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In this work, we investigate the N40°-50°E-oriented magnetic lineaments observed in the marine and aeromagnetic data from the Paraty-Mirim and Saco do Mamangua (RJ) coves. The main characteristic of these lineaments is the presence of normal and reverse magnetic polarity, indicating that the bodies responsible for the anomalies may be of different ages or have suffered remagnetization effects by metamorphism or have distinct lithologies. The interpretative model of the anomalies was obtained by the 3D Magnetization Vector Inversion (MVI) and by the study of the geology and tectonics of the area. Our results suggest, as a more likely hypothesis, that the magnetic lineaments can be correlated with two tholeiitic dykes swarms, Resende-Ilha Grande and Serra do Mar, which have intruded the region at different times. The age formation of those swarms can be placed at time intervals where the Earth's magnetic field has normal and reverse polarities. The contrasts of magnetic susceptibility obtained from the models are consistent with contrasts of the lithology of basic dykes intruded in the gneissic-granitic basement of the region. Keywords: magnetization vector inversion, magnetic properties, magnetic lineaments, remanent magnetization, tholeiitic dykes swarms Assinatura Magnetica do Saco do Mamangua e da Enseada de Paraty-Mirim, Rio de Janeiro, Brasil RESUMO. Neste trabalho, investigamos os lineamentos magneticos orientados a N40°-50°E observados nos dados marinhos e aeromagneticos das enseadas de Paraty-Mirim e do Saco do Mamangua (RJ). A principal caracteristica destes lineamentos e a presenca de polaridade magnetica normal e reversa, indicando que os corpos responsaveis pelas anomalias podem ter idades diferentes ou terem sofrido efeitos de remagnetizacao por metamorfismo ou apresentarem litologias distintas. O modelo interpretativo das anomalias foi obtido pela Inversao do Vetor de Magnetizacao (IVM) 3D e pelo estudo da geologia e tectonica da area. Nossos resultados sugerem, como hipotese mais provavel, que os lineamentos magneticos podem ser correlacionados com dois enxames de diques toleiticos, Resende-Ilha Grande e Serra do Mar, que tem idades diferentes de intrusao. As idades de formacao desses enxames podem ser colocadas em intervalos de tempo em que o campo magnetico da Terra apresentou polaridades normal e reversa. Os contrastes de susceptibilidade magnetica obtidos no modelo sao coerentes com os contrastes de litologia de diques basicos intrudidos no embasamento granitico-gnaissico da regiao. Palavras-chave: inversao do vetor de magnetizacao, propriedades magneticas, lineamentos magneticos, magnetizacao remanente, enxames de diques toleiticos

In December 2020, Chang’e-5 returned samples from mare basalt unit Em4/P58 in northern Oceanus Procellarum. With an absolute model age of 1.21-1.53 Ga (Hiesinger et al. 2003; Qian et al., 2018, 202...

Periodic mass discharges of icebergs from the Laurentide ice-sheet into the North Atlantic Ocean during the last glacial period deposited abundant ice-rafted detritus (IRD) accumulated in sequences of typically six major Heinrich Event layers, each with some tens of cm thickness, at all eastern slopes of the Grand Banks submarine platform of Newfoundland. Compositionally, it is well established that these IRD layers consist of varied rock contents emanating from distinct, but not yet clearly defined bedrock provinces of the Canadian Shield. The, most prominently reported constituent is detrital dolomite, but the entire lithological range of the IRD is much broader. Rock magnetic records, e.g. magnetic susceptibility logs of SE Grand Banks cores, therefore depict complex and partly repeating internal substructures across the Heinrich Event layers owing to distinct successions in IRD lithology over the course of every mass calving event.We investigated IRD sieve fractions (1mm – 4cm) of the entire glacial section (550–1054 cm) of SE Grand Banks slope gravity core GeoB 18530-1, sampled in 2.3 cm steps. Therefrom, we identified and classified distinct IRD rock types as well as monocrystalline rock-forming mineral particles, for which we established so far 24 well-defined lithological categories of sedimentary, igneous and metamorphic origin. This initial identification of IRD lithology was performed based on all available visual criteria including texture (crystallinity, grain-size), color and translucency (mineralogy), hardness and surface structures (e.g., cleavage) using a binocular microscope. This rock type classification is now being substantiated by polarized light microscopy of exemplary thin sections created from larger IRD clasts.To established cumulative rock magnetic fingerprints of all IRD magnetic mineral assemblages, isothermal remanent magnetization acquisition curves of all sieve fractions as well as individual specimens of all the classified rock types have been measured. These records systematically revealed higher concentrations of magnetic minerals at the tops and bottoms of most Heinrich Event layers and also clear variations in coercivity spectra. This finding is mirrored by the IRD rock count records, where magmatic rock types predominate mostly at Heinrich Event layer boundaries. Preferred deposition of these IRD rock types during the initiation and ending of events and their variation from older to younger events,- highlight repetitive patterns in the cyclic Laurentide ice-sheet collapses to be further explored.

Recent studies have demonstrated that rock-magnetic analysis may provide additional information to distinguish and characterize extreme marine inundation events such as tsunamis. Rock-magnetic proxies reinforce and improve the environmental evidences supplied by other methods, adding some decisive clues for the interpretation of the origin and genesis of the sedimentary deposits. Here we report rock-magnetic, XRD and SEM microscopy results obtained in the Palo Verde estuary (Colima Pacific coast, Mexico) in order to enhance the tools for identification and reconstruction of two tsunami-induced deposits. The sedimentary sequence includes two sand units, a tsunami deposit (PV1) associated with the 22 June 1932 tsunami and a deeper sandy layer (PV2) related to a possible palaeotsunami that occurred around 1300 CE. Both sandy units are topped by finer grained units. Magnetic properties exhibit a significant correlation with the stratigraphy. High susceptibility (χ) and high saturation isothermal remanence (SIRM) values typical of high concentrations of (titano)magnetite are a distinctive feature of the most recent sandy tsunamigenic unit PV1 and the overlaying soil. The lower sandy tsunamigenic unit PV2 shows significantly lower χ and SIRM values, indicating lower concentration of (titano)magnetite in this unit and the overlaying clayey-silt unit. The latter also shows a higher coercivity component associated to (titano)hematite. Magnetic grain-size differences are also observed between PV1 and PV2 suggesting differences in hydraulic conditions at the time of deposition. The bulk mineralogical composition and sediment texture of these units also supports the hypothesis of different provenances for each tsunamigenic unit as inferred from magnetic properties.

The Artemis Cu-Au-Zn-Ag deposit is located approximately 43 km southeast of Cloncurry and 19 km west of Eloise in northwest Queensland. The deposit, hosted within the Paleoproterozoic Mount Norna Quartzite unit which is part of the Lower Soldiers Cap Group, is a steeply-dipping massive sulphide body. The Artemis deposit was discovered by Minotaur in July 2014, only 9 months after taking possession of the project. The deposit is a new type of mineralisation that has previously not been identified in the Cloncurry district. The discovery was made by meticulously piecing together the historical geophysical and geological data, complimented by new geophysical data sets including airborne EM and ground EM to improve the understanding of prospective targets throughout the area. The dominant sulphide within the deposit is pyrrhotite, hence the deposit is highly conductive. Airborne and ground EM techniques were therefore the main tools in resolving the mineralisation. The pyrrhotite is non-magnetic and therefore the deposit has no discernible magnetic signature and magnetics played no part in the discovery process. The deposit had no associated surface geochemical anomalism and very little alteration, making this deposit geologically difficult to find and essentially reliant on EM techniques in making the discovery.

This study reports a multidisciplinary approach to determining hydrocarbon charge-points and migration in the Wessex Basin, southern England. Geochemical analysis of reservoir core material (Bridport Sandstone and Inferior Oolite) using gas chromatography-mass spectrometry (GC-MS), suggests that the oil in the Wessex Basin is from a single source, and that small variations in environmentally sensitive biomarkers are likely due to small differences in maturity or depositional conditions during the formation of the oil over millions of years. Using seismic data, basin modelling revealed two potential hydrocarbon migration pathways from the hanging wall of the Purbeck fault into the Sherwood Sandstone reservoir at Wytch Farm. One of these potential pathways is represented by cores termed Creech and the other Bushey Farm. To try to distinguish between the two potential pathways, cores were studied using mineral magnetic techniques. The magnetic signature was characterised using low-temperature (

Ground geomagnetic survey was conducted around Iseyin Area, part of Ado-Awaiye in order to map and ascertain the nature of variability in the magnetic signature of the subsurface rock, structure of the magnetic basement as well as the depth to basement top. This was aimed at revealing the subsurface basement configuration, lithology and its mineralization. Total field intensity were measured and recorded at each station with the value of the magnetic anomaly ranging from 30446.47 nT to 33536.10 nT (nanotesla). Regional-residual separation and half - width methods were applied and after necessary data processing, the results were presented in form of magnetic map and profiles. Qualitative and quantitative interpretations were made based on the maps and profiles. Variation in the magnetic data reveals variation in the basement lithology with the higher value corresponding probably to the amphibolite (basic) component and the lower value corresponding to the granitic or

Typhoon Morakot caused terrible flooding and torrential rains that severely damaged southern Taiwan. Swollen rivers wiped out roads and demolished buildings. Long-lasting and intense rainfall triggered landslides in many regions in southern Taiwan, including the landslide that buried Siaolin Village in Kaohsiung County and killed approximately 500 people. Locating buried buildings immediately after a landslide could be an emergent issue in life saving and hazard mitigation. Analyzing the magnetic signature of a buried area is an efficient and non-destructive way to detect subsurface buildings. This paper presents the results of a magnetic survey for the purpose of outlining subsurface images of Siaolin Village after the catastrophic landslide induced by Typhoon Morakot in 2009. We found that a high-resolution magnetic survey can reveal suspected building positions that match the initial locations of buildings in Siaolin Village. The estimated depths of the buried buildings are 5–10 m deep. The magnetic data further suggest a possible debris-flow direction of NE to SW because the northern part of the village was mostly destroyed, while buildings in the southern part of the village remained in place.

We present preliminary results of a multidisciplinary geophysical investigation applied to the subsoil imaging of the archaeological site of Cocciano, near Rome. We acquired capacitive coupled resistivity data along two parallel profiles and we performed a magnetic survey over a small subset of the survey area. The recovered resistivity models suggest the presence of a shallow, sub-horizontal, resistive layer ( r > 350 Ωm), of slightly variable thickness (2 – 3 m), which we interpret as the response of ancient substructions overlying a relatively low-resistive layer, which we ascribe to the geological substratum. Processed magnetic data show a clear magnetic signature aligned to form a curve-shaped anomaly right at the prosecution of a nearby, partially exposed, ancient wall.

Magnetic fields of naval vessels are widely used all over the world for detection and localization of naval vessel. Magnetic Anomaly Detectors (MADs) installed on air borne vehicles are used to detect submarine operating in shallow waters. Underwater mines fitted with magnetic sensor are used for detection and destruction of naval vessels in the times of conflict. Reduction of magnetic signature of naval vessels is carried out by deperming and installation of degaussing system onboard the vessel. Present paper elaborates details of studies carried out at Magnetics Division of Naval Science and Technological Laboratory (NSTL) for minimizing the magnetic signature of naval vessels by designing a degaussing system. Magnetic fields of a small ship model are predicted and a degaussing system is designed for reducing magnetic detection. The details of the model, methodology used for estimation of magnetic signature of the vessel and design of degaussing system is brought out in this paper with details of experimental setup and results.

EE-index (EDst, EUEL), has been used to study the longitudinal dependence and seasonal variation of the equatorial electrojet (EEJ). The EUEL data eliminates many sources of disturbances by subtracting the median value of horizontal component (H) and the E Dst from H component data. EUEL data at a chain of stations along the dip equator have been analyzed to provide a detailed study on the equatorial electrojet. Data from eight stations (ANC, ILR, AAB, TIR, LKW, BCL, DAV and YAP stations) have been used for a period of three years (2009, 2010 and 2011). The longitudinal dependence has been studied for each year, a very good agreement between each year results has been found. This study shows that the magnetic signature of the EEJ is stronger in South America with a maximum at about longitude 77°W; and weaker in Asia, with a minimum in India, between longitudes 70°E and 90°E. The seasonal variations of the equatorial electrojet have been studied by both the whole data set (disturbed and quiet days) and quiet days’ data. It has been proved that there is a semiannual variation in the equatorial electrojet with equinoctial maxima.

Exploration for Neoarchean intrusion-related gold deposits in the Abitibi greenstone belt of the Superior Craton, Canada, is of increasing interest as the two most recent Abitibi gold mines are intrusion-related. Late-Archean alkaline intrusions in the Abitibi Subprovince are separated into three groups based on their geophysical and geochemical signatures: (1) large, heterogeneous, unmineralized plutons; (2) small magnetite rich-syenites with magmatic gold, which is often remobilized along fault arrays; (3) small magnetite poor-quartz-syenites to alkali granites with magnetite-rich halos and magmatic gold mineralization. The positive, aeromagnetic high centred signature of some gold-bearing intrusions is related to the high content in magnetite of the more ferromagnesian intrusions resulting from their magmatic evolution. Intrusions with an annular shape record a lower Fe2O3t content in their less magnetite-rich cores and gold mineralization occurs within their magnetite-rich metasomatized host-rocks. If the aeromagnetic signature has an annular-shape, the lower-magnetic zones in the magnetic aureole are the more favourable zones. For those with a positive magnetic signature, the intrusion itself is the target.

Down-hole magnetic susceptibility techniques were explored as a means of improving near-surface geophysical surveys in historic grave detection. These techniques were used to document distinctive magnetic characteristics of grave shafts at three historic cemeteries first surveyed using various near-surface geophysical methods. Tests revealed a low magnetic susceptibility signature that soil magnetic studies indicated was largely related to differential soil compaction associated with the excavation and refilling of grave shafts. Most apparent at depths beyond those reached by soil penetrometers, this magnetic signature offers a way to assess anomalies identified by near-surface techniques that potentially represent graves. At one cemetery, magnetic studies of the interments themselves suggested spatially patterned, magnetically enhanced zones that might also aid in burial identification in certain contexts. While down-hole techniques will not be foolproof, they do provide another geophysical tool that can be used to improve grave detection. © 2010 Wiley Periodicals, Inc.

The La Gila Encantada Site is located on an isolated ridge top north of Silver City, New Mexico in an area defined culturally as the Mimbres Mogollon region. The 180 m × 80 m (14400 m2) sized site was originally recorded as a dense scatter of ceramics, lithics, and ground stone along with a number of surface depressions that appear to represent pithouses. Cesium magnetometer surveys were conducted to identify hearths, pithouse boundaries, and activity areas outside of pithouses in support of archaeological investigations, and to test this instrument's ability to image these features. This paper presents a characterization of the magnetic signature of a pithouse as a magnetic high in the center caused by the central hearth, low magnetic variability along the floor of the house, and then increased magnetic variability at the pithouse boundary. This characterization was successfully confirmed for three pithouses using cesium magnetometery and archaeological excavation.

Recently developed down-hole magnetic capabilities and laboratory soil magnetic techniques are combined in a geophysical approach to identify buried archaeological sites. Down-hole magnetic susceptibility measurements are used to locate palaeosols and soil magnetic studies are applied to evaluate whether an associated human occupation is likely. This geophysical approach has the potential to locate and explore buried archaeological deposits across varied environments in a cost-effective, efficient, and relatively non-invasive manner. This approach was developed and tested on sites in Minnesota and North Dakota, USA. Prototype Bartington Instruments MS2H down-hole sensors allowed identification of major stratigraphical units, buried occupation layers and non-cultural palaeosols at these sites. Buried soils correlated with enhanced susceptibilities. Buried cultural soils produced, in some cases, a signal over twice the magnitude of non-archaeological palaeosols. Although this magnitude shift suggested a possible avenue for determining if ancient land surfaces had been occupied by humans, laboratory soil magnetic techniques, which allow an understanding of changes in composition, concentration and/or grain size of the magnetic carrier that produce this contrast in magnitude, were found to be much more effective in making this determination. These soil magnetic tests were applied to sediments collected from the down-hole locations. An increase in magnetic remanence carried in the coarse soil fraction was found to consistently and reliably distinguish those soils associated with human occupation. Investigations into the origin of the magnetic signature have suggested this remanence resides in nodules of burnt clay found in the archaeological soils. As it is suspected that domestic fires produced these clay aggregations, this phenomenon may extend to a wide range of sites and thus provide a technique for buried site identification that is broadly applicable. Copyright © 2006 John Wiley & Sons, Ltd.

Aeromagnetic surveys play an important role in the detection and analysis of terrestrial impact structures as large semi-regional aeromagnetic surveys are widely available. Impact craters can be divided into two groups based on morphostructure, namely simple and complex. Simple craters are relatively small bowl-shaped depressions with an upraised and fractured rim whereas complex craters are larger with a central uplift zone. Magnetic signatures of terrestrial impact craters vary greatly, reflecting the target rocks, the impact-related magnetisation and effects of crater fill and post-impact sediments. In basement rocks, the common signature is a magnetic low, ranging in amplitude from a few nT up to a few hundred nT. The central peak or ring uplift of crushed basement may produce strong magnetic highs. The magnetic signature may be due to shock demagnetisation, shock remagnetisation, and thermal and chemical remanent magnetisation effects. Impact craters in sedimentary targets are usually subdued and amplitudes of a few nT up to 10 nT are common. Enhancement of magnetic signatures of impact structures using filtering techniques is an important part of detection and analysis. Derivatives and shaded relief techniques, along with separation filtering, are probably the most used methods. Algorithms for fractional order derivatives and circular shaded relief have dramatically improved filter results. The fractional derivative order can be varied to optimise separation of the impact magnetic signature. Circular shaded relief treats all directions equally unlike the fade-out for features sub-parallel to the shading direction evident in conventional shading. The fractional order derivative and circular shaded relief algorithms are illustrated from impact structures in Australia and Canada in both basement and sedimentary cover rocks.

It has been known for many years that entrapped hydrocarbons often coincide with shallow-sourced micromagnetic anomalies. Field studies have shown that these usually sub-nT amplitude anomalies are associated with near-surface magnetite deposits generated predominantly from microbial alteration of hematite to magnetite in the presence of hydrocarbon microseepage. These studies have demonstrated that hydrocarbon traps usually leak vertically as well as laterally. In the absence of significant cultural magnetic interference or shallow volcanic magnetic signature, and with favorable ground conditions, a high-resolution aeromagnetic survey can effectively locate seep-induced micromagnetic anomalies. One such survey has been recently performed over the Muglad Basin of the Sudd region of southern Sudan, flown with a relatively small line spacing (500 m) and low ground clearance (100 m). Analysis of these data has revealed characteristic annular micromagnetic anomalies (aureoles) that cluster along prospective high structures and coincide with the Jarayan oil field. Besides delineating exploration fairways, this method can be used to help prioritize seismic structural leads and to target non-structural hydrocarbon traps. Numerous researchers have studied this phenomenon over the past 70 years confirming its validity, and in turn have recommended the use of high-resolution aeromagnetic surveying as a valid exploration method for this particular purpose. Despite this, and highly effective complementary geo/biochemistry surface studies, the oil industry has not yet fully embraced this approach as a standard exploration tool. The micromagnetic exploration approach is likely to remain empirical due to the complex interplay of geologic, geochemical and biochemical factors involved. It is this empiricism that appears to deter oil companies. We hope that the southern Sudan case history presented here usefully contributes to the growing body of evidence that this is a valuable exploration technique that ought to be used with greater confidence in the field. The exact nature of the vertical microseepage process, …

We correlate oscillations in the hydrologic and/or cryologic balances of four Great Basin surface-water systems with Dansgaard–Oeschger (D–O) events 2–12. This correlation is relatively strong at the location of the magnetic signature used to link the lake records, but becomes less well constrained with distance/time from the signature. Comparison of proxy glacial and hydrologic records from Owens and Pyramid lakes indicates that Sierran glacial advances occurred during times of relative dryness. If our hypothesized correlation between the lake-based records and the GISP2 δ18O record is correct, it suggests that North Atlantic D–O stades were associated with relatively cold and dry conditions and that interstades were associated with relatively warm and wet conditions throughout the Great Basin between 50,500 and 27,000 GISP2 yr B.P. The Great Basin lacustrine climate records reinforce the hypothesis that D–O events affected the climate throughout much of the Northern Hemisphere during marine isotope stages 2 and 3. However, the absolute phasing between lake-size and ice-core δ18O records remains difficult to determine.

In this paper we examine the long-term temporal characteristics of palaeomonsoon dynamics in equatorial Africa from a continuous lacustrine sequence retrieved from Sacred Lake, Mount Kenya (0°03′N, 37°32′E, 2350 m a.s.l.), covering the last interglacial–glacial transition to the present. The trends in mineral magnetics and stable carbon isotopes are proxy indicators of changes in precipitation on the mountain over the last glacial–interglacial cycle. Spectral analysis by a fast fourier transform method revealed that the stable carbon isotope trend (δ 13 C) has strong signals at the 23,000 and 11,500 year frequencies. The mineral magnetic signature does not register the 23,000 year cycle observed in the δ 13 C signature. It has, however, a strong signal at an 11,500 year frequency, and sharp but relatively weak peaks at ca. 7500 and 5000 year frequencies are recorded. The dominant 23,000 year frequency recorded in the δ 13 C signature reflects the strong effect of the precessional cycle on tropical climate and ecosystems, and is most probably effected via global atmospheric pCO 2 and temperature changes. The shorter cycles at 11,500 year (indicated by both mineral magnetics and δ 13 C trends), and 7500 and 5000 years BP (apparent in the mineral magnetic record) are attributed to precipitation variations, whose temporal cycles are dominated by the higher precessional harmonics.

Wannaway, Mariners and Mt. Edwards are three of many type 1A komatiitic peridotite-hosted nickel sulphide deposits clustered in the Widgiemooltha area, approximately 560 km east of Perth. Magnetic susceptibility measurements show that the susceptibility of the ore is generally high but is extremely variable, from 4.5 ´ 10-3 SI units to 269 ´ 10-3 SI units in serpentinised environments, and 14.9 ´ 10-3 SI units in talc-carbonate altered environments, hangingwall ultramafic rocks have a generally high but variable susceptibility depending on the style of alteration. Serpentinised ultramafic rocks have a high susceptibility of 70 ´ 10-3 SI units, while talc-carbonate altered ultramafic rocks have a low susceptibility of 4.4 ´ 10-3 SI units due to the respective formation or destruction of magnetite. Footwall basaltic rocks have a consistently low susceptiblity average of 0.8 ´ 10-3 SI units. Susceptibility-temperature data indicate that ores with a high susceptibility contain monoclinic pyrrhotite and magnetite, and ores with a low susceptibility contain only small amounts of monoclinic pyrrhotite, minor magnetite and abundant, non-magnetic, hexagonal pyrrhotite. Components of remanent magnetisation in the ores and hangingwall ultramafic rocks lie in two dominant orientations. Remanent magnetisations held by pyrrhotite and magnetite at Wannaway and by pyrrhotite at Mt. Edwards are upwardly inclined toward the northwest. Vectors held by pyrrhotite at Mariners and magnetite at Wannaway are upwardly inclined toward the southeast. Two dimensional magnetic modelling of the Wannaway ore environment indicates that the anomaly over the Wannaway deposit is caused predominantly by the hangingwall ultramafic unit, with a small contribution from the near-surface hangingwall ore. Contact ore makes only a minor contribution to the anomaly due to the depth of the ore below the surface. Although the ore is significantly magnetic, it is difficult to directly detect on magnetic data due to the highly magnetic nature of the surrounding ore environment.

The Bottle Creek gold deposit lies on a north-trending structural break between two major terrains in the Ularring greenstone belt, north of Kalgoorlie. Aeromagnetic data in the vicinity of the deposit suggest several structures which appear to have controlled gold mineralisation. The mineralised horizon lies at a structurally discordant, north-northwest-trending internal junction within a thick mafic sequence in the eastern terrain. Lithological variations within this sequence have been inferred to represent distinct upper and lower groupings, and are structurally determined. Interpreted dips within the eastern terrain are very steep and can be contrasted with much shallower regional dips in the western terrain. Mineralisation within the Emu Formation host, which occupies a sheared zone, contains pyrrhotite but the observed magnetic responses along the structure appear to reflect magnetite-constructive alteration near the boundary. There are two shears; the largest separates western and eastern sequences, which meet acutely but are not mineralised along the shear. There is a lesser parallel structure further east. The structures are separated by a mafic unit which generates a chain of isolated magnetic responses along this structure. The Bottle Creek deposits are associated with sheared and altered junctions along the, apparently, lesser shear. Major deposits appear to be localised where this shear is intersected by large northeast-trending fractures.

An aeromagnetically mapped, buried, dyke swarm in southeastern Botswana is collinear with the Pilanesberg Dyke Swarm of South Africa. Further correlation of the two swarms is established by their common magnetic signature of double-peaked reversed anomalies with subsidiary highs to the south. The double-peaked anomalies are explained in terms of a two-stage emplacement process. The Pilanesberg Dykes have been dated at 1310±60 million years. They intrude the arenaceous strata of the Waterberg Group in Botswana, thus setting an upper age limit for the Waterberg. Quantitative modelling of the entire magnetic profile showed that it is not possible to explain all of the anomalies in terms of reversely magnetized dykes alone. A normally magnetized dyke swarm occupies the same zone of weakness and may be contemporaneous with the East-Rand Dykes, dated at 1120±45 million years.

[1] Using observations made by the Finnish chain of pulsation magnetometers, we select typical properties of the ULF response to sprite-associated lightning events taken from the EuroSprite 2003 summer campaign. We concentrate on frequencies below the Schumann resonance and compare the obtained results with properties obtained from positive cloud-to-ground lightning discharges which are not associated with sprites. The latter we take from listings of the Meteorage Lightning Detection Network, which correlate with strong impulsive events seen in the Finnish chain. The evidence shows no unique and identifiable ULF signature relating to the sprite causative discharges.

Aeromagnetic surveys play an important role in the detection and analysis of terrestrial impact structures as large semi-regional aeromagnetic surveys are widely available. Impact craters can be divided into two groups based on morphostructure, namely simple and complex. Simple craters are relatively small bowl-shaped depressions with an upraised and fractured rim whereas complex craters are larger with a central uplift zone. Magnetic signatures of terrestrial impact craters vary greatly, reflecting the target rocks, the impact-related magnetisation and effects of crater fill and post-impact sediments. In basement rocks, the common signature is a magnetic low, ranging in amplitude from a few nT up to a few hundred nT. The central peak or ring uplift of crushed basement may produce strong magnetic highs. The magnetic signature may be due to shock demagnetisation, shock remagnetisation, and thermal and chemical remanent magnetisation effects. Impact craters in sedimentary targets are usually subdued and amplitudes of a few nT up to 10 nT are common. Enhancement of magnetic signatures of impact structures using filtering techniques is an important part of detection and analysis. Derivatives and shaded relief techniques, along with separation filtering, are probably the most used methods. Algorithms for fractional order derivatives and circular shaded relief have dramatically improved filter results. The fractional derivative order can be varied to optimise separation of the impact magnetic signature. Circular shaded relief treats all directions equally unlike the fade-out for features sub-parallel to the shading direction evident in conventional shading. The fractional order derivative and circular shaded relief algorithms are illustrated from impact structures in Australia and Canada in both basement and sedimentary cover rocks.

An aeromagnetic and gravity investigation was conducted in the Rio do Peixe basin, NE Brazil. Several geological and geophysical evidence indicate a strong structural control of pre-existing upper crustal weakness zones in the development of the graben architecture of this basin. The digital processing of the aeromagnetic data supplied an analytic signal map of the area, which together with a three-dimensional gravity modeling revealed the magnetic signature of two major structural domains and an estimate of the internal geometry of the rift basin, respectively.

In several places in the world measurements of the magnetic susceptibility have been used to detect environmental changes. This study reassesses this problem with special emphasis on the validation of some general conclusions that are often found in the literature. This study links magnetic susceptibility measurements of Pampean soils with present environmental conditions. It could be shown that also in the Argentine Pampa, where such studies have just been started, susceptibility measurements can be used for paleoclimatic studies. In contrast to the results from the cold Chinese Loess plateau the calibration is however different.

The objective of this research is to investigate structural features based on geophysical data in the Tapajos river region (Amazon sedimentary basin). The research was based mostly on digital manipulation of geophysical images, gravimetric and magnetometric fields, and gravimetric model. Digital image processing improved the presentation of the data providing suitable means for the generation of new geologic information. Geoprocessing procedures allowed an integrated data analysis and a better understanding of their spatial and genetic relationships. This research made it possible to identify dense and magnetic bodies into the basement and to establish relationships between frequency, magnetic signature and depth of the bodies.

The Grenada Basin formed via back arc extension in Early Tertiary time The north-south orientation of major physiographic elements along the eastern margin of the Caribbean Plate suggests that simple east-west sea floor spreading resulted in the formation of the Basin. However gridded magnetic anomalies over the Basin exhibit prominent east-west trends, suggesting north-south sea floor spreading. Present models for the formation of the Grenada Basin vary from extension oriented north-south, to extension oriented northeastsouthwest, to extension oriented east-west. An integrated interpretation of magnetics, gravity. seismic (both reflection and refraction), geologic, and well data for the Grenada Basin support near east-west extension for the formation of the Basin. Subtle, magnetic anomaly trends, subparallel to the Lesser Antilles island arc, over the southern portion of the Basin are consistent with this conclusion. Late Tertiary tectonic movements may be responsible for disrupting the magnetic signature north of 15’N.

RESUMO A Provincia Alcalina Rio Verde-Ipora, na borda norte da Bacia do Parana, e caracterizada pela presenca de intrusoes alcalinas com forte assinatura aeromagnetica e gravimetrica, apresentando-se como anomalias quase circulares. O trabalho refinou o levantamento gravimetrico na regiao sudoeste de Goias incluindo um levantamento de detalhe sobre as intrusoes de Morro do Engenho (ME) e uma intrusao a poucos quilometros a NE de Morro de Engenho (A2), com informacoes gravimetricas e aeromagneticas, e sobre a anomalia gravimetrica proxima a cidade de Britânia, sem informacao de anomalia aeromagnetica. As anomalias gravimetricas variam de 15 a 30 mGal com relacao ao campo gravimetrico regional, enquanto que as anomalias aeromagneticas sao da ordem de -3000nT. Foram feitas tambem medidas das propriedades petrofisicas de amostras da area. Os resultados da inversao 2D e 3D dos dados gravimetricos e 3D dos dados magneticos indicam que os corpos alongam-se dentro da crosta superior ate a profundidade maxima de 10 km e diâmetro da ordem de 8 km, com geometria aproximadamente cilindrica. Os dados gravimetricos e aeromagneticos de ME foram tratados de forma conjunta por modelamento direto (2,5D) onde foi incluida a magnetizacao remanescente, os resultados obtidos indicam que o volume do corpo anomalo e da ordem de 980 km 3 . ABSTRACT The Alkaline Province Rio Verde - Ipora, in the northern border of the Parana Basin, is characterized by the presence of alkaline intrusions with strong gravity and magnetic signature. This project implemented a gravity survey in the Southwest of Goias, including a detail one at Morro do Engenho Complex (ME), and one a few kilometers towards NE from ME over an aeromagnetic anomaly known as A2, and on a gravity anomaly close Britânia city. ME and A2 have magnetic anomalies. The gravity anomalies vary from 15 to 30 mGal in relation to the regional gravity field, while the magnetic anomalies are of the order -3000nT. Measurements of petrophysical properties of samples from the area were also done. Results from the inversion of gravity and magnetic data suggested that the bodies are inside the superior crust until the maximum depth of 10 km, with a cylindrical configuration with 8 to 10 km in diameter. 2.5D direct modeling was also done for ME including remanent magnetization and was performed at the two data sets together, the obtained results indicate that the volume of the anomalous body is of 980 km 3 .

A low-pass filtered aeromagnetic map of southern Vancouver Island is inverted into a magnetization-contrast map that illustrates the lateral variations in the magnetization of the crust. The Island Intrusions and the West Coast Crystalline Complex are characterized by high magnetization relative to the other lithologic groups. The Island Intrusions form an almost continuous pluton in the deeper part of the crust. The low-magnetic Leech River Formation thickens northward. The almost uniform magnetic signature of the Leech River Formation in the east–west direction indicates that the crustal structure revealed from the Lithoprobe seismic results is typical throughout the formation west of the Survey Mountain fault. A low-magnetic zone, elongated in the northwest–southeast direction, cuts through the high-magnetic Metchosin basalts from the southern tip of the island to the Leech River fault. It coincides with the Sooke gabbros exposed in the southern tip of the island, implying that the gabbros probably cut the entire Metchosin basalts at depth.

Paleomagnetic measurements have been made on a continuously sampled, 5-m section of a core from Clear Lake, California. The sediments studied span an 8000-year interval centered at 25,000 years B.P., the approximate date of the large-scale, counterclockwise loop of the magnetic vector recorded in sediments from Mono Lake, California. The data from Mono Lake have been interpreted as a geomagnetic excursion with a duration of 600–1000 years. Because Clear Lake is only 320 km from Mono Lake and since each sample from Clear Lake represents 26 years of sedimentation, the magnetic signature of the Mono Lake geomagnetic excursion should be recorded in detail in the Clear Lake samples. Aside from a generally uniform shallowing of inclinations due to compaction, the paleomagnetic record from Clear Lake contains no anomalous features which would correspond to the Mono Lake excursion. Thus it has yet to be shown that the Mono Lake excursion was recorded anywhere besides Mono Lake. Even if the existence of the excursion is ultimately confirmed, its usefulness as a magnetostratigraphic horizon is limited.

The structural fabrics recorded from Devonian rocks and substrate in the Hasteinen area, Western Norway, show near E-W trends. In the Devonian rocks these fabrics are a consequence of post-depositional regional folding at a considerable depth, presumably attended by the aquisition of an thermoviscous (TVRM) or thermochemical (TCRM) remanence (group A). Post or possibly late syntectonic group A remanences (declination=210, inclination = +18, α95=9) constitute the principal remanence in both Devonian rocks and their substrate, whereas a younger group B magnetization (declination=047, inclination=+59, α95=7) relates to early Mesozoic shearing. The palaeomagnetic (group A) and structural data from the Hasteinen and Kvamshesten Old Red Sandstone Massifs favor a regional compressive N-S shortening event (Svalbardian-Solundian), which probably occurred during latest Devonian times. Some recent extensional-basin models, appealing to listric-normal faulting to account for both the sedimentation and the subsequent deformation of the Western Norwegian Devonian rocks, are rejected on the basis of inadequate structural information.

Poplar Bay, in southeastern Manitoba, is a circular lake approximately 3 km in diameter, 21 m in depth, and is framed by radial and arcuate photolineaments. Geophysical study of Poplar Bay shows a unique magnetic signature, and results of a gravity survey indicate a profound deficiency of mass under the lake. The form of the gravity anomaly is circular, and it has a maximum value of −7 mGal.There is no evidence of an igneous origin for the feature and data indicate the plausibility of a meteorite impact origin for Poplar Bay.

Late Caledonian granitoids, (c. 400 Ma) in the zone of the Iapetus suture provide a datum against which current models for the relative timing of deformation can be tested. One such granitoid adjacent to the proposed suture ‘line’ in eastern Ireland is now buried by an Upper Palaeozoic cover. It is identified geophysically by a negative Bouguer anomaly with no magnetic signature, and geologically by a hypabyssal dyke swarm and hornfels metamorphism. The timing of intrusion of the granitoid is shown to have occurred during the continuing end-Silurian/early-Devonian deformation. Other members of this widespread suite in Ireland show features consistent with diapiric intrusion during the later stages of the deformation. This evidence brackets the age of regional deformation as continuing during granite emplacement and cooling (c. 400 Ma). The unifying characteristics of the straddling granitoid suite, coupled with a sinistrally transpressive deformation, in this broad suture zone are interpreted in terms of a continental collision which climaxed in late-Silurian/early-Devonian time.

Some of the EM conductors, delineated in the greenstone belt of western Kenya during an airborne Barringer INPUT survey in 1977 and believed to be kimberlite pipe targets, have been geophysically investigated on the ground. Four of those initially investigated are reported in this paper. The results show that three of these are elliptically shaped and oriented in the general strike direction of the local geology. They are associated with a distinct magnetic signature. The magnetic source in each case seems to be deeply seated. The fourth conductor has a more or less circular surface outline. It is associated with a ground vertical magnetic pattern that appears to be of surficial origin and aligned in a north-south direction across the middle of the conductor. No geological outcrops occur on the conductors, except a few at their edges. Diamond drilling was carried out on the four conductors. Although not fully confirmed, drill core results from three of them show what appears to be vaguely defined kimberlitic material.

The northern margin of the basement complex east of the Tauern Window is characterized by a particular magnetic signature which connects the Penninic systems of Rechnitz-Bernstein with that of the Tauern Window. These partly ophiolitic structures are interpreted as belonging to the Penninic and reach depths in the range 3–7 (or even more) km. The ultramafic masses of Hochgrossen and Larchkogel are considered as exposed Penninic remnants. The crystalline basement complex east of the Tauern Window is dominated by anomaly patterns of an entirely different character. The anomalies are generally weaker and the locations of the magnetic sources are much shallower. Representative of this type is the rraubath mass. Starting from the analysis of the magnetic pattern, a model is presented for integrating the area east of the Tauern Window into an evolutive concept in terms of plate tectonic history.

Near 53°N the crest of the Mid-Atlantic Ridge is offset 370 km in a left lateral sense by a large fault. Smaller subsidiary and parallel fracture zones are found both to the north and south of the primary fault. There is no magnetic signature associated with the fracture zone.

A model has been proposed (Webster 1982) to utilize geophysical data for tin exploration. The model incorporates a diagnostic magnetic signature for tin deposits (Webster 1984) combined with the magnetic patterns of granitoids. Regional gravity and magnetic data can be utilized to define favourable geology and structural environments for epigenetic mineralization.

To accomplish inversion, it is necessary to have a so‐called forward model and its response. By response of a model, I mean that a geologic structure has been postulated and I have calculated its geophysical signature—its gravity signature, its magnetic signature, or its seismic signature.

Recently collected seismic reflection profiles reveal numerous emerged and buried diapiric structures along the Campeche Shelf and slope, south of 20° N. latitude. Slope diapirs have steep sides, relatively flat tops, and a subsurface reflector which may be an expression of caprock. Shelf diapirs are buried structures characterized by uniformly stratified sediments trapped between rising intrusives or displaced by numerous high-angle faults. The limited east-west distribution and the lack of magnetic signature suggest that Campeche Sheli and slope diapirs are salt structures co-linear with previously mapped salt domes in the Tabasco-Campeche Basin and the Sigsbee Knolls.