Ground Gravity Surveys Research Articles

Quantitative depth estimation using analytic signal at low-latitude for ground gravity survey of Gbede, Oyo State

This paper presents the insitu gravity survey of basement complex rock in Southwestern Nigeria. In the E-W direction, LaCoste and Romberg Gravity Meter type G309 was used to carry out a ground gravity survey where ten traverses were established over a distance of 1000 m by 500 m with station spacing of 20m and a traverse interval of 50 m. Observed gravity values were corrected, analyzed and interpreted quantitatively. The corrected bouguer gravity data were presented as bouguer anomaly graphs. Analytic Signal at low-latitude was adopted to compute the depth to source of iron-ore for a contact, a thin sheet (dyke) and a horizontal cylinder. The result revealed a depth range of 5.45 m-8.25 m for a contact, 9.44 m-14.29 m for a thin sheet (dyke) while a depth range of 12.31 m-18.05 m was estimated for a horizontal cylinder respectively. An average depth of 11.81±3.64 m was estimated for the entire area irrespective of the structural model, this was compared with published magnetic results of the study area and a small disparity of potential field measurements was recorded. The overall computed results signified the existence of iron mineral deposits at low depths across the study area.

Magnetic and gravity investigation of kimberlites in north-central Botswana

The Orapa kimberlite field of Botswana is one of the world’s major diamond producing regions. Within this field, there are several small kimberlite pipes that have not been completely explored in terms of their lateral extent, depth, and diamond potential. Two such pipes, BK54 and BK55, were found during a ground gravity and magnetic survey, and subsequent drilling confirmed the presence of kimberlite material. To determine the physical properties of these pipes, their lateral extent, depth, and thickness were estimated using a gravity and magnetic analysis and 2.5D and 3D modeling. Tilt derivatives of the magnetic data indicated that BK54 has a northwest-trending elliptical shape and BK55 has a roughly circular shape. Residual gravity anomaly maps indicate that BK54 does not have a density anomaly whereas BK55 is associated with a gravity maximum. The 3D gravity and magnetic inversion modeling constrained by magnetic susceptibility measurements indicates that BK54 is smaller in volume than BK55 and that neither pipe is thicker than 125 m. The difference in shape and the lack of a gravity anomaly over BK54 implies a different formation mechanism for each kimberlite pipe. Although several mechanisms are suggested, BK54 may have formed by a more explosive eruption producing more tuffistic material in the crater and diatreme facies. The gravity and magnetic analysis also found that the kimberlite pipes, while small, are larger in extent than was determined by drilling and warrant additional drilling to determine their economic potential.

Mapping New IOCG Mineral Systems in Brazil: The Vale do Curaçá and Riacho do Pontal Copper Districts

The Vale do Curaçá and Riacho do Pontal copper districts are located within the northern part of the Archaean São Francisco Craton and represent two pulses of mineralization. The copper districts have been identified as Iron-Oxide-Copper-Gold (IOCG) classes of deposits. An older metallogenic event associated with the Caraíba copper deposit, which is located in the Vale do Curaçá district, is related to Palaeoproterozoic (ca. 2 to 2.2 Ga) hydrothermal processes. A younger Neoproterozoic (ca. 750 to 570 Ma) episode of volcanism and associated plutonism is represented by the Riacho do Pontal mineral district. Seismic tomography data from across east-central Brazil show that the multiage Carajás province and Vale do Curaçá and Riacho do Pontal copper districts sit along either side of a prominent NW-trending upper lithospheric high-velocity zone. The edges of the high-velocity zone point to long-lived subparallel transcrustal structures that have been the focus of multiple reactivations and copper mineralization events. Regional gravity and magnetic maps show that the Vale do Curaçá copper district extends over an area greater than 110 km by 22 km. The magnetic and gravity values show significant variations correlated with this area. The district includes high gravity values associated with the Caraíba copper mine (>−35 mGal), which has a greater density (3.13 g/cm3) than the nonmineralized host rock density (2.98 g/cm3). The gravity anomaly signature over the Riacho do Pontal copper district is characterized by a 40-km long NW–SE trending Bouguer gravity low. The Ria4 occurrences of the Riacho do Pontal copper district are situated in these regional low-gravity domains. Data from regional airborne magnetic and ground gravity surveys were inverted to obtain a 3D magnetic susceptibility and density model, respectively, for the known districts. The results show that the Caraíba deposit is characterized by a both dense and magnetic source showing structural control by thrust shear zones. The 2D and 3D geological models show two main NNW prospective trends. Trends I and II have a sigmoidal shear shape and are positioned in the contact zone between domains with high magnetic susceptibility (SI > 0.005) and density > 0 g/cm3). Trend I is 40 km × 10 km in size and hosts the Caraíba, Surubim, and Vermelho copper mines and other minor deposits. The results obtained from the 3D magnetic inversion model for the region of the Riacho do Pontal district show weak magnetic anomaly highs extending along a NW–SE magnetic gradient trend. The gradient is related to mapped shear zones that overprint older and deeper NE–SW features of the São Francisco cratonic root. The area includes high gravity values associated with the Caraíba copper deposit, which has a greater density (3.13 g/cm3) than the nonmineralized host rock density (2.4 g/cm3).

Quantitative Interpretation of Gravity Anomalies in the Kribi-Campo Sedimentary Basin (South Cameroon) Based on Spectral Analysis and 2.5D Modelling: Structural Implications

Ground gravity survey was recently carried out in the Lolabe-Campo area, which constitutes the southern onshore portion of the Kribi-Campo sub-basin in South Cameroon. The obtained gravity data were processed and interpreted in order to elucidate the subsurface geology of the area in which preliminary studies highlighted the presence of an intrusive igneous body. In order to determine the nature and the characteristics of this body, spectral analysis, ideal body solution and 2.5D modelling methods were applied to the gravity data. The results suggest that the emplacement, breaking up and separation of the intrusive igneous body were the outcome of two tectonic events corresponding to an upward vertical translational tectonic movement followed by an anticlockwise rotational faulting tectonic movement. From a density contrast of 0.13 g&#183cm&#453, the two isolated bodies have an estimated density of 2.80 g&#183cm&#453 which is comparatively higher than the average density of 2.67 g&#183cm&#453 of the autochthonous rocks. These two blocks are completely surrounded by both sedimentary and metamorphic formations and are confined between the depths of 0.6 km and 3.5 km. The highly dense nature of the rocks and their location around the Atlantic Ocean led to the conclusion that the two intrusive bodies are composed principally of gabbro. The disposition and shape of the open zone resulting from the separation of the blocks are suggestive of a trap for the accumulation of hydrocarbons and mineral resources.

First horizontal derivative and Euler Deconvolution in application for reconstructing structural signature over the Blawan-Ijen Geothermal area

A ground gravity survey was conducted over the Blawan-Ijen geothermal area covering about 176 km2 area. A total of 72 stations were measured with 1.0 – 1.5 km spacing. The data obtained were then processed using several corrections for gravity data to get Complete Bouguer Anomaly (CBA). The gravity method was chosen because of the ability to detect geological structures corresponding to the rocks density of the field study. Two types of gravity technique, First Horizontal Derivative (FHD) and Euler Deconvolution (ED), have become widely used to interpret the structure condition. FHD technique delineates a geological boundary of different density body while ED technique estimates a source location and depth of structures. Furthermore, 3-D inversion of magetotelluric data result was also used as a reference to support 2-D gravity forward modeling. The main structure which probably controlled the emergence of Blawan-Ijen hotspring is indicated by maximum FHD value at the northern part area. Moreover, the ED result shows the depth of structures in Blawan-Ijen geothermal area is mostly about 250 – 1000 meters.

Airborne gravimetry takes off in the Western Australia ‘Generation 2’ reconnaissance gravity mapping project

In 1974, the Australian Bureau of Mineral Resources, Geology and Geophysics completed a 15-year systematic reconnaissance gravity survey of Australia with stations spaced at 11 km. The 1976 Gravity Map of Australia was a seminal product; half a century later, the data still provide the only coverage for substantial parts of the continent.In 2005, the Geological Survey of Western Australia, supported by Geoscience Australia, commenced a program of regional ground gravity surveys with 2.5 km station spacing, a sixteen-fold improvement of resolution over the ‘first generation’ BMR data. In 2013, GSWA declared its aim of completing ‘second-generation’ reconnaissance gravity coverage of WA by 2020.In 2016, with 45% of the State yet to be surveyed in the north and east, and ground access issues slowing progress and making uniform coverage increasingly difficult, GSWA and GA undertook the first government-commissioned regional aerogravity survey in Australia, using the Sander Geophysics AIRGrav system. The 38,000 line-km survey covering 84,000 km2 in the East Kimberley region was flown at 2.5 km line-spacing for compatible spatial resolution with GSWA’s regional ground surveys.We compare airborne with ground gravimetry in the context of the East Kimberley project and conclude that, for reconnaissance surveys: aerogravity costs now approach those of ground surveys; spatial resolution is equivalent; data precision is not a critical factor; and airborne and ground data can be merged seamlessly for interpretation.Consequently, new aerogravity surveys were commissioned over 264,000 km2 of northern WA in the Tanami, northeast Canning and Kidson regions.

Evaluating the groundwater resource potential of the Dundas buried bedrock valley, southwestern Ontario: an integrated geological and hydrogeological case study

Population growth in the groundwater-dependent municipalities of southwestern Ontario has prompted interest in the exploration for new, previously untapped, groundwater resources. In this study, the groundwater resource potential of the sediments infilling a deeply buried bedrock valley network centred beneath the Region of Waterloo and the counties of Brant and Hamilton–Wentworth are explored. The objectives of this study are to further refine valley location and geometry, understand infilling sediments and their hydrogeological properties, and characterize waters contained within the aquifers to inform future water management decisions. Results of a regional ground gravity survey were instrumental in locating buried bedrock valleys and guided follow-up drilling. Continuous sediment coring and monitoring well installations were completed to target thick and coarse-grained sediment packages that, based on existing borehole data, showed aquifer potential. Hydraulic testing and groundwater sampling results provided valuable insights into groundwater quantity and quality. Highly transmissive aquifers, some worth investigating further, have been identified within portions of the valley network. The aquifers appear to occur at a number of stratigraphic positions and do not necessarily occur as the deepest unit overlying bedrock. Bedrock topography likely played a role, however, in their preferential preservation. They are commonly overlain by thick sequences of relatively impermeable sediments, providing excellent protection from anthropogenic contamination. Information from water chemistry, however, does suggest hydraulic connection to the surface at some locations. Groundwater quality and quantity information combined with a conceptual three-dimensional geologic model aids in the selection of groundwater resource exploration targets within the untapped resources of the deep, Dundas buried valley sediments.

High resolution magnetic, regional gravity and petrophysical characterization of the Dominican Republic tectonic domains with special focus on the Central Cordillera

The aim of this study is twofold: to make a geological interpretation of available ground gravity and high resolution airborne magnetic surveys carried out over the Dominican Republic, and to define the spatial limits of key tectonic units of the Central Cordillera. Potential fields and petrophysical data allow the establishment of the spatial limits and the relationships of the key tectonic units at local and regional scale. Therefore, gravity and magnetic data have been integrated to produce models of the upper crustal structure across the Central Cordillera following corresponding geological cross-sections that have been extended in depth. Since there are no seismic or borehole data to constrain the models in our study area, modeling is based on potential field data and the analysis of the physical properties that have been measured from surface rocks (586 density and magnetic susceptibility samples). The main features of the potential fields maps are: (1) NW-SE-trending anomalies delineating the main tectonic units, which coincide with the major strike-slip fault zones that limit those units; (2) exceptional geophysical mapping examples of volcano-sedimentary units; and (3) significant anomalies due to a magmatism widely spread all over Hispaniola Island. The modeling results reveal the petrophysical heterogeneities and subsurface geometry of the tectonic units, faults, and shear zones. The model shows that Tireo Fm extends up to 7 km in depth. The Duarte Complex has been modeled as a layer of c. 5-7 km in thickness beneath the Tireo Fm and the S. Juan Basin. Some of the intrusive bodies may reach depths of c. 8 km.

Application of 3D Euler deconvolution and improved tilt angle to the aeromagnetic data of In Ouzzal terrane, western Hoggar, Algeria

The study area is located in the western Hoggar Shield (southern Algeria). It includes the In Ouzzal terrane, which consists of Archaean metamorphic rocks. By contrast to other rocks of the Hoggar Shield, the In Ouzzal terrane represents an exception of being neither deformed nor metamorphosed during the Pan-African event, remaining as a rigid block since 2 Ga. Although, previous geophysical works in the area include an airborne magnetometer and gamma-ray spectrometric survey as well as ground gravity and magnetotelluric survey structurally, the study area has not been very well understood. In this paper, we present the interpretation results of the airborne magnetic data by using the 3D Euler deconvolution and the improved Tilt-angle methods. These results reveal the existing of fault systems (FS) occurring within the center of the study area and along the latitude of 22°; the results also suggested that the deepest fault system is oriented NE–SW and is represented by parallel major faults splitting the In Ouzzal terrane into two different parts: northern and southern. The northern part moved northwards, whereas the southern part moved southwards colliding with the Iforas unit. The interpretation confirms that the In Ouzzal terrane and the surrounding Pan-African structures are bound two by two sub-vertical lithospheric faults with the existence of dextral and sinistral faults in the west and east of the terrane, respectively.

Review paper: Exploration geophysics for intrusion‐hosted rare metals

ABSTRACTIgneous intrusions, notably carbonatitic–alkalic intrusions, peralkaline intrusions, and pegmatites, represent significant sources of rare‐earth metals. Geophysical exploration for and of such intrusions has met with considerable success. Examples of the application of the gravity, magnetic, and radiometric methods in the search for rare metals are presented and described. Ground gravity surveys defining small positive gravity anomalies helped outline the shape and depth of the Nechalacho (formerly Lake) deposit within the Blatchford Lake alkaline complex, Northwest Territories, and of spodumene‐rich mineralization associated with the Tanco deposit, Manitoba, within the hosting Tanco pegmatite. Based on density considerations, the bastnaesite‐bearing main ore body within the Mountain Pass carbonatite, California, should produce a gravity high similar in amplitude to those associated with the Nechalacho and Tanco deposits. Gravity also has utility in modelling hosting carbonatite intrusions, such as the Mount Weld intrusion, Western Australia, and Elk Creek intrusion, Nebraska.The magnetic method is probably the most successful geophysical technique for locating carbonatitic–alkalic host intrusions, which are typically characterized by intense positive, circular to sub‐circular, crescentic, or annular anomalies. Intrusions found by this technique include the Mount Weld carbonatite and the Misery Lake alkali complex, Quebec. Two potential carbonatitic–alkalic intrusions are proposed in the Grenville Province of Eastern Quebec, where application of an automatic technique to locate circular magnetic anomalies identified several examples. Two in particular displayed strong similarities in magnetic pattern to anomalies accompanying known carbonatitic or alkalic intrusions hosting rare‐metal mineralization and are proposed to have a similar origin.Discovery of carbonatitic–alkalic hosts of rare metals has also been achieved by the radiometric method. The Thor Lake group of rare‐earth metal deposits, which includes the Nechalacho deposit, were found by follow‐up investigations of strong equivalent thorium and uranium peaks defined by an airborne survey. Prominent linear radiometric anomalies associated with glacial till in the Canadian Shield have provided vectors based on ice flow directions to source intrusions. The Allan Lake carbonatite in the Grenville Province of Ontario is one such intrusion found by this method. Although not discovered by its radiometric characteristics, the Strange Lake alkali intrusion on the Quebec–Labrador border is associated with prominent linear thorium and uranium anomalies extending at least 50 km down ice from the intrusion. Radiometric exploration of rare metals hosted by pegmatites is evaluated through examination of radiometric signatures of peraluminous pegmatitic granites in the area of the Tanco pegmatite.

Impact Craters in South America, by Acevedo R. D., Rocca M. C. L., Ponce J. F., and Stinco S. G. Heidelberg: Springer, 2015. 104 p. SpringerBriefs in Earth Sciences: South America and the Southern Hemisphere. ISBN 978-3-319-13092-7.

Impact Craters in South America, by Acevedo R. D., Rocca M. C. L., Ponce J. F., and Stinco S. G. Heidelberg: Springer, 2015. 104 p. SpringerBriefs in Earth Sciences: South America and the Southern Hemisphere. ISBN 978-3-319-13092-7.

Durbachites from the Eastern Moldanubicum (Bohemian Massif): erosional relics of large, flat tabular intrusions of ultrapotassic melts—geophysical and petrological record

The results of the airborne survey comprising gamma-ray spectrometry and proton magnetometry, ground gravity survey, and field geological observations (e.g., deep borehole profiles, contact aureole in dolomite marbles, distribution of granitic pegmatites within the Třebic pluton) suggest that the ultrapotassic Třebic and Jihlava plutons are flat intrusions. They intruded distinct deep levels of the crust, 2–4 kbar for Třebic pluton, and 5–7 kbar for Jihlava-pluton. Current thickness of the intrusions is generally less than 2 km, with two exceptions: (1) central part of the Jihlava pluton and (2) a small body near Věžnice, where the estimated depth of tube-shaped stocks of shoshonitic and ultrapotassic gabbros or monzogabbros is around 2.5 km. These stocks could represent feeding pipes of basic and alkaline and dry magmas protruding to the upper crust level. The NE part of the Třebic pluton is a bottom part of this body, whereas the NW corner and the southern promontory of the pluton could represent an upper (roof) part of the intrusion. Small isolated durbachite bodies located within the Moldanubian gneisses and migmatites of the Stražek Unit represent rootless remnants of a former large and flat durbachite body initially extending significantly more to N and NE and eroded since Lower Carboniferous. Discrepancy between the long-wave magnetic and gravity anomalies, and surface geological structure of the eastern part of the Moldanubian Zone indicates a crucial role of the thrust tectonics.

Magnetic and gravity data analysis of Rahat Volcanic Field, El-Madinah city, Saudi Arabia

The Rahat volcanic field represents one of the widely distributed Cenozoic volcanic fields across the western regions of the Arabian Peninsula. Its human significance stems from the fact that its northern fringes, where the historical eruption of 1256A.D. took place, are very close to the holy city of Al-Madinah Al-Monawarah.In the present work, we analyzed aeromagnetic data from the northern part of Rahat volcanic field as well as carried out a ground gravity survey. A joint interpretation and inversion of gravity and magnetic data were used to estimate the thickness of the lava flows, delineate the subsurface structures of the study area, and estimate the depth to basement using various geophysical methods, such as Tilt Derivative, Euler Deconvolution and 2D modeling inversion.Results indicated that the thickness of the lava flows in the study area ranges between 100m (above Sea Level) at the eastern and western boundaries of Rahat Volcanic field and getting deeper at the middle as 300–500m. It also showed that, major structural trend is in the NW direction (Red Sea trend) with some minor trends in EW direction.

The Role of GIS in Spatial Modeling of Multi-disciplinary Geoscientific Data for Uranium Exploration over the Kunjar-Darjing Basin, Odisha

Abstract: Application of Geographical Information System (GIS) for identifying the spatial locations of uranium exploration target areas using multi-disciplinary geoscientific data is presented in this paper. The data sets used in this study are Airborne Gamma Ray Spectrometric (AGRS), Aeromagnetic (AM), Satellite images, regional ground gravity and geochemical surveys over one of the promising mobile Proterozoic Kunjar-Darjing Basins to the west of Singhbum Uranium Province (SUP), Odisha, India. Analysis of the geochemical data indicated unconformity related uranium mineralization along the unconformity between Kunjar-Darjing sediments and the S-type per-aluminous Tamparkola granite with labile uranium. All the data sets are processed and interpreted independently in terms of geology based on characteristics such as intensity, frequency and texture of the images generated. Various ratio maps generated from AGRS data were used as training points for spatial modeling by building relationships (topology) with the structures and geology interpreted from the magnetic and gravity datasets. Index overlay method is adapted in spatial modeling. The study shows that integrating the geological, geophysical, geochemical and other geodata in a GIS environment provides valuable guidelines for geological mapping as well as identifying target areas for uranium exploration. The GIS study facilitated in identifying potential target areas for uranium exploration along the regional faults D1 and D2 around the villages Kelo, Tamra, northeast of Kunjar, Nuarali and Betajharan.

Geophysical and structural pattern of the Knížecí Stolec pluton and its host rocks in the south-western part of the Moldanubian Zone, Bohemian Massif

*Corresponding author A detailed airborne magnetic and gamma-ray spectrometric as well as ground gravity survey in the south-western part of the Moldanubian Zone (Bohemian Massif) provided detailed geophysical characteristics of the principal geological units and large-scale tectonic features. The Knižeci Stolec durbachitic pluton with high contents of natural radioactive elements (Th, U and K) represents a body with no magnetically anomalous response but a pronounced positive gravity anomaly. The Křisťanov granulite massif, which is a host rock of the Knižeci Stolec pluton, exhibits low concentrations of Th and U, high abundances of K, slight magnetic and a negative gravity anomalies. The Plechý composite pluton is characterized by strikingly low gravity and an extremely monotonous magnetic field. One of its petrographic varieties (the Třistolicnik granite) is enriched in natural radioactive elements, especially in Th and U. Metamorphic complexes of the Monotonous and Varied groups contain multiple magnetic anomalies mostly related to the intercalated metabasic rocks; they also cause positive gravity anomalies. The Linsser density boundaries at a depth of 0.5 km clearly delimit the Plechý composite pluton and the Lhenice zone and indicate several structurally and/or lithologically different domains within the Křisťanov granulite massif and in the Knižeci Stolec pluton. The 2.5D gravity model indicates an asymmetric shape of the Knižeci Stolec pluton, which is deepest in its SE part (at least 4 km). The joint interpretation of the geophysical and structural data revealed that the maximum depth of the pluton is c. 4.5 km, with gently to moderately dipping intrusive contacts in the north and subvertical orientation in the south. The asymmetric shape of the pluton is consistent with its inferred syn-tectonic emplacement coeval with the regional subvertical contraction and development of regional flat-lying fabric. The gravity model combined with structural analyses also implies a considerable depth of the south Bohemian granulite massifs. This indicates their steep exhumation path and is at variance with the model of the Moldanubian Zone as a sequence of flat-lying nappes. The south-western part of the studied area is reworked by the NE-directed compression, referred to as the ‛Bavarian’ deformation phase.

Gravity evidence for a mafic intrusion beneath a mineralized zone in the Bondy gneiss complex, Grenville Province, Quebec — Exploration implications

A ground gravity survey over the Bondy gneiss complex and its mineralized iron oxide- and copper-rich hydrothermal system(s) in the Grenville Province of SW Quebec was undertaken to aid mineral exploration in mapping subsurface intrusions. Several kilometric-scale positive Bouguer anomalies were identified that coincide with outcropping mafic and intermediate intrusive rocks of the post peak-metamorphic, 1.17–1.16 Ga mafic to intermediate Chevreuil suite intrusions and a 1.09–1.07 Ga Rolleau ultramafic stock. An additional 4 × 3 kilometre positive gravity anomaly indicates a mafic body underlies part of the metamorphosed hydrothermal system in the area of magnetite , pyrite , pyrrhotite , and chalcopyrite mineralization . Advanced argilic alteration associated with sulphide enrichment here is however indicative of an epithermal system with a felsic intrusion fluid source. As a felsic intrusion cannot explain the positive Bouguer gravity anomaly both felsic and mafic bodies must be present beneath the mineralized zone. Our preferred interpretation based on integrating gravity data and 2D forward gravity modelling with the results of field and geochemical studies is that this anomaly corresponds to a ca. 500 m deep mafic 1.17–1.16 Ga Chevreuil suite pluton that may have provided the source for hydrothermal fluids associated with late ductile shear- and fault-related mineralization or remobilization of early mineralization associated with a felsic pluton into late structures. This interpretation is compatible with gabbro xenoliths in the 1.07 Ga Rivard lamprophyre dyke on the NW margin of the gravity anomaly that bear significant similarities with those of the Chevreuil intrusive suite. The presence of both early felsic and late mafic intrusions beneath a group of three mineral occurrences in the Bondy gneiss complex strengthens their prospectivity in comparison to other mineral occurrences in the area. That early, pre-metamorphic mineralization was upgraded late in the tectonothermal evolution during a subsequent igneous and deformation event agrees with interpretations of other IOCG-style deposits in the Lac Manitou area of the eastern Grenville Province. ► A ground gravity survey provides new data that aids mineral exploration. ► Most gravity anomalies are associated with mafic to intermediate intrusive rocks. ► A Gravity anomaly over mineralized outcrops may be generated by a deep gabbro. ► Mafic intrusion may be the driving source for mineralization or remobilisation.

Gravity changes before large earthquakes in China: 1998–2005

Although it is well known that coseismic gravity changes take place during an earthquake, previous research has not yielded convincing evidence demonstrating that significant gravity changes occur before large earthquakes. Furthermore, even if we suspect that gravity changes occur before large earthquakes, we have yet to demonstrate how to consistently observe these changes for useful earthquake forecast that would bring benefits to society. We analyzed ground gravity survey data obtained in 1998, 2000, 2002, and 2005 at stations of the Crustal Movement Observation Network of China (CMONOC) and examined gravity changes before the occurrence of nine large (Ms⩾6.8) earthquakes that ruptured within or near mainland China and Taiwan from November 2001 to August 2008. Results from this analysis show that significant gravity changes occurred across a large region before each of these nine large earthquakes, and these changes were detected by repeated ground gravity surveys through CMONOC. Although these gravity changes were significant, more research is needed to investigate whether these gravity changes could be viewed as precursors of large earthquakes. Limitations and uncertainties in the data include sparseness of the gravity monitoring network, long time intervals between consecutive gravity surveys, inevitable measurement errors, hydrological effects on gravity, and effects of vertical crustal movements on gravity. Based on these observations, we make several recommendations about possible future directions in earthquake-related research using gravity monitoring data.

Gravity Measurements and Their Variations before the 2008 Wenchuan Earthquake

It was recognized 50 years ago that coseismic gravity changes take place during a large earthquake. In the past 30 years, there have been research efforts devoted to investigating possible associations between gravity variations and earthquakes in China. Before the Wenchuan ( M w 7.9) earthquake, a group of researchers at the Second Crust Monitoring and Application Center of China Earthquake Administration noticed significant gravity changes in a region covering the well-known south–north earthquake belt in China and suggested in 2006 that the possibility for a major earthquake to occur near Wenchuan in either 2007 or 2008 was high. These researchers used gravity changes as the primary earthquake precursor to make the suggestion. In this article, we report the method used for repeated regional gravity survey, the procedures for gravity survey data analysis, and the characteristics of gravity variations before the Wenchuan earthquake that ruptured on 12 May 2008. Although gravity changes at a number of locations in the region surrounding Wenchuan were significant, more research is needed to investigate whether these gravity variations could be viewed as a precursor of the Wenchuan earthquake. Uncertainties in the reported gravity variations include inevitable measurement errors related to ground gravity surveys covering a large region, hydrologic effects on gravity, and the effects of vertical crustal movements on gravity. Other limitations of the data are that the density of the gravity observation stations is coarse and that the time intervals of the surveys were two to three years long. Based on these observations, we make several recommendations about data collection and data analysis procedures that would enhance future earthquake research using gravity monitoring data in China.

Geophysical signatures of some recently discovered large (> 40 ha) kimberlite pipes on the Alto Cuilo concession in northeastern Angola

This paper presents a comparison of geophysical responses from several large kimberlite complexes discovered and delineated on the Alto Cuilo concession in the diamond fields of northeastern Angola in the years 2005 to 2008. Several geophysical methods were used in combination with geochemical and mineralogical prioritization techniques to guide exploratory, delineation and bulk sample drilling, in order to rapidly identify and evaluate the kimberlite bodies. The kimberlites were emplaced through Karoo Supergroup sandstones and shales, have eruption ages contemporaneous with the sand-dominated Cretaceous-age Calonda Formation, and are covered by sand-dominated poorly consolidated sediments of the Kalahari Group. Given that sand-dominated non-kimberlite lithologies are magnetically transparent, a low level, high resolution helicopter-borne magnetic gradiometer survey proved to be exceptionally effective in discriminating kimberlite targets, even for low-amplitude anomalies (e.g. 1–2 nT). The helicopter magnetic data outlined approximately 244 probable kimberlite targets and drilling of 103 targets confirmed 80 new kimberlites greater than 5 ha in area. Most kimberlites take the form of well-preserved crater edifices containing a full range of crater-related kimberlite lithologies. Ground gravity and electromagnetic surveys were conducted over all kimberlites prioritized for follow-up investigation. Geophysical responses were ground-truthed against magnetic susceptibility and density measurements, which were routinely collected on all drill cores. The geophysical signatures resolved by the three independent geophysical methods were surprisingly variable and are inferred to be sourced primarily in the crater facies materials, which demonstrate characteristically variable lithologies. Geophysical interpretations guided the drill targeting at all stages of the program at Alto Cuilo, from exploration to evaluation. Combined with geochemical and mineralogical prioritization techniques, the geophysical signatures in magnetic, gravity and electromagnetic data provide a sound basis to guide exploratory, delineation and mini bulk sample drilling of kimberlites buried under 10–70 m of overburden. The significant variability in the geophysical responses from kimberlites that are similar in size, structure and geometry highlights the importance of applying all three independent geophysical methods in order to effectively achieve kimberlite exploration and evaluation goals. The effectiveness of a multidisciplinary approach to kimberlite evaluation is demonstrated in the rapid assessment of a cluster of large kimberlites discovered at Project Alto Cuilo.

Measuring gravity on ice: An example from Wanapitei Lake, Ontario, Canada

Large lakes have always represented a problem for regional gravity databases; the difficulty of access means gaps or coarse spacing in the sampling. Satellite, airborne, and shipborne gravity techniques are options, but the resolution and/or cost of these systems make them impractical or inaccurate for exploration or environmental studies, where the required resolution is [Formula: see text]. In this study, the feasibility of a ground gravity survey over a frozen lake where ice moves because of windy conditions is assessed. Lake Wanapitei, widely accepted as resulting from the impact of a meteorite 37 million years ago, is one of these cases in which the necessity of expanding coverage over poorly sampled regions arose from a significant gap between surface and airborne geophysical maps. Two gravity surveys were completed on the ice of Lake Wanapitei in the winters of 2003 and 2004. To study the structure, longtime series of gravity field measurements were recorded for 98 stations, allowing for improved control over the noise sources in the data. Final processing and integration with an existing regional data set in the area and the application of terrain corrections reduced the amplitude of the circular anomaly from 15 to [Formula: see text] and its diameter from 11 to [Formula: see text]. The feasibility of gravity surveys on ice was assessed, and we determined that for large-scale studies such as this one, the quality of the data, even under noisy conditions, was acceptable. However, for more detailed mapping, calm wind conditions and long time series are required.