Dyke-like Structures Research Articles

Imaging magmatic intrusions using derivatives of high-resolution aeromagnetic data over the Nigerian sector of the chad Basin

Magmatic activities can either positively or negatively affect hydrocarbon generation in sedimentary basins. The effect of these activities can be positive when they prevent further migration and escape or negative when extreme pressure and high temperature force the hydrocarbon to evaporate at the time of emplacement. The aim of the study is to confirm the presence of intrusive structures in the Nigerian sector of the Chad Basin using derivatives of aeromagnetic data. The enhanced body and edges suggest a dyke-like structure intruding the Chad Basin. The edges are identified as areas that are skewed on the first vertical derivative and maxima over the analytic signal generated by the dyke-like intrusion. Three profiles with lengths ranging from 68.4 km to 263.9 km were generated on the derivatives of aeromagnetic data, and the analytical results indicate dyke-like structures of 0.3 km to 0.4 km in size.

Utilization of Landsat-8 Imagery and Aeromagnetic Data for Deciphering Alteration Zones and Structures: Implications for Mineral Exploration in the Southeastern Desert of Egypt

Remote-sensing (RS) and aeromagnetic data implemented notable information for mineralization potentiality detection of ore deposits within Al-Allaqi region of the southeastern Desert of Egypt. Several approaches of remote-sensing and aeromagnetic data were implemented. Results of band ratios (BR) and Principal Component Analysis (PCA) practicing Landsat-8 successfully imaging the hydrothermally altered zones and the structures outlined by lithologic contacts, faults, and porphyry features (dyke-like structures). Lineaments well extracted using aeromagnetic datasets. CET system method was applied for generating a heat map of the structural complexity. Likewise, the CET Porphyry Analysis system was performed to extract the likely near-circular porphyry features that revealing a probable site of mineralization and to validate the remote sensing observations. Integrated outcomes showed that areas of highly complex structures, density (fractures/faults) are consistent with the detected hydrothermally altered areas that are rivaled with mining zones in Al-Allaqi region of the Southeastern Desert of Egypt. This approach can be broadly appropriate to other divisions of the Southeastern Desert of Egypt for mineral prospecting.

Very low frequency electromagnetic (VLF-EM) study over Wajrakarur kimberlite Pipe 6 in Eastern Dharwar Craton, India

The Wajrakarur kimberlite Pipe 6 in Eastern Dharwar Craton, is hardly explored using latest ground-based geophysical techniques. The present study uses the Very Low Frequency Electromagnetic (VLF-EM) method for understanding the aerial extension, depth and geometry of the kimberlite pipe. The VLF-EM data have been analyzed using Fraser filtering of in-phase component, 3D Euler deconvolution of Fraser filtered in-phase data, radially average power spectrum (RAPS) of VLF data (raw data) and 2D inversion of VLF data (raw data). The Fraser filtered in-phase grid anomaly map has witnessed as an effective tool for mapping extension of the kimberlite pipe. The maxima of Fraser filtered in-phase component has been observed as a key parameter to delineate the conducting bodies. The high apparent current density in Karous–Hjelt (K–H) pseudo section locate relatively conducting body possibly associated with kimberlite pipe. Two depth interfaces at about 15 and 32 m have been delineated using RAPS. 3D Euler solution indicate dyke-like structure associated with kimberlite pipe having depth solutions ranging from 6 to 40 m with mode of depth 17 m in the study area. 2D resistivity sections indicate that causative bodies are in the depth range of 15–50 m. The results of VLF-EM study are well validated using geological borehole data over the study area reported by Geological Survey of India.

Groundwater prospecting by the inversion of cumulative data of Wenner–Schlumberger and dipole–dipole arrays: A case study at Turamdih, Jharkhand, India

The present study deals with groundwater prospecting in hardrock terrain. Initially, the Wenner–Schlumberger array and the dipole–dipole array data have been acquired using Syscal Junior Switch-48. Furthermore, data acquired using both arrays have been merged using Prosys-II data handling software for the inversion of the cumulative data for possible mapping of water-bearing fracture rock masses with different structural distribution in a complex geological environment. The data have been analysed using RES2DINV software, based on the smoothness constrained least-square technique. Two numbers of 2D electrical resistivity tomography profiles (AA $$'$$ and BB $$'$$ ) have been selected over an official colony of the Turamdih uranium mine for groundwater prospecting, which is located at about 24 km west of Jaduguda, Jharkhand, India. High-resistivity features associated with a dyke-like structure have been delineated in both the profiles. Three low-resistivity features have been delineated as water saturated alluvium/aquifers in profile AA $$'$$ . A low-resistivity feature associated with the water-saturated fracture zone has been identified in profile BB $$'$$ , which is well correlated with the surficial location of an ephemeral channel at the bottom of the hill across the slope. It is observed that geoelectric sections generated by the inversion of cumulative data of both arrays provide superior results compared with the Wenner–Schlumberger and dipole–dipole arrays, separately.

Geomorphic and lithologic characteristics of Wadi Feiran basin, southern Sinai, Egypt, using remote sensing and field investigations

Wadi Feiran is an important drainage basin in southern Sinai Peninsula covering an area of about 1785 \(\hbox {km}^{2}\), its streams drain into the Gulf of Suez crossing variety of rocks and sedimentary units varied in age from Precambrian to Quaternary. Field investigations, geographic information systems (GIS) and remote sensing studies including Landsat-7 ETM+, Radarsat-1, and SRTM DEM were integrated to reveal its lithologic, geologic and geomorphic features. Besides the field investigations, rock units including basement and pre- and syn-rift sedimentary units were discriminated using band ratios and principal component analysis techniques (PCA). Such techniques revealed that the crystalline rocks covering W. Feiran are unaltered rocks lacking OH-bearing minerals. Radar data successfully displayed the structures and geomorphic features related to topography. Moreover, the techniques allowed the extraction of the dyke-like structures along faults and shear zones. This also characterized the topographic variations through analysis of the shaded terrain and the altitudinal profiles. The results of data integration, lineament analysis and lineament density maps revealed that the structural grain in the present study has four different trends: N20–45E, N30–45W, N–S and E–W. Based on analysis of radar data and geomorphic indices, W. Feiran is an asymmetrical basin, its left side occupies \(\sim \)34% of the total area that leads to a supposedly massive tilt towards the south which caused the southwestward slope.

Global nonlinear optimization for the interpretation of source parameters from total gradient of gravity and magnetic anomalies caused by thin dyke

An efficient approach to estimate model parameters from total gradient of gravity and magnetic data based on Very Fast simulated Annealing (VFSA) has been presented. This is the first time of applying VFSA in interpreting total gradient of potential field data with a new formulation estimation caused due to isolated causative sources embedded in the subsurface. The model parameters interpreted here are the amplitude coefficient ( k ), exact origin of causative source ( x 0 ) depth ( z 0 ) and the shape factors ( q ). The results of VFSA optimization show that it can uniquely determine all the model parameters when shape factor is constrained. The model parameters estimated by the present method, mostly the shape and depth of the buried structures were found to be in excellent agreement with the actual parameters. The method has also the proficiency of evading highly noisy data points and improves the interpretation results. Study of histogram and cross-plot analysis also suggests the interpretation within the estimated uncertainty. Inversion of noise-free and noisy synthetic data for single structures as well as field data demonstrates the efficacy of the approach. The technique has been warily and effectively applied to real data examples (Leona Anomaly, Senegal for gravity and Pima copper deposit, USA for magnetic) with the presence of ore bodies. The present method can be extremely applicable for mineral exploration or ore bodies of dyke-like structure embedded in the shallow and deeper subsurface. The computation time for the whole process is very short.

Melt production, redistribution and accumulation in mid-crustal source rocks, with implications for crustal-scale melt transfer

Abstract Ascent of granitic melt initiates under suprasolidus conditions in the mid- to lower crust before continuing through subsolidus rocks to higher crustal levels. Whereas migration of melt in suprasolidus rocks can occur in pervasive net-like structures and involve relatively small melt volumes, ascent through the subsolidus crust requires more focused, dyke-like structures and larger volumes to prevent freezing. Migmatites in the Aus granulite terrain, southern Namibia, preserve evidence that large-scale melt redistribution and accumulation occurred in the near-source region under suprasolidus conditions. Melt that was mainly produced in metapelitic rocks utilised pervasive small-scale leucosome networks to migrate to areas surrounding pre-tectonic granite sheets. These areas are dominated by metapsammitic rocks, and abundant and voluminous leucogranite sheets attest to melt accumulation and residence occurring over a protracted period while the area was undergoing anatexis. However, the leucogranites have an anhydrous mineralogy and the surrounding rocks only preserve evidence for limited, high-temperature retrogression, consistent with substantial melt loss from the accumulation structures. We speculate that melt batches leaving the accumulation sites are likely to have been large, allowing for substantially more efficient ascent through subsolidus crust. Our results suggest that a degree of near-source melt accumulation is likely to occur during the early stages of meltmigration, and that this can significantly enhance the effectiveness of subsequent melt ascent.

The Auckland volcanic field, New Zealand: Geophysical evidence for structural and spatio-temporal relationships

Geophysical data from the monogenetic Auckland volcanic field reveal complex structural and spatio-temporal relationships at different scales. The volcanic field is coincident with regional magnetic and gravity anomalies that mark a major crustal suture and with a discontinuity marking a significant structural asperity. Here, the linear regional magnetic anomaly splays into a wide band of NNW-trending lineaments, arising from serpentinised shear zones in the upper crust, that matches the extent of the volcanic field and that may reflect a region of crustal weakness creating preferential permeability. However, there appears to be no simple correlation between the locations of individual vents and these lineaments that might delineate more shallow structural controls with this orientation, probably as a consequence of other structural influences. High-resolution aeromagnetic data over the volcanic field show that the volcanoes have a wide range of magnetic signatures indicating a variability of subsurface structure. Scoria cone volcanoes typically have strong anomalies (up to several 100 nT) whilst tuff-ring volcanoes typically have weak anomalies (less than 50 nT), though the surface geology is not always an indicator of the nature and extent of the subsurface deposits. Both cone and tuff-ring volcanoes in the Auckland field appear to be underlain by subsurface bowl-shaped bodies of basalt, implying that their eruption histories commonly involve lava ponding into early excavated craters. The present geophysical data give no evidence for subsurface dyke-like structures or for substantial near-surface volumes of basaltic rocks where there are no known eruption centres or buried flows. Aeromagnetic and palaeomagnetic data suggest that a number of adjacent vents with an implied structural linkage may be contemporaneous, though other examples occur where vents of clearly different ages exploit the same apparent structure. A unique feature of the Auckland field is that at least 5 widely separated and structurally unrelated volcanoes are contemporaneous. These observations highlight the spatial and temporal heterogeneities that can occur in monogenetic fields and have an important bearing on recurrence rates and temporally linked eruptions, which are probably more common than is generally supposed, and are key factors in statistically based hazard assessments.

Two-dimensional non-linear inversion of VLF-R data using simulated annealing

SUMMARY The VLF-R (very low frequency-resistivity) data, i.e. the apparent resistivity (r a ) and phase (Q) data, were inverted individually and jointly using the VFSA (very fast simulated annealing) global inversion approach. Global inversion results for synthetic data without and with various amounts of random and normally distributed Gaussian noise reveal that the inversion of neither the r a nor Q data alone yields the true parameters of the structures. However, the joint inversion of the r a and Q data yields very good estimates of the model parameters. Five models, representing typical subsurface structures in the shield areas, are studied here. Various models achieved after 10 VFSA runs were used to compute the mean model and the corresponding covariance and correlation matrices, which were used to estimate the uncertainties in the mean model parameters and correlations between the model parameters. We observe that these correlations follow the physics associated with the problem. VLF-R field data due to a nearly vertical contact structure and a very thick dyke-like structure were also inverted to demonstrate the eYcacy of the approach in the delineation of the parameters of 2-D structures.

A strategy of tectonomagnetic observation for monitoring possible precursors to earthquakes in the western part of the North Anatolian Fault Zone, Turkey

In order to investigate changes in the geomagnetic field associated with seismic activity in the western part of the North Anatolian Fault Zone (NAFZ), seven continuous measurement stations and eighteen repeated-survey sites were set up in the İnik-Geyve region, taking into account the fault trace and its features. Simple differences in the total intensity between these continuous stations have been derived for detecting possible anomalous phenomena associated with earthquake occurrences. Theoretical studies imply that changes in the total intensity associated with fault activity are larger in the case of non-uniform distribution of rock magnetization than those in the uniform case. Meanwhile, inversion analyses of magnetic anomalies at the North Anatolian Fault Zone (NAFZ) suggest a dyke-like structure extending parallel to the fault strike. This structure enables us to test our strategy, based on non-uniform magnetization, for detecting pre- and/or co-seismic changes in the geomagnetic field. We made a theoretical estimation of geomagnetic changes for the case of a magnetic dyke structure intruding a non-magnetic body parallel to the strike of a fault. It turns out that, unlike the uniform case, the magnetic field arising from stress-induced magnetization is enhanced. A tectonomagnetic observation system talcing into account such a non-uniform magnetization structure in the İnik-Geyve region of the NAFZ is also described.

Multidisciplinary research on fault activity in the western part of the North Anatolian Fault Zone

Westward migration of large earthquakes along the North Anatolian Fault Zone seems to have been suspended after the 1967 Mudurnu Valley earthquake, which may imply the formation of a seismic gap. However, the westward extension of the North Anatolian Fault Zone has not been clearly defined; two branches have been proposed but which one of them is the true extension is a matter of controversy. In this paper we review some of the results which we have obtained so far from field work in order to derive some basic information as to the characteristic features of the respective branches. The northern branch, which we call the Izmit-Sapanca fault zone, is characterized by a wide, graben-like structure. In the deformation zone, however, the topography corresponding to folding prevails, which implies compressional deformation in late Quaternary. The southern branch, which we call the Iznik-Mekece fault, is a simple, continuous strand and is displaced right-laterally. There also exist Holocene offset features indicating that strike-slip faulting has been dominant. Temporary seismic observations were made twice; for one month in 1986 and for two months in 1988. During these two observation periods, no significant seismic activity was found in the vicinity of the Iznik-Mekece fault, whereas many small earthquakes occurring at the Izmit-Sapanca fault zone were recorded. Although the observation period is rather short, we conclude that current seismic activity at the Iznik-Mekece fault is very low. In view of some geological evidence showing the past activity of this fault, we interpret low seismic activity as indicating that seismicity is currently at the stage of quiescence. We attempted to estimate recurrence intervals of large earthquakes at the Iznik-Mekece fault by excavating trenches at two locations. We carefully selected trench sites but they were not good enough to provide crucial information; the only result is that the latest earthquake at the eastern portion of the Iznik-Mekece fault must have taken place 200–500 years B.P. Trenching was also attempted at a location where the surface rupture associated with the 1967 Mudurnu Valley earthquake is clearly recognized. We found evidence showing the last two faulting events. The most marked feature is extensive magnetic anomalies along the Iznik-Mekece fault. Detailed features of the observed anomalies indicate that they are closely related to the fault structure; in particular, weak magnetization zones are found to correspond to fractured zones. These magnetic anomalies can be interpreted in terms of dyke-like structures. Similar fault-related anomalies have been found in the distributions of electric potential and radon concentration in soil gas. These anomalies provide important information as to a strategy for continuous measurements; fault-related anomalies should be taken into consideration when an effective way is pursued for detecting anomalous changes prior to faulting.

CSAMT measurements across the 1986 C craters of Izu-Oshima Island, Japan.

We carried out CSAMT (controlled-source audiofrequency magnetotelluric) measurements across the 1986 C craters of Izu-Oshima Island one year after the eruptions. After 2D analyses of the two profiles, we obtained basically two-layered structures, except for shallow resistivity variations. We found a deep conductive layer below the resistive lava which corresponds to thermal water, compared with nearby wells. We also found shallow (<200m) conductive bodies whose locations correspond to an old vent (8th century) and one of the 1986 craters. We think these are fracture zones containing meteoric water. We did not get affirmative evidence for dyke-like structures, partly because we used TM mode excitation, and partly because the dyke hadn't transferred enough heat to the surrounding rocks to decrease resistivity.Away from the profiles, we found a three layered structure. We think the three layers correspond to (top) resistive lava, (middle) a fresh water lens, and (bottom) sea water. We think the two profiles lacked the fresh water lens due to mixing of fresh and saline water by thermal convection.

A possible crust-mantle configuration near Trivandrum deduced from Indian geomagnetic data

Anomalies in H and Z, primarily attributable to subsurface features, were obtained from the annual mean values at five Indian magnetic stations for the period 1965–1973, after correcting for the secular and normal variations. When the average anomaly at each station over the nine years chosen, after projecting on to a reference longitude (78°E), was plotted against its geographic latitude, the resulting ΔZ and ΔH latitudinal profiles were found to reach their maximum values near Trivandrum ( TRV). This rise in anomaly near TRV has been interpreted in terms of a probable plutonic dyke-like structure, superposed on the general upwarp of the mantle known to be associated with a land-to-ocean transition. A Bouguer gravity profile along the same reference longitude, supporting this inference, is also presented. A sample calculation has been done with assumed parameters for the model and the theoretical and observed anomalies are compared.

Error Analysis for Gravity Interpretation using Stanley’s Gradient Method

In a paper read at the 25th International Geological Congress, Sydney, 1976 Stanley described a simplified method for interpreting gravity anomalies due to contact and dyke-like structures. The method involved the assumptions of considerable depth extent relative to thickness of overburden, infinite strike length, and in the case of a dyke, a depth of burial much greater than the width. In real situations these assumptions may not always be justified. We now evaluate the limits to which the approximations may be extended before unsatisfactory errors are introduced to the interpreted structural parameters.

Simplified Gravity and Magnetic Interpretation of Contact and Dyke-Like Structures

This paper describes one simple procedure which can be applied to the interpretation of gravity or magnetic profiles measured perpendicular to the strike of contact or dyke-like structures or to structures which can be considered as a combination of both. The method is very easy and rapid to apply, is unambiguous within the assumed contact or dyke-like model, is applicable to airborne or ground level data and does not necessarily require computing facilities but may be automated if desired. As the interpretation requires only a relatively short length of traverse (usually less than 3 depth units) in close proximity to the structure, the procedure results in a very high rejection of neighbouring disturbances. In the gravity case the method has the particular advantage of freedom from regional, topographic and latitude corrections and in most applications only a minimum of elevation surveying is necessary.