Analysis of a large extent aeromagnetic survey near the geomagnetic equator (Minas Gerais,Brazil)

A large extent aeromagnetic survey, covering a 9°×9° degree of a Precambrian shield area in SE Brazil, was used to analyse long wavelength magnetic anomalies with the purpose of determining the magnetic structure of the lithosphere.Data analysis based on a novel approach to the estimation of the parameters in the statistical model ofSpector andGrant (1970), reveals a highly magnetized lower crust, bounded by an essentially non-magnetic upper mantle. The highly magnetized layer can be associated with a higher density lower crust model, derived from gravity and seismic data available in the area, suggesting a compositional transition from upper to lower crust.It was also verified that the operation of reduction to the pole, which is singular at the geomagnetic equator, performs quite satisfactorily at the low geomagnetic inclination of the area, bringing into accordance the long wavelength anomalies with the structural trends.     

Public awareness and attitudes towards naval sonar mitigation for cetacean conservation: a preliminary case study in Fairfax County, Virginia. (the DC Metro area)

The potential impacts of naval sonar on cetaceans has led to a series of court cases and statements of concern by international organizations. However, there has been no research conducted on attitudes of the general public with respect to this issue. To investigate this, a preliminary public survey was conducted in Fairfax, Virginia (the Washington, DC Metro region). The majority of the public sampled believed that naval sonar impacted marine mammals (51.3%), that the US Navy should not be exempt from environmental regulations in time of peace (75.2%), and that sonar use should be moderated if it impacts cetaceans (75.8%). Individuals who were conservative, Republican, and have served in the military were more likely to believe the Navy should be exempt from marine mammal protection regulations. In addition, expert interviews were conducted to gain opinions on the potential ramifications of the recent US Supreme Court case on naval sonar mitigation.     

Subsurface structure of the volcanic centre of the České Středohoří Mountains, North Bohemia, determined by geophysical surveys

Former geophysical surveys performed in the region of the volcanic centre of the České Stř edohoří Mts. in North Bohemia (the Ohře Rift zone) showed that anomalous volcanic bodies and features can be effectively identified within sedimentary environment. For this reason we carried out new geophysical measurements in the area of the main mafic intrusion of essexitic character. The target was the exact location and geometry of the intrusion and its relation to other components of the volcanic centre. We used gravity, magnetic, shallow seismic and electromagnetic techniques. The new gravity and magnetic data were tied to the old databases so that we could investigate the area as a whole complex. Electromagnetic measurements were applied in the area of the expected extent of the intrusion, and the seismic measurements in the central part of the intrusion. Based on all the data, mainly on gravity modelling, we delineated not only the surface and subsurface extent of the intrusion, but we also defined the hidden relief of the intrusion. It was found that the intrusion is formed by a single body that has a few protrusions, and not by a set of separate individual intrusions, as indicated by surface outcrops. However, the body of the intrusion is affected by a major fault that caused lithological differences on both sides (essexite/monzodiorite). In detail we show the depth of the debris cover and the thickness of the weathered zone in the central part of the essexite body. We also derived indications of tectonic elements in the area of the intrusion in the main structural/tectonic direction in the region. The results will be utilized to establish a 3D geological model of the whole volcanic centre. This investigation may serve as an example of non-seismic geophysical exploration applied to the study of volcanic centres surrounded by sedimentary rocks.     

A geophysical survey on the archaeological site of Mugardos (NW Iberian peninsula)

Archaeological indications near Mugardos (Ferrol, NW Spain) suggest the existence of a Roman settlement. In fact, in the area were found pavements, walls with north-south and east-west orientations and some structures that endured heating. These remains are covered by soil, more than 1 m thick, and lie over schists. In order to determine the archaeological potential of the area and to delimit future excavations a geophysical survey, consisting of a joint resistivity and magnetic survey, was planned and carried out. The square array of electrodes was used and the data are discussed as apparent square array resistivity maps and azimuthal inhomogeneity ratio (AIR) maps. The magnetic survey included total field measurements using sensor heights of 0.30 and 2.30 m above the ground, so that a magnetic gradient could be computed.A combined interpretation of both resistivity and magnetic data is discussed. Later excavations have confirmed the geophysical interpretation.     

Delineation of a quick clay zone at Smørgrav,Norway, with electromagnetic methods under geotechnical constraints

In many coastal areas of North America and Scandinavia, post-glacial clay sediments have emerged above sea level due to iso-static uplift. These clays are often destabilised by fresh water leaching and transformed to so-called quick clays as at the investigated area at Smørgrav, Norway. Slight mechanical disturbances of these materials may trigger landslides. Since the leaching increases the electrical resistivity of quick clay as compared to normal marine clay, the application of electromagnetic (EM) methods is of particular interest in the study of quick clay structures.For the first time, single and joint inversions of direct-current resistivity (DCR), radiomagnetotelluric (RMT) and controlled-source audiomagnetotelluric (CSAMT) data were applied to delineate a zone of quick clay. The resulting 2-D models of electrical resistivity correlate excellently with previously published data from a ground conductivity metre and resistivity logs from two resistivity cone penetration tests (RCPT) into marine clay and quick clay. The RCPT log into the central part of the quick clay identifies the electrical resistivity of the quick clay structure to lie between 10 and 80 Ω m. In combination with the 2-D inversion models, it becomes possible to delineate the vertical and horizontal extent of the quick clay zone. As compared to the inversions of single data sets, the joint inversion model exhibits sharper resistivity contrasts and its resistivity values are more characteristic of the expected geology. In our preferred joint inversion model, there is a clear demarcation between dry soil, marine clay, quick clay and bedrock, which consists of alum shale and limestone.     

Role of Lithology and Subsurface structures detected by potential field data in controlling the radioactive mineral accumulation at Natash area,Eastern Desert,Egypt

Wadi Natash area is located in the southern part of the Eastern desert of Egypt. It has a great importance for containing accumulations from the radioactive minerals of Uranium, Thorium and Potassium. An integrated potential study was carried out on the study area with the aim of locating depths to causative bodies with sufficient magnetic susceptibility that may represent magmatic intrusions with relation to the radioactivity location and delineate the subsurface structures affecting the area. Both magnetic and Bouguer data as well as radiometric data were interpreted rapidly for source positions and depths using Euler deconvolution, Werner deconvolution and 3D modeling techniques. The results deduced from the trend analyses show that the major fault trend affecting the area have NNW-SSE (Red Sea–Gulf of Suez trend) direction intersected by the less predominant NNE-SSW(The Gulf of Aqaba–Dead Sea trend) and WNW-ESE (Najd Fault System) fault trends. The causative bodies were imaged at depths ranging from 0.3 km to about 1.5 km. The depths along the interpreted profiles display discontinuities in potential field markers due to presence of the NNW-SSE fault trends act as pass channels for the hydrothermal solutions.It can be stated that the radioactive mineral accumulations were caused by the hydrothermal solutions rich with radioactive minerals as a result of intruding Natash volcanic to the granitic rocks. The Qouseir clastics and the Nudian sand stone were affected by these solutions and show a positive response for the radioactive minerals.     

保山隆阳5.0级地震永昌城区房屋震害统计与分析

对保山隆阳5.0级地震中,保山市永昌城区14个街区、占房屋建筑总面积的47%的房屋震害进行了调查,获得了调查区各类房屋震害统计资料。将这些资料与城区工程地质条件、抗震防灾规划等资料进行对比和分析,结果表明,大多数震害调查结果与预测结果基本一致。     

Anomalous magnetic field of the Lomonosov Rise according to data of the north pole 19 drifting station

The technique for determining the normal and anomalous magnetic fields from magnetovariation observations has been applied to the measurements on the SP19 drifting station. As a result, the regional profile of three magnetic field components has been obtained to an accuracy considerably exceeding the accuracy of other methods of marine magnetic survey. These data make it possible to estimate the geometrical parameters of the Earth’s crust blocks composing the Lomonosov Rise.     

New magnetic data of the southern East China Sea and some geological interpretations

In September and October of 2011, a marine magnetic survey was conducted in the southern East China Sea between 120° and 126°E, 26° and 28°N with the research vessel Kexue-3. New magnetic maps with 5 × 5 km grid spacing were generated after meticulous data processing. Based on an analysis of magnetic anomaly features and further calculation results, including the total horizontal derivatives (THD), second order vertical derivatives (SVD) and analytic signal amplitude (ASA), several new tectonic insights on magnetic sources, basement structures and fault properties were obtained. The NE-SW or NNE-SSW oriented magnetic anomaly highs have different sources: (1) The high amplitude, short wavelength magnetic anomalies of the Zhemin Uplift are caused by shallow buried igneous rocks intruded along NE-NEE oriented faults. (2) The high amplitude, moderate wavelength magnetic anomalies of the Diaoyudao Uplift are caused by the metamorphic basement intruded by magmatic rocks. (3) The magnetic sources of the Yandang Uplift and Taibei Uplift in the shelf basin are shallow buried metamorphic basements and deep buried volcanic bodies, respectively. Several NW-SE or NWW-SEE oriented dextral strike-slip fault belts were identified as important tectonic boundaries. Each is composed of several en echelon and partly overlapping secondary faults. Initially formed in the Cretaceous, these fault belts have evolved through multiple periods, propagated seaward with the migration of the basement rifting and accommodated local stress fields in the Cenozoic.     

Comparative review of marine mammal guidance implemented during naval exercises

This article reviews the types and effectiveness of marine mammal mitigation measures used during some naval activities worldwide. The three main standard methods used to mitigate the potential impacts of naval sonar sound on marine mammals are (1) time/area planning (of exercises/active sonar use) to avoid marine mammals; (2) implementation of operational procedures (e.g. ‘soft start’ - where sound levels are gradually increased over time); and (3) monitoring of animals for the purpose of maintaining an ‘exclusion zone’ around the sound source. Suggestions towards a minimum worldwide mitigation standard are made.     

Design,modelling and imaging of marine seismic swarm surveys

Autonomous marine vehicles instrumented with seismic sensors allow for new efficient seismic survey designs. One such design is the swarm survey, where a group, or swarm, of slow moving autonomous marine vehicles record seismic data from shots fired by a source vessel sailing around circles within the swarm. The size of the swarm is dictated by the maximum offset requirement of the survey, and it can be shaped to acquire wide‐ and full‐azimuth data. The swarm survey design equation describes the relationship between the source and receiver positions of the survey and the subsurface coverage or fold. It is used to adapt the swarm to the seismic survey requirements and to calculate survey duration time estimates as function of available equipment. It is shown that a survey conducted by a slowly moving swarm requires six times fewer shots than an equivalent seabed node survey conducted over 85.5 km². Swarm surveys can also be adapted to efficiently conduct infill surveys and replace multi‐vessel undershoots. The efficiency of the survey can further be increased when the autonomous marine vehicles are towing short streamers with multiple receivers. Synthetic tests show that the seismic images for swarm surveys are comparable to those from streamer surveys, while little variation in image quality is found when reducing the number of autonomous marine vehicles but equipping them with a short streamer with multiple receivers.     

A STRATIGRAPHIC CASE HISTORY USING THREE-DIMENSIONAL SEISMIC DATA IN THE GULF OF THAILAND *

A three-dimensional marine seismic survey was conducted in the Gulf of Thailand to aid in the development of a gas field indicated by three wildcat wells. The results and interpretation reported previously demonstrated improved fault resolution and better structural definition. Five successful appraisal wells have now been drilled, and these show that most of the sands have limited extent. Widespread character changes in the seismic data also support stratigraphic variations in many of the sands. Several new methods of 3D stratigraphic interpretation have been developed while investigating the depositional history of this area. Anomalous seismic amplitudes, tied to sands penetrated by wells and mapped from Seiscrop^TM horizontal sections in time and depth, have indicated the distribution of bars and channels. Horizon Seiscrop sections, each sliced through a single bed, have been used to delineate these depositional features directly. G-LOG^TM sections, displaying seismic logs derived by rigorous wave equation inversion, confirm the existence of these features. Sands greater than 10 m thick have proved mappable.     

Geophysical study of the Peinan Archaeological Site,Taiwan

The Peinan archaeological site is the most intact Neolithic village with slate coffin burial complexes in Taiwan. However, the area that potentially contains significant ancient remains is covered by dense vegetation. No reliable data show the distribution of the ancient village, and no geophysical investigation has been performed at this site. To evaluate various geophysical methods under the geological setting and surface condition of the site, the physical properties of the remains were measured and four geophysical methods involving magnetic, electromagnetic (EM), electrical resistivity tomography (ERT), and ground-penetrating radar (GPR) were tested along three parallel profiles. The results imply that the EM and magnetic methods are much cost-effective and suitable for investigating the entire area. GPR and ERT methods can provide high resolution subsurface image, which are much suitable for subsequently detail investigation.The EM and magnetic surveys were thus conducted over the entire Peinan Cultural Park to understand the distribution of the ancient building remains at the Peinan site. The results of this study were verified by subsequent excavations, which indicate that the EM survey was successful in delineating the majority of the ancient village because the basements of building are highly resistive in comparison to the background sediment. The results of this investigation suggest that the ancient village was broadly distributed over the eastern part of the Peinan Culture Park and extended to the southeast.     

Propagation of an accreting plate boundary: a discussion of new aeromagnetic data in the Gulf of Tadjurah and southern Afar

A detailed aeromagnetic survey of the Republic of Djibouti and immediate surroundings was performed in 1977. This paper summarizes the reduction techniques which are used in order to produce a magnetic anomaly map and discusses the accuracy of this map, which is presented as an insert at a scale of 1/250,000. Two distinct magnetic styles are recognized: linear anomalies with both large amplitude and short wavelength, considered to be typical of oceanic lithosphere, contrast with areas of lower-amplitude longer-wavelength anomalies, which are found mostly in the northern part of the survey. This quiet zone of subdued magnetic style is thought to have undergone major tectonic deformation in the last millions of years. The general morphology of magnetic anomalies is interpreted in terms of a propagating crack model, as proposed by Courtillot [23]. The crack propagates westwards at approximately 3 cm/yr and the crack tip is thought to lie close to Lake Asal, both on the basis of the magnetic data and of other geophysical evidence. The land section of the survey is a central topic of this paper and is interpreted in terms of the crack propagation model in the light of other available geological, geochemical and geophysical data.     

Damage to residential buildings in Hveragerði during the 2008 Ölfus Earthquake: simulated and surveyed results

This study analyses the performance of residential buildings in the town of Hveragerði in South Iceland during the 29 May 2008 Mw 6.3 Ölfus Earthquake. The earthquake occurred very close to the town, approximately 3–4 km from it. Ground shaking caused by the earthquake was recorded by a dense strong-motion array in the town. The array provided high-quality three-component ground acceleration data which is used to quantify a hazard scenario. In addition, surveys conducted in the town in the aftermath of the earthquake have provided information on macroseismic intensity at various locations in the town. Detailed information regarding the building stock in the town is collected, and their seismic vulnerability models are created by using building damage data obtained from the June 2000 South Iceland earthquakes. Damage to buildings are then simulated by using the scenario hazard and vulnerability models. Damage estimates were also obtained by conducting a survey. Simulated damage based on the scenario macroseismic intensity is found to be similar to damage estimated from survey data. The buildings performed very well during the earthquake—damage suffered was only 5 % of the insured value on the average. Correlation between actual damage and recorded ground-motion parameters is found to be statistically insignificant. No significant correlation of damage was observed, even with macroseismic intensity. Whereas significant correlation was observed between peak ground velocity and macroseismic intensity, neither of them appear to be good indicators of damage to buildings in the study area. This lack of correlation is partly due to good seismic capacity of buildings and partly due to the ordinal nature of macroseismic intensity scale. Consistent with experience from many past earthquakes, the survey results indicate that seismic risk in South Iceland is not so much due to collapse of buildings but rather due to damage to non-structural components and building contents.     

Problems associated with quantitative magnetic sourcing of sediments of the scarborough to mablethorpe coast,Northeast England,U.K.

There are many problems associated with beach tracing and sand provenance in coastal studies over short and long timescales. Here, an attempt has been made to trace the source sediments of the beaches along the Holderness and Humberside coastline of northeast England, between Scarborough and Mablethorpe, using rapid field and laboratory magnetic techniques. The methodology employed involves classification and modelling of sediment sources and mixtures based on their magnetic properties. Focus is given to a strong ferrimagnetic black sand (heavy mineral) which dominates the magnetic signal of the beach mixtures. Delimiting the spatial extent, quantifying and tracing of this mineral may give a good indication of wave energy and sediment movement processes along this coastline. This paper represents results obtained during a short-time-scale survey.     

Geophysical investigations of the Olonium Roman site (Northern Como Lake)

The study area is located at S. Agata (Gera Lario), a small center at the northern end of Como Lake, near the junction of Valchiavenna and Valtellina Valleys. This site played a strategic role since ancient times, providing the control on the communications routes to both the Como Lake and the Spluga and Septimer alpine passes. Since the end of the last century archaeological findings are reported in literature, also supported, from the early XI century, by archival documents confirming the existence of the `Olonium' settlement, an administrative and fiscal center of primary importance, as well as a parish amongst the most influential in the Como Lake area. Within an area of 45,000 m² an electrical survey has been carried out in conjunction with magnetic and GPR investigations. These studies have indicated the presence of a number of sub-areas characterized by significant anomalies defined by the overlapping of the results obtained from two or more geophysical methods. In two of such sub-areas, excavation tests have been conducted, which have brought to light a number of archaeological findings of interest. In one of the two sub-areas, which is characterized by the superimposition of electrical and radar anomalies, a deposit of large pebbles has been found. The origin of this deposit has not been ascertained, whether it is of fluvial origin, related to the deviation of the Adda river in the Pian di Spagna region in Roman times, or it is part of reclamation works, still of Roman times, of paleolacustrine marshes. The overlapping stratigraphy, however, suggests the development of fluvial channels between Roman and Low-Medieval times. In the other sub-area, excavations were carried out on sites defined by electrical and radar anomalies, and confirmed by the results from magnetic survey. The excavations brought to light, below the fluvial deposits, a large medieval edifice, which could be identified as the S. Stefano church abandoned in 1444. The church is built on earlier structures, amongst which an apse with a single aisle has been interpreted as a paleochristian sacellum.     

Geoprospecting Survey in the Archaeological Site of <Emphasis Type="Italic">Aquinum</Emphasis> (Lazio,Central Italy)

In this paper, the results of a ground penetrating radar (GPR) prospecting survey performed in the Roman archaeological site of Aquinum are shown. The prospecting was performed in two distinct areas. The first one was close to the remains of the ancient thermal baths, where a public project intends to build a car park. The second area is close to an apsidal Roman building, and it was of interest to establish whether, and possibly up to what extent, the remains of this partially revealed structure continue, or maybe whether meaningful further structures are present.     

A gravity study of the Island of vulcano,tyrrhenian sea,Italy

A detailed gravity survey was carried out on the island of Vulcano, Aeolian Islands, Italy. Gravity was measured on 107 stations and the Bouguer anomalies were computed by assuming geological densities. Aim of this survey was to complete the island structural pattern relatively to the shallower structures. Separation of the gravity anomaly field was carried out by means of data filtering, and two main components were discerned. The λ>2.2 km wavelength component, filtered out of the longer wavelength components, was interpreted quantitatively along a NW profile. The best fitting model consists of an upper layer of recent pyroclastic products (p=2.1 g/cm³) lying upon a highly compacted pyroclastic series or lavas (p=2.4 g/cm³). The shorter wavelength residual gravity field (λ<2.2 km) is characterized by two anomalies, located on Vulcanello and the «Fossa di Vulcano» crater. Vulcanello anomaly could be interpreted, given the geothermal state of the area, as due to an increase of the rock density consequent to propylization processes by high temperature fluids (T>200°C). «Fossa di Vulcano» anomaly is instead attributable to the local volcanic chimney. A schematic comprehensive model of Vulcano is also presented, which accounts for the available main geological and geophysical data.