Surface Geophysical Data Research Articles

Determination of Aquifer Hydraulic Properties, Aquifer Potential and Vulnerability Characteristics Using Pumping Test and Surface Geophysical Data: A Case Study of Udi and Ezeagu Areas, Southeastern Nigeria

Determination of Aquifer Hydraulic Properties, Aquifer Potential and Vulnerability Characteristics Using Pumping Test and Surface Geophysical Data: A Case Study of Udi and Ezeagu Areas, Southeastern Nigeria

Upscaling digital outcrop models to infer well connectivity in carbonates with karstic features

We construct an innovative static–dynamic integrated workflow capable of bridging the gap between input geological data, inherent to a lacustrine carbonate outcrop containing karst geobodies, and the description of the flow patterns and quantification of the multi-well productivity index (MWPI) for a particular well configuration in the outcrop. The workflow incorporates additional features stemming from the use of Machine Learning-based methods to mitigate lack of data in the locations away from the sections of input signals, along with the construction of new upscaling methods to assess the MWPI matrix. The ML-enhanced geostatic model hinges upon shallow surface geophysical data collected using Ground-Penetrating Radar (GPR) techniques. Furthermore, by discretizing the flow equations and adopting a flow-based upscaling method, we construct correlations between well flow rates and pressure drawdown in a typical five-spot well configuration. In this setting, we analyze the sensitivity of each well productivity with respect to heterogeneity distribution and correlations in the karst system within the outcrop. Computational simulations illustrate the ability of the integrated workflow proposed herein to improve prediction of hydraulic-connectivity between well pairs, which appear manifested in the entries of the MWPI matrix, whose magnitude aims at quantifying the effects of the karst geobodies upon geofluid production.

Combined Magnetic, Transient Electromagnetic, and Magnetotelluric Methods to Detect a BIF-Type Concealed Iron Ore Body: A Case Study in Gongchangling Iron Ore Concentration Area, Southern Liaoning Province, China

The detection and evaluation of concealed mineral resources deep in metallic mines and in the surrounding areas remain technically difficult. In particular, due to the complex topographic and geomorphic conditions on the surface, the detection environments in these areas limit the choices of detection equipment and data collection devices. In this study, based on metallogenic theory and the metallogenic geological characteristics of banded iron formation (BIF)-type iron ores, equipment for surface geophysical surveys (i.e., the high-precision ground magnetic survey method, the transient electromagnetic method, and the magnetotelluric method) and data collection devices capable of taking single-point continuous measurements were employed to detect the concealed iron ore bodies in the transition zone CID-1 between the Hejia and Dumu iron deposits in the Gongchangling iron ore concentration area in the Anshan-Benxi area (Liaoyang, China), a representative area of BIF-type iron ores. The results showed that an optimal combination of these geophysical survey methods accurately determined the anomalous planar spatial locations and anomalous profile morphologies of the concealed iron ore bodies. On this basis, we determined their locations, burial depths, and scales. Two anomalous zones induced by concealed iron ore bodies, YC-1 and YC-2, were discovered in zone CID-1. Two concealed iron-bearing zones, one shallow (0–150 m) and one deep (300–450 m), were found in YC-1. A 100 m scale drilling test showed that the cumulative thickness of the shallow iron-bearing zone was over 23.6 m.

Petrophysical characterization of deep saline aquifers for CO2 storage using ensemble smoother and deep convolutional autoencoder

Carbon dioxide sequestration in deep saline aquifers requires accurate and precise methods to monitor carbon capture and storage and detect leakage risks. The assessment of the CO2 plume location during injection and storage depends on the accuracy of the spatial distribution model of petrophysical properties, such as porosity and permeability. This work focuses on stochastic methods for petrophysical characterization and presents a method for the prediction of porosity and permeability using borehole observations and surface geophysical data. This study utilizes injection and monitoring measurements at the borehole locations and time-lapse seismic surveys. The proposed method is based on a stochastic approach to inverse problems with data assimilation, namely the ensemble smoother with multi-data assimilation. Ensemble-based methods are generally unfeasible when applied to large geophysical datasets, such as time-lapse seismic surveys. In the proposed approach, a machine learning method, namely the deep convolutional autoencoder, is applied to reduce the dimension of the seismic data. The ensemble smoother is then applied in a lower dimensional data space to predict the aquifer petrophysical properties. This method updated predictions of porosity and permeability every time new data, either seismic surveys or borehole data, are available, to reduce the uncertainty in the CO2 plume prediction. The method is tested and validated on a synthetic geophysical dataset generated for the Johansen formation, a potential large-scale offshore site for CO2 storage.

Investigating Microtopographic and Soil Controls on a Mountainous Meadow Plant Community Using High‐Resolution Remote Sensing and Surface Geophysical Data

AbstractThis study aims to investigate the microtopographic controls that dictate the heterogeneity of plant communities in a mountainous floodplain‐hillslope system, using remote sensing and surface geophysical techniques. Working within a lower montane floodplain‐hillslope study site (750 m × 750 m) in the Upper Colorado River Basin, we developed a new data fusion framework, based on machine learning and feature engineering, that exploits remote sensing optical and light detection and ranging (LiDAR) data to estimate the distribution of key plant meadow communities at submeter resolution. We collected surface electrical resistivity tomography data to explore the variability in soil properties along a floodplain‐hillslope transect at 0.50‐m resolution and extracted LiDAR‐derived metrics to model the rapid change in microtopography. We then investigated the covariability among the estimated plant community distributions, soil information, and topographic metrics. Results show that our framework estimated the distribution of nine plant communities with higher accuracy (87% versus 80% overall; 85% versus 60% for shrubs) compared to conventional classification approaches. Analysis of the covariabilities reveals a strong correlation between plant community distribution, soil electric conductivity, and slope, indicating that soil moisture is a primary control on heterogeneous spatial distribution. At the same time, microtopography plays an important role in creating particular ecosystem niches for some of the communities. Such relationships could be exploited to provide information about the spatial variability of soil properties. This highly transferable framework can be employed within long‐term monitoring to capture community‐specific physiological responses to perturbations, offering the possibility of bridging local plot‐scale observations with large landscape monitoring.

Three-dimensional stochastic assimilation of gravity data in Lalor volcanogenic massive sulphide, Manitoba, Canada

We propose a new numerical workflow based on stochastic data integration where we merge a conceptual geological model, drillhole geophysical and geological logs, and surface geophysical data to compute a unified numerical model of a volcanogenic massive sulphide (VMS) deposit. The first step of the workflow consists in building a three-dimensional (3D) numerical conceptual model of the geology. This conceptual model, as well as geological logs, is then used to generate multiple equiprobable scenarios of the geology by means of multiple-point simulation (MPS). The MPS method studies high-order statistics in the space of a numerical conceptual model, making it possible to reproduce complex geological structures. We then use conventional conditional sequential Gaussian simulation, which is a method based on a node-by-node sequential process, to stochastically populate the geological grid with densities. For this purpose we use available density logs to simulate multiple equiprobable spatial distributions of the density at high spatial resolution within each geological unit separately. The stochastic high-resolution density models are iteratively combined by the gradual deformation method to minimize the difference between measured Bouguer anomaly data and the data computed on the combined realizations of density. Application of the proposed method to the Lalor deposit, a VMS deposit in Manitoba, Canada, produces a density model that honours the geology of the deposit and the Bouguer anomaly data. This unified model has the advantage to include all the available information (geological and density logs and surface geophysics) at scales appropriate for mining applications.

Integration of Borehole Data in Geophysical Inversion Using Fuzzy Clustering

Borehole data is critical information to constrain geophysical inversions. Borehole information is usually used as a petrophysical constraint of the inversion scheme. Therefore, borehole information is only valuable if the borehole features are the same as the physical parameters of geophysical models. In fact, we may have many drilled holes, but the physical parameter of the geophysical model is only available in a few holes. Thus, the question is how we can exploit other borehole features to assist the inversion. We present an application of fuzzy clustering to incorporate multiple borehole features such as lithological, assay and wireline logs in the geophysical inversion. The integration of this extra information assist inversion process to build a model that fit surface geophysical data and simultaneously honour the prior information of borehole data. We apply this approach to the case study over the Kevitsa deposit within the Kevitsa ultramafic intrusion in northern Finland. The inversion of seismic reflection data with assistance from borehole information produces a more geologically interpretable image than seismic reflection data. The integration of both petrophysical and spatial attribute of the borehole enable the inversion to build a better model than only petrophysical constraint.

Near-surface geophysical surveys at the Duport gold deposit, Ontario, Canada: Relating airborne responses to small-scale geologic features

Near-surface geophysical measurements using magnetometer, magnetic susceptibility, terrain conductivity, and time-domain electromagnetic instruments were made at the shear-hosted Duport gold deposit on Cameron Island in Shoal Lake, western Ontario, Canada, to help relate airborne total magnetic intensity (TMI) and helicopter electromagnetic survey data to small-scale geologic features. The magnetic airborne response provides a weak indication of a narrow anomaly within the Duport deformation zone, and the airborne electromagnetic response provides an indication of enhanced conductivity in the northwest of Cameron Island. In contrast, surface magnetic responses are dominated by the narrow 10,000–15,000 nT magnetic anomaly of a talc-chlorite-dolomite schistose basalt unit, a feature barely visible in the airborne TMI data. This geologic unit hosts the veins containing gold mineralization, so the surface TMI data provide a valuable response for delineating the corresponding rocks. Modeling of the TMI data indicates that the unit has a susceptibility of up to 0.7 SI and a corresponding magnetite content of up to 20%. The TMI data also reveal along-strike variations in the magnetic anomaly providing information on the component of ultramafic rocks in the protolith of the unit. The surface geophysical data allow the enhanced conductivity in the northwest part of Cameron Island to be attributed to several narrow sulfidized zones containing up to 10% pyrrhotite. Additional conductive and positively magnetized zones are associated with concentrations of 5%–10% of magnetite. The near-surface geophysics at the Duport deposit provided a rapid and inexpensive method for defining the magnetic and electrical properties of the geologic units at the site and for defining the exact location, width, and internal structure of features observed in the airborne geophysical data. Availability of magnetic susceptibility measurements from a single drill core enhanced the accuracy of the interpretations and the ability to relate the near-surface geophysical responses to geologic features.

Correlation of Surface Geophysical and Logging Data of Some Selected Boreholes in Imo State, South Eastern Nigeria

Boreholes have become a major source of water supply in South Eastern Nigeria, an area with diversity in geology, topography and climatic conditions. The common approaches in borehole investigations are surface survey and logging. The two approaches are supposed to be complementary. However, for small schemes of groundwater development, logging is hardly considered.. Six boreholes were selected from locations at the three geographical zones of Imo state. The selected locations are Umueze, Umuduru, Ogbor-Ugiiri, Ngor Okpuala and Eziama. Geophysical survey and logging were carried out. Electrical resistivity method was adapted for geophysical survey. ABEM Terrameter (SAS) 300B with digital read-out was used for logging. The degree of correlation between the variables was determined by computing the coefficient of correlation denoted as R<sup>2</sup>. The results indicate generally poor correlations between logging and geophysical surface values for the selected boreholes except for the one located at Ngor Okpala with R<sup>2</sup> value of 0.7408. At this location, geophysical surface method for borehole locations can be carried without any Logging exercise to establish the Total Drilling Depth (TDD). This will help reduce the total cost for the drilling of the boreholes and also save time and much desired energy. In those areas where no correlation exists, there is need for Government Financial support in drilling sustainable borehloes as much costs are involved in investigation and construction works.

Late Cenozoic paleovalley fill sequence from the Southern Liverpool Plains, New South Wales—implications for groundwater resource evaluation

The Liverpool Plains in northern New South Wales contain some of the best agricultural land in Australia and are underlain by extensive smectite clay-dominated soils sourced from weathering the alkali basalts of the Liverpool Ranges. It had been thought that a relatively simple geological model explained the underlying Cenozoic sequence with salt-rich clays of the Narrabri Formation overlying sands and gravel aquifers comprising the Gunnedah Formation. Extensive groundwater modelling based upon this simple conceptualisation has been used in management plans proposed by the mining and agricultural industries. A 31.5 m core has been recovered using minimally disturbed triple-tube coring methods at Cattle Lane (Latitude –31.52° S, Longitude 150.47° E) to resolve uncertainty concerning the aquitard status of the upper layer. Recovered core has been examined and tested to determine grainsize, cation-exchange capacity, X-ray diffraction, X-ray fluorescence and microscopic examination of granular components. These measurements complement surface and borehole geophysical techniques, hydrogeological data and hydrochemical analysis of water samples recovered from a series of specially constructed piezometers adjacent to the cored hole. The sequence overlies a Late Cretaceous channel cut into Permian bedrock at 91 m depth with sands and clays below 31.5 m considered to represent various alluvial fill events mostly occurring since the Early Pliocene. Erosion of Late Eocene alkali basalts on the Liverpool Ranges, with the formation of smectite clays, pedogenic carbonates and with the addition of quartz from both eolian sources and locally derived from adjacent Triassic sandstone hills, provides the great majority of the sediment recovered from the cores. Late Pleistocene (114 ka) to Holocene ages were determined for the core from three optically stimulated luminescence (OSL) measurements on fine sands (13, 23 and 29 m BG). Detailed examination has failed to detect any evidence of a boundary between Narrabri and Gunnedah formations revealing rather a gradual change in dominance of clays and silts over sands and gravels embedded in a clay-rich matrix. This result challenges the conceptualisation used to conduct groundwater modelling on the Liverpool Plains.

Coupling geophysical investigation with hydrothermal modeling to constrain the enthalpy classification of a potential geothermal resource

An appreciable challenge in volcanology and geothermal resource development is to understand the relationships between volcanic systems and low-enthalpy geothermal resources. The enthalpy of an undeveloped geothermal resource in the Karckar region of Armenia is investigated by coupling geophysical and hydrothermal modeling. The results of 3-dimensional inversion of gravity data provide key inputs into a hydrothermal circulation model of the system and associated hot springs, which is used to evaluate possible geothermal system configurations. Hydraulic and thermal properties are specified using maximum a priori estimates. Limited constraints provided by temperature data collected from an existing down-gradient borehole indicate that the geothermal system can most likely be classified as low-enthalpy and liquid dominated. We find the heat source for the system is likely cooling quartz monzonite intrusions in the shallow subsurface and that meteoric recharge in the pull-apart basin circulates to depth, rises along basin-bounding faults and discharges at the hot springs. While other combinations of subsurface properties and geothermal system configurations may fit the temperature distribution equally well, we demonstrate that the low-enthalpy system is reasonably explained based largely on interpretation of surface geophysical data and relatively simple models.

Hydrogeophysical evaluation of aquifer hydraulic characteristics using surface geophysical data: a case study of Umuahia and environs, Southeastern Nigeria

The determination of aquifer hydraulic characteristics of Umuahia area from Dar-Zurrouk parameters was carried out using 30 vertical electrical sounding (VES) data and information from ten well-log data. The Schlumberger electrode configuration was employed for the field data acquisition with a maximum half current electrode separation (AB/2) of 500 m. The VES data were interpreted using the conventional partial curve matching technique to obtain initial model parameters which were used as input for computer iterative modeling using the OFFIX software. Aquifer apparent resistivity in the area varies from 109 to 850 Ωm with a mean value of 253.2 Ωm. The depth to water table varies from 0.7 m around Ajata Ibeku to 82.67 m at Mater Dei Umuahia. The aquifers of the study area are highly variable in thickness with Ajata Ibeku having the least thickness of 1.2 m. The hydraulic conductivity across the area ranges from 0.64 m/day to about 10.22 m/day, while the transmissivity have mean values of 308.16 and 6.257 m2/day for zones A and B, respectively. The mean storativity values of 1.92 × 10−4 and 1.74 × 10−5 for zones A and B, respectively, suggest that zone A has a fairly productive aquifer unlike zone B which revealed very poor hydraulic properties. Information extracted from iso-resistivity models, geo-electric cross-sections, litho logs, and electric logs revealed a sandy clay lithology at very shallow depths with a thick layer of shale/clay extending to over 300 m within zone B. These potential shallow aquiferous units are however perched, partially saturated and may not have sufficient yield.

Quantifying and relating land-surface and subsurface variability in permafrost environments using LiDAR and surface geophysical datasets

The value of remote sensing and surface geophysical data for characterizing the spatial variability and relationships between land-surface and subsurface properties was explored in an Alaska (USA) coastal plain ecosystem. At this site, a nested suite of measurements was collected within a region where the land surface was dominated by polygons, including: LiDAR data; ground- penetrating radar, electromagnetic, and electrical-resis- tance tomography data; active-layer depth, soil tempera- ture, soil-moisture content, soil texture, soil carbon and nitrogen content; and pore-fluid cations. LiDAR data were used to extract geomorphic metrics, which potentially indicate drainage potential. Geophysical data were used to characterize active-layer depth, soil-moisture content, and permafrost variability. Cluster analysis of the LiDAR and geophysical attributes revealed the presence of three spatial zones, which had unique distributions of geomor- phic, hydrological, thermal, and geochemical properties. The correspondence between the LiDAR-based geomor- phic zonation and the geophysics-based active-layer and permafrost zonation highlights the significant linkage between these ecosystem compartments. This study suggests the potential of combining LiDAR and surface geophysical measurements for providing high-resolution information about land-surface and subsurface properties as well as their spatial variations and linkages, all of which are important for quantifying terrestrial-ecosystem evolution and feedbacks to climate.

High resolution reflection seismic profiling over the Tjellefonna fault in the Møre-Trøndelag Fault Complex, Norway

Abstract. The Møre-Trøndelag Fault Complex (MTFC) is one of the most prominent fault zones of Norway, both onshore and offshore. In spite of its importance, very little is known of the deeper structure of the individual fault segments comprising the fault complex. Most seismic lines have been recorded offshore or focused on deeper structures. This paper presents results from two reflection seismic profiles, located on each side of the Tingvollfjord, acquired over the Tjellefonna fault in the southeastern part of the MTFC. Possible kilometer scale vertical offsets, reflecting large scale northwest-dipping normal faulting, separating the high topography to the southeast from lower topography to the northwest have been proposed for the Tjellefonna fault or the Baeverdalen lineament. In this study, however, the Tjellefonna fault is interpreted to dip approximately 50–60° towards the southeast to depths of at least 1.3 km. Travel-time modeling of reflections associated with the fault was used to establish the geometry of the fault structure at depth, while detailed analysis of first P-wave arrivals in shot gathers, together with resistivity profiles, were used to define the near surface geometry of the fault zone. A continuation of the structure on the northeastern side of the Tingvollfjord is suggested by correlation of an in strike direction P-S converted reflection (generated by a fracture zone) seen on the reflection data from that side of the Tingvollfjord. The reflection seismic data correlate well with resistivity profiles and recently published near surface geophysical data. A highly reflective package forming a gentle antiform structure was also identified on both seismic profiles. This structure could be related to the folded amphibolite lenses seen on the surface or possibly by an important boundary within the gneissic basement rocks of the Western Gneiss Region. The fold hinge line of the structure is parallel with the Tjellefonna fault trace suggesting that the folding and faulting may have been related.

Field appraisal and accurate resource estimation from 3D quantitative interpretation of seismic and CSEM data

The key questions in field appraisal are: What is the hydrocarbon volume, and how are the hydrocarbons distributed in the field? The ability to answer these questions accurately is critical for deciding whether to produce a field and for developing a production plan. Wells drilled during the appraisal phase provide well and flow-test data, which are combined with structural knowledge from seismic surveys to map the extent of the field and generate a reservoir model. The cost for appraising an offshore field can exceed US $100 million, and it is desirable to obtain the information required with fewer wells if possible. Quantitative interpretation of surface geophysical data provides reservoir properties between well locations and can, therefore, significantly reduce appraisal costs. A quantitative analysis of seismic data using well-log information will typically determine reservoir rock porosity. Other important parameters are hydrocarbon saturation, permeability, and net-to-gross ratio. Quantitative interpretation of several reservoir properties using only the seismic data is associated with significant ambiguity. To determine several of these parameters accurately, complementary geophysical data sets with different sensitivity characteristics are needed. Controlled-source electromagnetic (CSEM) data are sensitive to the fluid type and can provide additional information to determine the hydrocarbon saturation more accurately. We have developed a new quantitative interpretation workflow integrating seismic and marine 3D CSEM data for estimating the hydrocarbon volume and obtaining 3D distributions of the hydrocarbon pore volume. A performance test of the workflow has been carried out on the Troll West Oil Province (TWOP) in the Norwegian North Sea (Figures 1 and 2). This article describes our methodology and presents encouraging results.

Biographies of the guest editors

Laura Valentina Socco took her PhD in environmental geo-engineering in 1996 at Politecnico di Torino, where she is presently Assistant Professor of Applied Geophysics. Her main research activity is focused on characterization techniques based on seismic surface wave analysis and geophysical data integration. Xander’s affinity with near-surface geophysical methods is a result of his thesis work on 3D ground-penetrating radar data imaging, in the Mining Department at Delft University, the Netherlands. His interest in surface waves started during his PhD in the Applied Mathematics Dept, also at Delft University, where from 2000-2005, he worked on the development of a method to predict scattered surface waves in exploration seismic data.

Relationships between water flow rate and geophysical measurements in an alluvial aquifer

Our study focuses on the potential usefulness of surface geophysical data to constrain the water content within an alluvial aquifer. On a study area where two wells have been drilled, we have performed several geophysical measurements, including ground penetrating radar, DC resistivity prospecting, seismic refraction survey and magnetic resonance soundings. From these data, we estimated several parameters, namely, the water height in the deposits, the effective porosity, the water content, the permeability, and the transmissivity of alluvial deposits. These physical parameters allow us to characterize the alluvial deposits in order to constrain the estimation of the potential water flow. The lithology and water flow rate known from the wells enabled us to compare geophysical results obtained in a high water flow rate zone to those in a low water flow rate zone. Correlation has been found between the water flow rate observed in both wells and the geophysical data obtained in the vicinity of these wells.

Lateral variations in mantle rheology: implications for convection related surface observables and inferred viscosity models

SUMMARY Over the past decade numerous analyses of convection-related observables, such as horizontal surface divergence, geoid or gravity anomalies and dynamic surface topography, have been carried out in the context of tomography-based mantle flow models. One of the major objectives of this modelling has been the inference of the rheological structure of the mantle. With few exceptions, these studies have been conducted in the framework of a viscous flow theory which assumes that the mantle rheology may be represented in terms of an effective viscosity which varies with depth only. Here, we present a detailed assessment of the impact of lateral variations in viscosity on global convection related observables using forward modelling of buoyancy induced flow in a 3-D spherical shell. We find that the resulting dynamic topography at the surface and the core‐mantle boundary, as well as the gravitational response of the earth, are affected relatively little by the inclusion of lateral viscosity variations (LVV) when compared with results for a purely 1-D radial viscosity model. In particular, we found that the effect of LVV on the global observables is significantly smaller than the variability due to uncertainties in the current seismic tomography models. We also quantify the effect of LVV in the context of the viscosity inverse problem using two synthetic data sets generated with a 1-D viscosity profile and with a fully 3-D viscosity model in which the LVV span across three orders of magnitude. We compared the 1-D viscosity profiles recovered from the inversions and found that LVV have virtually no effect on our inversion results. The synthetic viscosity inversion further revealed that the effect of LVV is small in comparison to the uncertainties arising from the seismic tomography models. The inversions also suggest that the 1-D viscosity profiles derived from actual surface data represent the depth variation of the horizontally averaged logarithm of the 3-D viscosity distribution in the mantle. INTR O DUCTION Constraining the rheological properties of the mantle using surface geophysical data has traditionally involved separate treatments of short(seismic), intermediate- (solid Earth tides), long- (postglacial rebound) and very-long- (mantle convection) timescale processes. In each case the initial focus, historically speaking, has been on the inversion of the data to determine a one-dimensional (1-D), purely radial variation of the relevant rheological parameter (e.g. elastic moduli, intrinsic attenuation and viscosity) which provides the best fit to the data. It has long been recognized, particularly in the context of the seismic inverse problem, that the search for a 1-D earth model is subject to possible bias due to the presence of lateral variations in structure which may be as significant as the radial variations, especially at shallow depths. This paper is concerned with the mantle viscosity field. The earliest inferences of the radial (i.e. horizontally averaged) profile of viscosity based on glacial isostatic adjustment (GIA) data (e.g. McConnell 1968; Parsons 1972) were characterized by a pronounced increase of viscosity with depth in the top half of the mantle; however, these were followed by two decades of GIA research that argued for an essentially isoviscous

Faulting and volcanism in the axial valley of the slow‐spreading center of the Mariana back arc basin from Wadatsumi side‐scan sonar images

We analyzed in detail the geology of the median valley floor of the Mariana Basin slow‐spreading ridge using sea surface geophysical data and a high‐resolution deep‐tow side‐scan sonar survey over one spreading segment. Analysis of surface magnetic data indicates highly asymmetric accretion, with the half‐spreading rate on the western side of the basin being two to three times larger than on the eastern side. Surface magnetic and reflectivity data together suggest that asymmetric spreading is accomplished through eastward ridge jumps of ∼10 km of amplitude. Deep‐tow backscatter data indicate along‐axis variations of the volcanic processes with the emplacement of smooth and hummocky flows at the segment center and end, respectively. This variation likely relates to changes in the effusion rate due to the deepening or even disappearance of the magma chamber toward the segment end. Concerning tectonic processes, we find a power law distribution of the fractures, with an exponent of 1.74. This suggests that within the inner valley floor, fracture growth prevails over fracture nucleation and coalescence and that fractures accommodate less than 8% of the strain. According to our calculation based on a ratio of 0.02 to 0.03 between the vertical displacement and the length of faults, the amount of tectonic strain accommodated in the inner valley floor would consistently be ∼1.1–3.4%. Data also show two distinct sets of fractures. One trend is parallel to the rift direction at the segment center (∼N160°E) and perpendicular to the plate separation direction. Another set trends ∼17° oblique to this direction (∼N175°E) and is located over the eastern part of the valley, in the vicinity of a major bounding fault also trending ∼N175°E, that is, obliquely to the direction of plate motion. We modeled the stress field near a major fault that is oblique to the regional stress field associated with plate separation using a three‐dimensional boundary element approach. We found that the orientation of the predicted fissuring near the oblique fault is locally rotated by ∼15° due to a flexure of the bending plate close to this fault.

Buried Neogene volcanic structures in Hungary

Surface Neogene volcanics in Hungary are abundantly documented in the literature, but buried volcanic structures are little known. Early burial of the volcanic centers beneath latest Miocene to Pliocene sediments preserved much of their original relief, permitting their classification into genetic types. More than two-thirds of Hungary is covered by thick Neogene and Quaternary sediments, below which buried volcanic eruptive centers and the extent of their products may only be recognized by complex geologic-geophysical methods. Our study is based on the data of several thousand wells, more than 60,000 km of seismic sections, as well as airborne and surface geophysical (gravimetric, magnetic, electromagnetic, radiometric) data. Results of chemical, mineralogical studies and K/Ar dating of deep cores were also included. The data were evaluated in terms of the regional deep structure of the Carpathian-Balkan region, the Miocene evolution of which was determined by the position, movement and welding of indivi...