°N.Combined with previous authors’results,we propose that the stress produced by the early spreading of the PVB transmitted westward and promoted the final stage amagmatic extension of the CBR.The eastward propagation of the CBR destroyed the KPR,of which the magmatism had decayed or ceased at that time.The destruction mechanism of the KPR associated with the rifting of the PVB varies along strike the KPR.Adjacent to the triple-junction,the KPR was destroyed mainly due to the oblique intersection of the PVB rifting center.Whereas south of the triple-junction,the KPR was destroyed by the E-W-directional extensional faulting on its whole width.

Gravity and magnetic field characteristics and regional ore prospecting of the Yili ancient continent, West Tianshan, Xinjiang Uygur Autonomous Region, China

Gold,iron,copper,lead-zinc and other mineral exploration in West Tianshan,Xinjiang Uygur Autonomous Region,has made remarkable progress in recent years.However,due to the dispute on the tectonic division of West Tianshan,the ore-controlling factors and the regional metallogenic laws are controversial.The authors analyze regional gravity data and notice that the high-value region corresponds to the Yili ancient continent,thus the southeastern boundary of the Yili ancient continent is delineated.Comparative analysis of gravity,aeromagnetic and geologic data reveals that the Tulasu basin,where some medium to large epithermal gold deposits locate,lies above the Yili ancient continent;the Yili Carboniferous-Permian rift extends in E-W direction,numbers of copper deposits have been found in the mid-west section of the rift which lies above the Yili ancient continent,whereas few copper deposits have been discovered in the east section which is outside the Yili ancient continent.Accordingly,the Yili ancient continent may be rich in gold,copper and other metal elements;the metal-bearing hydrothermal solution moves up with the activity of magmatism,and deposits in the favorable places(the Tulasu basin and the Yili Carboniferous-Permian rift),forming numerous small and medium gold,copper deposits,as well as some large and super-large gold deposits.Therefore,the tectonic-magmatic hydrothermal zone above the Yili ancient continent should be the prospective area for epithermal gold and copper polymetallic deposits.

Multi-component joint inversion of gravity gradient based on fast forward calculation

With the development of gravity gradient full tensor measurement technique,three-dimensional( 3D) inversion based on gravity gradient tensor can provide more accurate information. But the forward calculation of 3D full tensor sensitivity matrix is very time-consuming,which restricts its development and application.According to the symmetry of the kernel function,the authors reconstruct the underground source of geological body to avoid repeat computation of the same value,and work out the corresponding relationship between the response of geological body to the observation point and the response of reconstructed geological body to the observation point. According to the relationship,rapid calculation of full tensor gravity sensitivity matrix can be achieved. The model calculation shows that this method can increase the speed of 30-45 times compared with the traditional calculation method. The sensitivity matrix is applied to the multi-component inversion of gravity gradient. The application of this method on the measured data provides the basis for the promotion of the method.

Characteristics of gravity and magnetic fields and deep structural responses in the southern part of the Kyushu-Palau Ridge

The southern part of the Kyushu-Palau Ridge(KPR)is located at the conjunction of the West Philippine Basin,the Parece Vela Basin,the Palau Basin,and the Caroline Basin.This area has extremely complex structures and is critical for the research on the tectonic evolution of marginal seas in the Western Pacific Ocean.However,only few studies have been completed on the southern part,and the geophysical fields and deep structures in this part are not well understood.Given this,this study finely depicts the characteristics of the gravity and magnetic anomalies and extracts information on deep structures in the southern part of the KPR based on the gravity and magnetic data obtained from the 11th expedition of the deep-sea geological survey of the Western Pacific Ocean conducted by the Guangzhou Marine Geological Survey,China Geological Survey using the R/V Haiyangdizhi 6.Furthermore,with the data collected on the water depth,sediment thickness,and multichannel seismic transects as constraints,a 3D density model and Moho depths of the study area were obtained using 3D density inversion.The results are as follows.(1)The gravity and magnetic anomalies in the study area show distinct zoning and segmentation.In detail,the gravity and magnetic anomalies to the south of 11°N of the KPR transition from high-amplitude continuous linear positive anomalies into low-amplitude intermittent linear positive anomalies.In contrast,the gravity and magnetic anomalies to the north of 11°N of the KPR are discontinuous and show alternating positive and negative anomalies.These anomalies can be divided into four sections,of which the separation points correspond well to the locations of deep faults,thus,revealing different field-source attributes and tectonic genesis of the KPR.(2)The Moho depth in the basins in the study area is 6-12 km.The Moho depth in the southern part of KPR show segmentation.Specifically,the depth is 10‒12 km to the north of 11°N,12‒14 km from 9.5°N to 11°N,14-16 km from 8.5°N to 9.5°N,and 16‒25 km in the Palau Islands.(3)The KPR is a remnant intra-oceanic arc with the oceanic-crust basement.which shows noticeably discontinuous from north to south in geological structure and is intersected by NEE-trending lithospheric-scale deep faults.With large and deep faults F3 and F1(the Mindanao fault)as boundaries overall,the southern part of the KPR can be divided into three zones.In detail,the portion to the south of 8.5°N(F3)is a tectonically active zone,the KPR portion between 8.5°N and 11°N is a tectonically active transition zone,and the portion to the north of 11°N is a tectonically inactive zone.(4)The oceanic crust in the KPR is slightly thicker than that in the basins on both sides of the ridge,and it is inferred that the KPR formed from the thickening of the oceanic crust induced by the upwelling of deep magma in the process of rifting of remnant arcs during the Middle Oligocene.In addition,it is inferred that the thick oceanic crust under the Palau Islands is related to the constant upwelling of deep magma induced by the continuous northwestward subduction of the Caroline Plate toward the Palau Trench since the Late Oligocene.This study provides a scientific basis for systematically understanding the crustal attributes,deep structures,and evolution of the KPR.

A marine gravimeter based on electromagnetic damping and its tests in the South China Sea

A new gravity sensor based on electromagnetic damping for the JMGrav marine gravimeter is presented.The new gravity sensor considered the advanced construction methods of the electromagnetic damping system of the gravimeter.The design features of the new system are discussed and the research survey data in the South China Sea are shown.Numerical simulations are applied to model the magnetic and mechanical characteristics of the system using finite element analysis and to evaluate the force distribution and the resulting damping effects.The performance characteristics of the system were tested on a motion simulator in laboratory,and the gravimeter was subjected to vertical accelerations of up to 100 Gal in 1-1000 s.It was found that the amplitude reduction of vertical accelerations in 3-15 s is 30-45 dB,with a time lag of 2-5 s,while the effect on gravity in period greater than 600 s is less than 0.5 dB,with a time lag of less than 100 s.The accelerations cause discrepancies of approximately only 1 mGal between the static value and the mean dynamic value.The sea tests were conducted in September 2020.Gravity measurements were taken with a JMGrav marine gravimeter onboard the R/V Dongfanghong 3,and the effective survey line exceeded 2000 km.Completely irregular accelerations with peaks up to 100 Gal yielded a reduction of approximately 40 dB in amplitude.The survey data were evaluated using ocean gravity field models and grid line tests.The results show that the accuracy of the gravity measurements is better than 2 mGal.

Development of Integrated and Intelligent Geodetic and Photogrammetry Satellites with Corresponding Key Technologies

Aerospace surveying and mapping has become the main method of global earth observation.It can be divided into the geodetic observation satellites and the topographic surveying satellites according to the disciplines.In this paper,the geodetic satellites and photographic satellites are introduced respectively.Then,the existing problems in Chinese earth observation satellites are analyzed,and the comprehensive satellite with integrated payloads,the intensive microsatellite constellation and the intelligent observation satellite are proposed as three different development ideas for the future earth observation satellites.The possibility of the three ideas is discussed in detail,as well as the related key technologies.

Review:Progress in SQUID⁃Based Geophysical Precision Measurement Technology

Superconducting quantum interference device(SQUID),with the advantages of ultra⁃high sensitivity,low noise,broad frequency bandwidth,and excellent low⁃frequency response,is widely used in several geophysical methods,such as vector magnetic survey,electromagnetic method,gravity and gravity gradient measurement.In this paper,the latest technological progress of SQUID and SQUID⁃based geophysical precision measurement technology are described.In addition,the advantages,characteristics,and existing problems of each measurement technology are analyzed.Combined with the requirements of current geophysical technology,the future application prospect is discussed and development suggestions are given.

Geophysical prospecting of copper-nickel deposits in Beishan rift zone, Xinjiang

The Beishan rift zone in Xinjiang Uygur Autonomous Region was formed due to strong activities of faults on the basement of the Tarim continental crust.Despite the fact that many geological research results of the rift zone have been achieved,only a few studies have been conducted on its regional geophysical characteristics.In this paper,the gravity and magnetic anomalies of the rift zone were highlighted through specific data processing of 1∶50000 high-precision aeromagnetic data and gravity data with a grid spacing of 2 km×2 km.Based on this,the geophysical evidence for the scope and internal structures of the Beishan rift zone was obtained for the first time.The distinct characteristics of magnetic and gravity fields in the areas to the north and south of the Beishan rift zone reveal that deep faults exist between the Beishan rift zone and the geological units on the southern and northern sides.Furthermore,the faults on the two areas contain the bidirectional thrusts and have flower-shaped structures according to the characteristics of the magnetic and gravity fields.The Beishan rift zone can be divided into two tectonomagmatic zones,namely the Zhongposhan-Bijiashan-Cihai-Baishanliang zone(the northern zone)and the Bayiquan-Qixin-Baishan zone(the southern zone).The northern zone can be further subdivided into three comet-shaped anomaly groups(tectonomagmatic areas),while the southern zone can be further subdivided into two tectonomagmatic areas.According to the characteristics of aeromagnetic anomalies and gravity field,19 mafic-ultramafic complexes were delineated.The known Pobei,Hongshishan,and Qixin complexes are all located within the inferred complexes,with estimates of total explored resources of Ni,Cu,and Au of 3×10^(6) t,10×10^(3) t and 10 t,respectively.The prospecting of high-grade copper-nickel deposits should focus on the periphery and deep parts of the known and inferred mafic-ultramafic complexes.Among them,the peripheral strata of the complexes specifically have great prospecting potential of large-scale high-grade copper-nickel deposits of magma injection type.Finally,this paper analyzed the application effects of the rapid airborne-ground-drilling synergetic exploration method in the prospecting of copper-nickel deposits in Qixin,Beishan,Xinjiang,which will provide references for further exploration of copper-nickel deposits in Beishan area,Xinjiang.

PRECISE LEVELING OF THE VERY LONG QINLING MOUNTAIN TUNNEL

The Qinling tunnel with length of 18. 488km is located on the railway line from Xi’ an (Shanxi Province) to Ankan (Sichuan Province) in the middle of Qinling moun- tain. It is the longest double track railway tunnel in China and takes the third place in the world. According to the design, the break-through error in vertical direction caused by the al- timetric control surveying is limited to 18mm for the case of one piercing face. Because the leveling route reaches over 120km in length and must go over two mountains in 2 800m height, the first-order precise leveling and precise gravity measurement should be carried out in the construction stage. In this paper the field leveling approach, the application of new technology,some experience as well as the office calculation with final results analysis are in- troduced. By meticulous planning, organization and observation, the final accuracy of vertical difference between two tunnel end points is only 8mm, and it provides reliable surveying guarantee for this great tunnel engineering. Finally it is pointed out that this vertical differ- ence distinguishes obviously from the primary measurement result as high as 114mm. It means that the primary result is not accurate enough.

Deep structural research of the South China Sea: Progresses and directions

The South China Sea(SCS)is the hotspot of geological scientific research and nature resource exploration and development due to the potential for enormous hydrocarbon resource development and a complex formation and evolution process.The SCS has experienced complex geological processes including continental lithospheric breakup,seafloor spreading and oceanic crust subduction,which leads debates for decades.However,there are still no clear answers regarding to the following aspects:the crustal and Moho structure,the structure of the continent-ocean transition zone,the formation and evolution process and geodynamic mechanism,and deep processes and their coupling relationships with the petroliferous basins in the SCS.Under the guidance of the“Deep-Earth”science and technology innovation strategy of the Ministry of Natural Resources,deep structural and comprehensive geological research are carried out in the SCS.Geophysical investigations such as long array-large volume deep reflection seismic,gravity,magnetism and ocean bottom seismometer are carried out.The authors proposed that joint gravitymagnetic-seismic inversion should be used to obtain deep crustal information in the SCS and construct high resolution deep structural sections in different regions of the SCS.This paper systematically interpreted the formation and evolution of the SCS and explored the coupling relationship between deep structure and evolution of Mesozoic-Cenozoic basins in the SCS.It is of great significance for promoting the geosystem scientific research and resource exploration of the SCS.

CRUSTAL STRUCTURE IN EASTERN REGION OF QINGHAI—TIBET PLATEAU

A geophysical survey including Magnetotelluric sounding(MT), gravity and geomagnetism was carried out in eastern region of Qinghai—Tibet plateau in 1998. It is to be known from tectonic map that study area was divided into four blocks by three large sutures, but we have very little information on deep\|seated structure which have close relationship with evolution of Tibet plateau, our purpose is to find some deep crustal structure evidences of the blocks and sutures for understanding deep tectonic feature in this region. The geophysical survey region is located in about 97～98°E and 35～26°N, the profile with the length of 760km runs through Bayanhar, Qingtang and Gandise block in NS direction, 349 gravity and geomagnetic data were acquired simultaneously, and 31 MT sites are be placed along the profile with about 18km\|interval to detect electrical conductive feature.

Geophysical Data Bearing on Hydrocarbon Traps and Resource Potential of the Galisteo Basin, New Mexico

Land-based gravity and magnetic data were obtained at 56 gravity stations and 250 magnetic stations throughout the northern-central part of the Galisteo Basin south of Santa Fe, NM. The study area extends south from the village of Eldorado to the foothills of the Ortiz Mountains and east from I-25 to Highway 285, an area of approximately 2200 square kilometers. These data offered an approach to integrating surface geological mapping and subsurface geophysical surveys to characterize the distribution, depth, and potential hydrocarbon trap geometries in parts of the Galisteo Basin using the Talwani geophysical modeling program. Both sets of potential field data were integrated into ArcGIS and Surfer to generate extrapolated surfaces and derivative maps, which allowed for the characterization of the subsurface geology along with specific profiles across the mapped area. These data revealed several anomalies within the central portion of the basin that are interpreted as likely hydrocarbon trap-structures.

Preliminary Analysis for Ground Structure around Source Region of 2008 Iwate-Miyagi Nairiku Earthquake

To obtain a detailed model of the three-dimensional ground structure, the microtoremor and gravity surveys were carried out around Kurikoma area, Japan, where is the source reagion of 2008 Iwate-Miyagi nairiku earthquake and is located about 90 km north of Sendai. Using the microtremor data, velocity structures of shallow sediments are estimated and the results at some of the sites are reported preliminarily. We could not find the detailed velocity structure to bedrock, because small arrays for the microtremor observation were applied, However, significant structures are observed for shallow velocity structure at some sites. Furthermore, gravity data provided Bouguer anomaly, which is one of gravity anomalies, around the target area. From the Bouguer anomaly, area with low anomaly is tbund around the south-eastern area of the fault plane.