Analysis Of Magnetic Anomalies Research Articles

Oceanic plateau formation by seafloor spreading implied by Tamu Massif magnetic anomalies

Tamu Massif is an immense Mesozoic submarine volcano, the main edifice of the Shatsky Rise oceanic plateau. It is located at a spreading ridge triple junction, but considered to be a shield volcano formed by effusive volcanism from an emerging mantle plume. However, it is unclear how Tamu Massif eruptions interacted with the spreading ridges, which are enormous linear volcanoes themselves. Here we create a magnetic anomaly map for Tamu Massif, which can provide clues about crustal formation. For Tamu Massif, we find dominantly linear magnetic field anomalies caused by crustal blocks of opposite magnetic polarity. This pattern suggests that Tamu Massif is not a shield volcano, but was emplaced by voluminous, focused ridge volcanism. If the magma source at the Shatsky Rise was a plume, it was closely connected to and controlled by seafloor spreading. By implication, even the largest oceanic plateau edifices can be formed by seafloor spreading. We suggest that the widely accepted analogy between continental flood basalts and oceanic plateaus requires reconsideration. The Shatsky Rise oceanic plateau formed by spreading ridge volcanism, according to analyses of linear magnetic anomalies over the Tamu Massif submarine volcano.

Radiometric Surveys for Detection of Uranium in Dhofar Region, Sultanate of Oman

The Proterozoic basement of the Dhofar region of the Sultanate of Oman might be considered as a potential source of Uranium mineralization like other Proterozoic basins of the world, which collectively contribute 30%-40% to the global production. Uranium exploration assumes great importance in this part of the world and therefore geochemical and geophysical exploration for Uranium was initiated. Initial, random sampling of the study area with Gamma ray spectrometry as well as in situ XRF (Xray Florescence) analysis indicated the presence of around 30-40 ppm of Uranium. Since Gamma ray spectrometry results in the calculation of the heat produced during radioactive decay of Potassium (K), Uranium (U) and Thorium (Th) within the rocks in the study area, measurements were acquired along eleven traverses of varying length from 100-250 m at a traverse interval of 20 m and sampling interval of 10 m. Individual plots of the concentrations of radioelements such as U, K and Th have shown favorable anomalous radioactive sources. Besides this, Uranium to Thorium (eU/eTh) and Uranium to Potassium (eU/K) ratio plots have been presented and the alteration zones associated with Uranium mineralization through all traverses have been identified. Further, the generated composite ternary image based on the combination of K, Th and U from Gamma ray spectrometry determines different relative amounts of radioelements and paves the way for deciphering the level of radioactivity in the study area. However, there seems to be no presence of strong Uranium anomalies in the near surface of the limited study area. As the study area is heavily faulted, the depth to concealed subsurface fault structures is estimated to be around 64 m based on Hartley spectral analysis of total magnetic anomaly (line-6).

Contrastive analysis of gravity and magnetic anomalies between North China Craton and Indian Shield

The evolution correlation between the North China Craton (NCC) and the Indian Shield (IND) is very important for plate reconstruction. Recently, more and more research results show that the NCC and IND had been pieced together in the Paleoproterozoic. In order to analyse the correlation between the NCC and IND, we use the latest gravity and magnetic data to study deep geophysical characteristics of the NCC and IND respectively, and we also use a two‐dimensional discrete wavelet transform method to discuss the division scheme of the basement tectonic unit in the Paleoproterozoic at the point of gravity and magnetic field. On the basis of gravity and magnetic anomalies, the NCC is divided into the Eastern Block, Western Block, and the Trans‐North China Orogen, which are further divided into the secondary tectonic zones and blocks. The IND is divided into the Southern Block, the Northern Block, and the Central Indian Tectonic Zone, and they also are further divided into secondary tectonic zones and ancient cratons. On the basis of that, according to the four Paleoproterozoic plate assemblage schemes proposed by predecessors, we compared the geophysical characteristics and the correlation through the regional gravity and magnetic anomalies between the two continents. The results show that the faults and tectonic boundaries in the Trans‐North China Orogen of the NCC correspond well to those in the Central Indian Tectonic Zone of IND. Moreover, the gravitational and magnetic anomalies in the Eastern Block of the NCC are quite similar to those in the Southern Block of IND, which shows that the NCC and IND might have been pieced together during the Paleoproterozoic. The geophysical result that verifies the correlation between the NCC and IND based on gravity and magnetic fields provides the evidence for Paleoproterozoic plate reconstruction and constrains the assembling patterns between the NCC and the IND.

Magnetic field modelling and analysis of uncertainty in archaeological geophysics

AbstractOne of the major problems in the forward modelling of magnetic anomalies is the assessment of a minimum level of acceptable accuracy in the fit between observed and theoretical anomalies. We present a new approach to the analysis and interpretation of archaeological magnetic anomalies, based on classical algorithms of forward modelling and a new technique of error assessment. This approach allows us to determine geometry, physical properties, and location of buried archaeological features, as well as the occurrence of fires or other historical events that may have affected the observed magnetic signal. Our method starts from the acquisition of total field data, usually in a regular grid arrangement, and proceeds through their reduction to archaeological magnetic anomalies. This reduction is performed subtracting from the observed total field data a polynomial representation of the regional field, on the basis of a rigorous criterion that tries to separate archaeological anomalies from geological (crustal) contributions. At the next step, a map of the maximum allowed misfit is built, which depends from the estimated uncertainty at each point of the magnetic anomaly field. This map specifies the maximum allowed deviation of theoretical anomalies from the observed values. The last step is the analysis of these anomalies through a new forward modelling tool, with the objective to reconstruct the three‐dimensional arrangement of buried features and possibly obtain some information about the history.

Ridge propagation, oceanic core complexes, and ultramafic-hosted hydrothermalism at Rainbow (MAR 36°N): Insights from a multi-scale magnetic exploration

A high-resolution magnetic survey conducted over the apparently inactive ultramafic-hosted hydrothermal site Clamstone reveals a positive magnetic anomaly, in accordance with the magnetic response of other ultramafic-hosted sites known to date. Here this magnetic signature is mainly the result of in-place bulk-magnetized serpentinite underlying the site rather than any concentration of magnetite within the stockwork zone, the latter being probably negligible due to the limited fluid venting. Conversely, the nearby high-temperature active ultramafic-hosted hydrothermal site Rainbow illustrates the key contribution of hydrothermal activity in producing large amounts of magnetite in stockwork zones within ultramafics. At a larger scale, we propose from the analysis of sea-surface magnetic anomalies that the underlying Rainbow Hill has been formed as an oceanic core complex associated with the Amar Minor North segment, later dismantled by the northward propagation of the Amar Minor South segment and the coeval recession of the Amar Minor North segment. This model solves the apparent paradox of a high-temperature hydrothermal site sitting on an oceanic core complex usually associated with cold, poorly magmatic spreading environments: the core complex belongs to the cold, receding Amar Minor North segment whereas the heat fueling site Rainbow comes from the hot, propagating Amar Minor South segment. It also explains why the Rainbow Hill, although interpreted as an oceanic core complex, lacks the typical corrugations of such core complexes – the latter have been dismantled by the deformation associated with the northward propagation of the non-transform offset.

Variations of Earth Magnetic Field Intensity for the Past 5 Myr Derived From Marine Magnetic Anomalies in a Slow‐to‐Intermediate Spreading South Atlantic Ridge

AbstractWe investigated sea‐surface magnetic anomaly data for the past 5 Myr in the slow‐to‐intermediate spreading South Atlantic Ridge. After error analysis and signal processes, the sea surface magnetic anomaly profiles were downward continued to 1.5 km below sea level, and then the distance from spreading axis was converted to time by matching the observed anomalies with forward models of known polarity reversals. The stacked anomaly profiles from South Atlantic Ridge exhibit short‐wavelength wiggles superimposed on the first‐order variations of magnetic anomalies. The stacked anomaly profiles were first compared with commonly used geomagnetic palaeointensity data synthesized from sedimentary relative intensity records in the past and were further correlated with other available magnetic profiles derived from Central Indian Ridge, East Pacific Rise, and Pacific‐Antarctic Ridge. We found that short wavelength variations in the South Atlantic Ridge can be ascribed to geomagnetic intensity variations from excursions or short polarity events. Spectral analysis of marine magnetic anomalies records reveals that there is no obvious dominant periodicity for the geomagnetic field behavior during the past 5 Myr.

Subsurface Structure Determination of Geotermal Area in Siogung-ogung Samosir District by Using Magnetic Method

Underwater research often uses geomagnets. It is one of the geophysical methods for measuring magnetic field variations. This research was done to identify how the subsurface rock structure is and determine kinds of rock based on its susceptibility value in Siogung-ogung geothermal area, Pangururan, Samosir District. The tool measurement of total magnetic field called Proton Precission Magnetometer, positioning using Global Position System, and north axis determination using geological compass. Data collection was done randomly with total 51 measuring points obtained. Data analysis started with International geomagnetics Reference Field correction to obtain the total magnetic field anomaly. Then, the data analysis of total magnetic anomaly was done by using surfer program 12. To get a magnetic anomaly cross section used Magdc For Windows program. Magnetic measurement results indicated that the variation of magnetic field strength in each point with the lowest magnetic intensity value of 41785.67 nano tesla. The highest magnetic intensity value is 43140, 33. From the results of qualitative interpretation, the magnetic anomaly value is at -200.92 to 1154.45 whereas the quantitative interpretive results of model show the existence of degradation and andesitic rocks, with the value of susceptibility

Analysis of the lithospheric magnetic anomalies and tectonics in continental China and the adjacent regions using CHAMP satellite data

In this paper, the CHAMP satellite data from the last decade are used to map the lithospheric magnetic field in continental China at altitudes of 300 and 400 km using the spline function model technique. The results show that our model has higher amplitude than the spherical harmonic magnetic field model MF7. The resulting lithospheric anomaly distribution of continental China agrees with the geotectonic structure and crustal thermal structure. The magnetic anomaly distribution clearly indicates extrusion of the Tibetan Plateau towards the northeast and northwest directions. Northward movement is impeded by the relative rigid Tarim Basin featured with positive magnetic anomalies. The positive magnetic anomalies in Tarim basin extend to the northern Tibetan Plateau, suggesting that the southern part of Tarim Basin underthrusted the northern Tibetan Plateau.

Global Centroid Distribution of Magnetized Layer From World Digital Magnetic Anomaly Map

Global distribution of the centroid of magnetic layer, Zo, was estimated from spectral analysis of aeromagnetic and marine magnetic anomalies using recently released new global digital magnetic anomaly data, World Digital Magnetic Anomaly Map (WDMAM) version 2.0. Zo distribution provides a comprehensive view of regional‐scale trends of lithospheric thickness; Zo is generally shallower in oceanic ridges and deeper older continents, which is consistent with previous works. Zo is closely related to reasonable average thermal regime, and its correspondence with tectonic regime indicates that Zo is useful to delineate regional crustal thermal structure. It is expected that Zo combined with multidisciplinary data should help to infer geophysical and geological information in less explored regions.

Dyke emplacement and crustal structure within a continental large igneous province, northern Barents Sea

Abstract We perform an integrated analysis of magnetic anomalies, multichannel seismic and wide-angle seismic data across an Early Cretaceous continental large igneous province in the northern Barents Sea region. Our data show that the high-frequency and high-amplitude magnetic anomalies in this region are spatially correlated with dykes and sills observed onshore. The dykes are grouped into two conjugate swarms striking oblique to the northern Barents Sea passive margin in the regions of eastern Svalbard and Franz Josef Land, respectively. The multichannel seismic data east of Svalbard and south of Franz Josef Land indicate the presence of sills at different stratigraphic levels. The most abundant population of sills is observed in the Triassic successions of the East Barents Sea Basin. We observe near-vertical seismic column-like anomalies that cut across the entire sedimentary cover. We interpret these structures as magmatic feeder channels or dykes. In addition, the compressional seismic velocity model locally indicates near-vertical, positive finger-shaped velocity anomalies (10–15 km wide) that extend to mid-crustal depths (15–20 km) and possibly deeper. The crustal structure does not include magmatic underplating and shows no regional crustal thinning, suggesting a localized (dyking, channelized flow) rather than a pervasive mode of magma emplacement. We suggest that most of the crustal extension was taken up by brittle–plastic dilatation in shear bands. We interpret the geometry of dykes in the horizontal plane in terms of the palaeo-stress regime using a model of a thick elastoplastic plate containing a circular hole (at the plume location) and subject to combined pure shear and pressure loads. The geometry of dykes in the northern Barents Sea and Arctic Canada can be predicted by the pattern of dilatant plastic shear bands obtained in our numerical experiments assuming boundary conditions consistent with a combination of extension in the Amerasia Basin sub-parallel to the northern Barents Sea margin and a mild compression nearly orthogonal to the margin. The approach has implications for palaeo-stress analysis using the geometry of dyke swarms.

Identifikasi Jalur Sesar Opak Berdasarkan Analisis Data Anomali Medan Magnet dan Geologi Regional Yogyakarta

<p>This research was aimed to know the magnetic field anomaly pattern upon Opak’s fault, to identify the direction of Opak’s fault and the structure of Opak’s fault based on Geomagnetic data. It was conducted in the vicinity of Opak’s fault in Pundong sub-district, Yogyakarta. It used magnetic method which is based on rocks magnetic susceptibility. The data were taken by G-856AX Proton Precision Magnetometer (PPM) using looping method. Data analysis was done using diurnal variation and IGRF correction, reduction to pole, upward continuation and under surface structure modelling. From magnetic field anomaly analysis, it was discovered that magnetic field anomaly on Opak’s fault has -50 nT negative closure in the East, and 100 nT positive closure in the West. According to the result interpretation of 2.5D model on two slices using Mag2DC software, it can be obtained that the under surface structure of Opak’s fault consist of Young Alluvial formation, Nglanggran formation, Semilir formation, Kebo Butak formation, and Wungkal Gamping with various thickness. The location of this Opak’s fault is almost the same with the location of Opak’s fault in Yogyakarta geology map. Opak’s fault around Pundong sub-district is a normal fault directed to N 35º E. The eastern part of the fault has a relatively constant movement, while the western part is relatively moving down.</p>

Curie Point Depths in North China Craton Based on Spectral Analysis of Magnetic Anomalies

Magnetic anomaly analysis is an important method to study the structure of deep crust. With the assumption of random magnetization sources, the structure of a magnetic layer can be inverted via spectral analysis. Curie point depth (CPD), the depth at which rocks lose their ferromagnetic properties, is the bottom of a magnetic layer. In this study, we estimate from the magnetic anomaly data of EMAG2 dataset the CPD and the top of the magnetic layer in North China. With a moving window of 180 km × 180 km, we calculate the average top and centroid depth of the magnetic layer in each window and determine the regional CPD distribution across North China. The CPD of North China varies from 18 to 32 km. In addition, the CPD in the western part of the North China Craton is deeper than that in the eastern part. The shallowest CPD is located near the Bohai Sea. When compared to available heat flow data, the estimated CPD values are consistent with thermal conductivity of 1.8–3.2 Wm−1 K−1 and on heat production value of 0.4–1.3 µWm−3.

Spherical cap harmonic analysis of regional magnetic anomalies based on CHAMP satellite data

We used CHAMP satellite vector data and the latest IGRF12 model to investigate the regional magnetic anomalies over mainland China. We assumed satellite points on the same surface (307.69 km) and constructed a spherical cap harmonic model of the satellite magnetic anomalies for elements X, Y, Z, and F over Chinese mainland for 2010.0 (SCH2010) based on selected 498 points. We removed the external field by using the CM4 model. The pole of the spherical cap is 36N° and 104°E, and its half-angle is 30°. After checking and comparing the root mean square (RMS) error of ΔX, ΔY, and ΔZ and X, Y, and Z, we established the truncation level at K max = 9. The results suggest that the created China Geomagnetic Referenced Field at the satellite level (CGRF2010) is consistent with the CM4 model. We compared the SCH2010 with other models and found that the intensities and distributions are consistent. In view of the variation of F at different altitudes, the SCH2010 model results obey the basics of the geomagnetic field. Moreover, the change rate of X, Y, and Z for SCH2010 and CM4 are consistent. The proposed model can successfully reproduce the geomagnetic data, as other data-fitting models, but the inherent sources of error have to be considered as well.

Shallow geophysical imaging of the Olympia anomaly: An enigmatic structure in the southern Puget Lowland, Washington State

Gravity and magnetic anomalies suggest that the Olympia structure beneath the southern Puget Lowland (western Washington State, U.S.) vertically displaces Eocene Crescent Formation strata. Northeast of the Olympia structure, middle Eocene Crescent Formation is beneath 4–6 km of Paleogene–Neogene and Quaternary strata of the Tacoma basin, whereas the Crescent Formation is exposed at the surface immediately to the south. Although numerous marine seismic reflection profiles have been acquired near the surface location of the Olympia structure as defined by potential field anomalies, its tectonic character remains enigmatic, in part because inlets of southern Puget Sound are too shallow for the collection of deep-penetration marine seismic profiles across the geophysical anomalies. To supplement existing shallow-marine data near the structure, we acquired 14.6 km of land-based seismic reflection data along a profile that extends from Crescent Formation exposed in the Black Hills northward across the projected surface location of the Olympia structure. The reflection seismic data image the Crescent bedrock surface to ∼1 km depth beneath the southern Tacoma basin and reveal the dip on this surface to be no greater than ∼10°. Although regional potential field data show a strong linear trend for the Olympia structure that implies folding over a blind thrust and/or bedrock juxtaposed against a weakly to nonmagnetic sediment section, high-resolution magnetic anomaly analysis along the land-based profile suggests that the structure is more complex. Overall, seismic and potential-field profiles presented in this study identify only minor shallow faulting within the projected surface location of the Olympia structure. We suggest that the mapped trace of the Olympia structure along the northern flank of the Black Hills, at least within the study area, is constrained by juxtaposed normal and reversely magnetized Crescent Formation units and minor tectonic deformation of Crescent Formation bedrock.

An analysis of crustal magnetic anomaly and Curie surface in west Himalayan syntaxis and adjacent area

In this paper, the magnetic model NGDC-720-V3 is used to investigate the distribution of crustal magnetic anomaly and the depth of Curie surface in west Himalayan syntaxis and its adjacent area. The Curie surface is compared with regional heat flow, the Moho, and seismicity. The results show that the magnetic anomaly and Curie surface are both consistent with the regional tectonic setting. Tarim craton, Tadjik Basin, and Indian Plate have positive magnetic anomaly and a Curie surface deeper than 36 km, corresponding to low heat flow there. In contrast, orogenic belts such as west Himalayan syntaxis, Tianshan Mountain, Afghanistan, and Tibetan Plateau have negative anomaly and a Curie surface shallower than 32 km, corresponding to high heat flow. The north of the syntaxis presents a positive anomaly on the surface, resulting from Ferghana Basin, while the negative anomaly in large depth is probably associated with the subduction of the slab and the resulting heat. The depth of the Curie surface has an inverse relationship to the Moho depth, which is attributed to temperature and isostatic balance. The distribution of earthquakes with epicenter deeper than 100 km clearly suggests the subduction of the Indian plate deep down to the asthenosphere and the deep earthquakes were caused by shear motion within the subducting slab.

A Comprehensive View from Geophysical Signatures over Chitradurga Schist Belt, Karnataka

Abstract Geology of Chitradurga Schist Belt (CSB) from Gadag to Srirangapatna, as inferred from the gravity and magnetic data analysis and supportive evidences from digitized Airborne Total count map is presented in this paper. Significant inferences have emerged from the integrated geophysical analysis coupled with geological information. Pattern, continuity and intensity of anomalies indicate that CSB can be viewed as three sectors; northern (Gadag), central (Chitdradurga) and southern (Nagamangala). Qualitatively geophysical signatures are diagnostic for the purpose of broad classification of the area into different geological domains such as granite-gneissic (gravity lows, fluctuating magnetic values and moderate radiometric counts), metavolcanic (gravity highs, magnetic highs and radiometric lows), metasedimentary (moderate gravity low, low magnetic values and moderate radiometric values) and granitic (gravity low, magnetic high and radiometric high) regions. Images prepared from gravity, magnetic and radiometric values indicated possibility of an additional arm of the schist belt from Huliyar to Hosdurga between the known Kibbanahalli and Chitradurga arms of CSB. From the gravity image it appears that the eastern margin of CSB may be encompassing Bukkapatna granite in the central sector of CSB. Gravity low closures and gravity high gradient between the granites of Chitradurga and those near Jagalur suggest different nature of geological continuity. Structural features, particularly the sheared contacts of metavolanics with metasedimentaries are demarcated based on gravity gradients. Based on these inferred features coupled with known geological controls of mineralization, a few target areas for gold / base metal exploration are identified. A major structural feature affecting the strike of arms of CSB may be the possible regions for emplacement of ultramafic bodies as observed near Sasival and Ghattishosalli. This in turn is significant for mineral exploration in Chitradurga Schist belt. Gravity and magnetic profiles drawn across CSB for every 15’ latitude indicated crustal architecture beneath CSB. Significant rise is noticed in the background gravity field from the arms of CSB to the main schist belt suggesting that the Bababudan volcanics have generated from different depths compared to the Chitradurga volcanics. Further, long wavelength magnetic anomaly coinciding with the eastern margin of schist belt suggests the differences in nature of crust on either side. Frequency analysis of gravity and magnetic anomalies and the modeling indicated the depth persistence of geological units.

Efficient Mining Maximal Subspace Differential Co-expression Patterns in Matrix Datasets: a General Earthquake Analysis Approach

The electromagnetic anomaly observations before earthquake, have been confirmed by many cases of strong earthquakes. The analysis of earthquake magnetic anomaly is an effective approach for seismo-precursor detection. Traditional frequent mining methods for electromagnetic matrix datasets analysis often find the co-related items. However, these methods may miss the items which are differential co-related patters under different datasets. Mining these differential co-related patterns is more valuable for inferring potential knowledge. In this paper, we develop an algorithm, MSPattern, to mine maximal subspace differential co-expression patterns. MSPattern constructs a weighted undirected item-item relational graph firstly. Then all the maximal co-related patterns would be mined using item-growth method in above graph. MSPattern also utilizes several techniques for producing maximal patterns without candidate patterns maintenance. Evaluated by real electromagnetic matrix datasets and the gene expression datasets, the experimental results show our algorithm can find some potential knowledge for earthquake analysis, and MSPattern algorithm is more efficient than traditional ones. The performance of MSPattern is also evaluated by empirical p-value and gene ontology, the results show our algorithm can find statistical significant and biological differential co- expression patterns.

Quantitative interpretation of magnetic anomalies from bodies approximated by thick bed models in complex environments

Magnetic prospecting is a rapid and inexpensive geophysical tool and one of the most widely used methods for geophysical prospection throughout the world. However, the noise factors—such as inclined magnetization, complex geological structure of investigated areas, and uneven terrain relief—strongly obscure the interpretation of observed magnetic anomalies. The developed methodology of magnetic anomalies’ interpretation from models of thin beds and horizontal circular cylinders (spheres) in complex environments (oblique magnetization, rugged relief and unknown levels of the normal magnetic field) using improved versions of characteristic points and tangents was presented in detail in the author’s previous publications. However, many geological targets have a geometrical form of thick beds and thin horizontal plates and intermediate forms that crop up between these two models. In this paper, a methodology for interpreting magnetic anomalies produced by thick bed models in complex environments is explicitly described. It shows that quantitative analysis of magnetic anomalies due to intermediate targets could be successfully carried out using the methodology developed for the thick bed model. In the case of a thin horizontal plate with a large horizontal size, two distinct anomalies (from the left and right ends) may be interpreted as anomalies from thin beds. The interpretation methodology was successfully tested, both on typical models and on real geological targets. It was concluded that these methods could be effectively applied for quantitative analysis of magnetic surveys for geological–geophysical mapping, archeological target delineation, ore body searching, revealing oil and gas traps, and solving other geological and environmental problems.

Quantitative Interpretation of Magnetic Anomalies from Thick Bed, Horizontal Plate and Intermediate Models Under Complex Physical‐Geological Environments in Archaeological Prospection

AbstractMagnetic prospecting is one of the most widely used methods for archaeological prospection in the world. Noise both of natural [main factors are inclined magnetization, complex geological (archaeological) structure of investigated sites, and uneven terrain relief] and artificial origin (different iron‐containing targets, electric power lines, etc.) strongly obscure interpretation of observed magnetic anomalies. For quantitative analysis of magnetic anomalies produced by archaeological targets under aforementioned conditions a non‐conventional interpreting system has been developed. Methodology of magnetic anomalies interpretation from models of thin bed and horizontal circular cylinder (sphere) in conditions of oblique magnetization, rugged relief and unknown level of the total magnetic field by the use of improved versions of characteristic point and tangents has been earlier suggested. However, many archaeological targets have geometrical form of thick bed, thin horizontal plate and intermediate between these two models. In this paper methodology of magnetic anomalies produced by thick bed models in complex environments is explicitly described. It is shown that quantitative analysis of magnetic anomalies due to intermediate (between the thick bed and thin horizontal plate) targets could be successfully carried out by the use of methodology developed for the thick bed model. In the case of thin horizontal plate with a large horizontal extent, the two measured anomalies may be interpreted as anomalies from thin beds. The interpretation methodology was successfully tested both on typical models and on real archaeological targets with some success. Copyright © 2015 John Wiley & Sons, Ltd.

Geophysical surveys to help map buried igneous intrusions, Snowdonia, North Wales, UK

The geology of the Snowdonia National Park in North Wales comprises a mixture of Lower Palaeozoic shallow marine sediments, acidic igneous rocks and basic intrusions of the Welsh Basin that were subsequently deformed during the Caledonian Orogeny. Thin igneous intrusions are challenging to map due to variable surface exposures, their intrusive origin, structural deformation and burial by glacial sediments. This study used a combination of traditional geological techniques, near-surface geophysical surveys and remote sensing to detect and map a buried dolerite sheet intrusion. Both simple and mathematical analysis of magnetic anomalies and numerical modelling allowed the dolerite position, depths and target widths to be determined. Results showed that calibrated magnetic surveys can characterize buried igneous bodies in such mountainous environments.