Application of the 3D magnetotelluric inversion code in a geologically complex area

The WSINV3DMT code makes the implementation of 3D inversion of magnetotelluric data feasible using a single PC. Audio‐magnetotelluric data were collected along two profiles in a Cu‐Ni mining area in Xinjiang, China, where the apparent resistivity and phase curves, the phase tensors and the magnetic induction vectors indicate a complex 3D conductivity structure. 3D inversions were carried out to reveal the electrical structure of the area. The final 3D model is selected from the inversion results using different initial Lagrange values and steps. The relatively low root‐mean‐square (rms) misfit and model norm indicate a reliable electrical model. The final model includes four types of low resistivity areas, the first ones coincide with the known location of an orebody and further forward modelling indicates that they are not in full connectivity to form a low resistivity zone. The second ones are not controlled by magnetotelluric sites and embody little information of the observed data, they are considered as tedious structures. The third one is near to the regional Kangguer fault and should be treated carefully considering the effect of the fault. The last ones are isolated and existing at a limited level as the first ones, they should be paid more attention to.     

2D data modelling by electrical resistivity tomography for complex subsurface geology

A new tool for two‐dimensional apparent‐resistivity data modelling and inversion is presented. The study is developed according to the idea that the best way to deal with ill‐posedness of geoelectrical inverse problems lies in constructing algorithms which allow a flexible control of the physical and mathematical elements involved in the resolution. The forward problem is solved through a finite‐difference algorithm, whose main features are a versatile user‐defined discretization of the domain and a new approach to the solution of the inverse Fourier transform. The inversion procedure is based on an iterative smoothness‐constrained least‐squares algorithm. As mentioned, the code is constructed to ensure flexibility in resolution. This is first achieved by starting the inversion from an arbitrarily defined model. In our approach, a Jacobian matrix is calculated at each iteration, using a generalization of Cohn's network sensitivity theorem. Another versatile feature is the issue of introducing a priori information about the solution. Regions of the domain can be constrained to vary between two limits (the lower and upper bounds) by using inequality constraints. A second possibility is to include the starting model in the objective function used to determine an improved estimate of the unknown parameters and to constrain the solution to the above model. Furthermore, the possibility either of defining a discretization of the domain that exactly fits the underground structures or of refining the mesh of the grid certainly leads to more accurate solutions. Control on the mathematical elements in the inversion algorithm is also allowed. The smoothness matrix can be modified in order to penalize roughness in any one direction. An empirical way of assigning the regularization parameter (damping) is defined, but the user can also decide to assign it manually at each iteration. An appropriate tool was constructed with the purpose of handling the inversion results, for example to correct reconstructed models and to check the effects of such changes on the calculated apparent resistivity. Tests on synthetic and real data, in particular in handling indeterminate cases, show that the flexible approach is a good way to build a detailed picture of the prospected area.     

GPR to constrain ERT data inversion in cavity searching: Theoretical and practical applications in archeology

I used theoretical forward models to show that a cavity embedded in a stratified sedimentary sequence can induce an equivalence problem in the ERT data inversion. Conductive top soil increases the misfit between the ground feature and the ERT model. The misfit depends on array and stratigraphy sequences. The latter induce an equivalence problem that manifests itself as wrong cavity depth positioning. The misfit is greater in the data acquired with Schlumberger array than with dipole–dipole.The ambiguity of ERT data inversion problems was tested in the detection of cavities linked to an 8th–6th century B.C. Sabine tomb, 3 m wide × 3 m long × 2 m high, excavated from a shaly gray volcanic ash (cinerite) layer covered by semi-lithoid tuff and top soil layers. In the real study I reduced the ambiguity in the inverse problem of ERT data using a priori information on geometry and resistivity of the cavity. The constrains were carried out from georadar data acquired with 80 and 200 MHz antenna. I demonstrate that this procedure has a practical application in cavity detection, and is a key to the reduction of the uncertainty inherent in the inversion process of ERT data.     

Very high resolution 3D marine seismic data processing for geotechnical applications

The processing of a small-scale, very high resolution (VHR) shallow marine 3D data volume is described. The data were acquired over a small clay diapir, on the river Schelde, in 1990. Using an array of 12 dual-channel microstreamers towed from a catamaran, a network of 1 m × 1 m bins could be produced over an area of 50 m × 180 m (< 100 m penetration). Positioning was performed with an auto-tracking laser ranging system, assuring an absolute accuracy of a few decimetres.
Preliminary processing steps included tidal correction and multiple removal. An important step concerned the application of 3D prestack Kirchhoff depth migration. Indeed this processing allows easy handling of the exact positions of both source and receivers as the latter were not set out on a conventional regular grid due to navigation difficulties. Because of the restricted data volume and the more-or-less stratified medium, a 1D velocity model could be used. This allowed a considerable simplification of the migration algorithm, based on summation. Traveltimes were calculated only once, using a 2D time grid with 0.1 m intervals.
This migration method proved very efficient, greatly improving the seismic image, and involved only limited CPU time on a small computer (Sparc 10 workstation). It clearly demonstrates that advanced seismic processing can form a valuable and economically feasible tool for VHR shallow subsurface 3D seismics, as long as the velocity field is not too complex. This method should therefore no longer be restricted to large computers and hydrocarbon exploration, but should also become a routine for VHR 3D shallow seismic work.     

A 3D ERT study of solute transport in a large experimental tank

A high resolution, cross-borehole, 3D electrical resistivity tomography (ERT) study of solute transport was conducted in a large experimental tank. ERT voxels comprising the time sequence of electrical images were converted into a 3D array of ERT estimated fluid conductivity breakthrough curves and compared with direct measurements of fluid conductivity breakthrough made in wells. The 3D ERT images of solute transport behaviour were also compared with predictions based on a 3D finite-element, coupled flow and transport model, accounting for gravity induced flow caused by concentration differences.The tank (dimensions 185×245×186 cm) was filled with medium sand, with a gravel channel and a fine sand layer installed. This heterogeneous system was designed to complicate solute transport behaviour relative to a homogeneous sand tank, and to thus provide a challenging but insightful analysis of the ability of 3D ERT to resolve transport phenomena. Four ERT arrays and 20 piezometers were installed during filling. A NaCl tracer (conductivity 1.34 S/m) was injected and intensively monitored with 3D ERT and direct sampling of fluid chemistry in piezometers.We converted the bulk conductivity estimate for 250 voxels in the ERT imaged volume into ERT estimated voxel fluid conductivity by assuming that matrix conduction in the tank is negligible. In general, the ERT voxel response is in reasonable agreement with the shape of fluid conductivity breakthrough observed in six wells in which direct measurements of fluid conductivity were made. However, discrepancies occur, particularly at early times, which we attribute to differences between the scale of the image voxels and the fluid conductivity measurement, measurement errors mapped into the electrical inversion and artificial image roughness resulting from the inversion.ERT images revealed the 3D tracer distribution at 15 times after tracer injection. The general pattern and timing of solute breakthrough observed with ERT agreed with that predicted from the flow/transport modelling. However, the ERT images indicate a vertical component of tracer transport and preferential flow paths in the medium sand. We attribute this to transient vertical gradients established during tracer injection, and heterogeneity caused by sorting of the sand resulting from the filling procedure. In this study, ERT provided a unique dataset of 250 voxel breakthrough curves in 1.04 m³. The use of 3D ERT to generate an array of densely sampled estimated fluid conductivity breakthrough curves is a potentially powerful tool for quantifying solute transport processes.     

Imaging a 3D geological structure from HEM, airborne magnetic and ground ERT data in Kalat-e-Reshm area, Iran

A set of geophysical data collected in an area in Iran are analyzed to check the validity of a geological map that was prepared in connection to a mineral prospecting project and also to image the spatial electrical resistivity distribution. The data set includes helicopter electromagnetic (HEM), airborne magnetic and ground electrical resistivity measurement. Occam approach was used to invert the HEM data to model the resistivity using a layered earth model with fixed thicknesses. The algorithm is based on a nonlinear inverse problem in a least-squares sense.The algorithm was tested on a part of an HEM dataset acquired with a DIGHEM helicopter EM system at Kalat-e-Reshm, Semnan in Iran. The area contains a resistive porphyry andesite that is covered by Eocene sedimentary units. The results are shown as resistivity sections and maps confirming the existence of an arc like resistive structure in the survey area. The resistive andesite seems to be thicker than it is indicated in the geological maps. The results are compared with the reduced to the pole (RTP) airborne magnetic anomaly field data as well as with two ground resistivity profiles. We found reasonable correlations between the HEM 1D resistivity models and 2D models from electrical resistivity tomography (ERT) inversions. A 3D visualization of the 1D models along all flight lines provided a useful tool for the study of spatial variations of the resistivity structure in the investigation area.     

2-D and 3-D processing and interpretation of multi-fold ground penetrating radar data: a case history from an archaeological site

A 2.5-D and 3-D multi-fold GPR survey was carried out in the Archaeological Park of Aquileia (northern Italy). The primary objective of the study was the identification of targets of potential archaeological interest in an area designated by local archaeological authorities. The second geophysical objective was to test 2-D and 3-D multi-fold methods and to study localised targets of unknown shape and dimensions in hostile soil conditions. Several portions of the acquisition grid were processed in common offset (CO), common shot (CSG) and common mid point (CMP) geometry. An 8×8 m area was studied with orthogonal CMPs thus achieving a 3-D subsurface coverage with azimuthal range limited to two normal components. Coherent noise components were identified in the pre-stack domain and removed by means of FK filtering of CMP records. Stack velocities were obtained from conventional velocity analysis and azimuthal velocity analysis of 3-D pre-stack gathers. Two major discontinuities were identified in the area of study. The deeper one most probably coincides with the paleosol at the base of the layer associated with activities of man in the area in the last 2500 years. This interpretation is in agreement with the results obtained from nearby cores and excavations. The shallow discontinuity is observed in a part of the investigated area and it shows local interruptions with a linear distribution on the grid. Such interruptions may correspond to buried targets of archaeological interest. The prominent enhancement of the subsurface images obtained by means of multi-fold techniques, compared with the relatively poor quality of the conventional single-fold georadar sections, indicates that multi-fold methods are well suited for the application to high resolution studies in archaeology.     

Possibilities of interpretation of the magnetotelluric data,obtained on a single profile over 3D resistivity structures

Magnetotelluric soundings are frequently carried out on a single profile or on profiles remote from each other. Interpretation of the obtained data is difficult in the presence of spatially heterogeneous geoelectric structures. We evaluate its capabilities on the basis of the synthetic data, that correspond to a geoelectric model, which consists of a three-layered section in the background and three rectangular prisms, differently arranged relative to the profile. Using the simple methods of analysis of magnetotelluric data, we succeeded in allocating all three heterogeneities over the area that surrounds the profile of observations. As a result of the fast smoothed-structure 1D and 2D inversion of different components of data, taking into account their specific features, the depths of the occurrence of anomalies and the order of the values of their electrical resistivity were evaluated, and the background section was also reconstructed. On this basis, and, also, with the use of a priori geological-geophysical information, the construction of a 3D model in a more or less broad band around the profile and its correction with the aid of 3D data inversion are possible.     

三维AVO解释技术在洪泽地区岩性油藏勘探中的应用

洪泽地区由于沉积的特点，储层横向变化快，油藏受构造、岩性、油源多因素控制。在对该区三维AVO属性体解释中，利用多元回归方法求取了横波曲线，分岩性和含油气性统计了纵、横波、泊松比参数分布规律，建立了本区的含油砂岩的正演模型，从而降低了AVO解释的多解性。通过井-震结合对四种AVO属性数据体进行了标定，并确定了各属性体应用范围，进而进行了储层和含油气检测。实践表明，该方法能有效地利用AVO属性数据体进行储层预测及油气检测，具有一定的推广价值。     

A pattern-based approach for multiple removal applied to a 3D Gulf of Mexico data set

Surface‐related multiples are attenuated for one sail line and one streamer of a 3D data set (courtesy of Compagnie Générale de Géophysique). The survey was carried out in the Gulf of Mexico in the Green Canyon area where salt intrusions close to the water‐bottom are present. Because of the complexity of the subsurface, a wavefield method incorporating the full 3D volume of the data for multiple removal is necessary. This method comprises modelling of the multiples, where the data are used as a prediction operator, and a subtraction step, where the model of the multiples is adaptively removed from the data with matching filters. The accuracy of the multiple model depends on the source/receiver coverage at the surface. When this coverage is not dense enough, the multiple model contains errors that make successful subtraction more difficult. In these circumstances, one can either (1) improve the modelling step by interpolating the missing traces, (2) improve the subtraction step by designing methods that are less sensitive to modelling errors, or (3) both. For this data set, the second option is investigated by predicting the multiples in a 2D sense (as opposed to 3D) and performing the subtraction with a pattern‐based approach. Because some traces and shots are missing for the 2D prediction, the data are interpolated in the in‐line direction using a hyperbolic Radon transform with and without sparseness constraints. The interpolation with a sparseness constraint yields the best multiple model. For the subtraction, the pattern‐based technique is compared with a more standard, adaptive‐subtraction scheme. The pattern‐based approach is based on the estimation of 3D prediction‐error filters for the primaries and the multiples, followed by a least‐squares estimation of the primaries. Both methods are compared before and after prestack depth migration. These results suggest that, when the multiple model is not accurate, the pattern‐based method is more effective than adaptive subtraction at removing surface‐related multiples while preserving the primaries.     

Internal structure of Ustica volcanic complex (Italy) based on a 3D inversion of magnetic data

A ground magnetic study of Ustica Island was performed to provide new insights into subsurface tectonic and volcanic structures. The total-intensity anomaly field, obtained after a data-reduction procedure, shows the presence of a W–E-striking magnetic anomaly in the middle of the island and another two intense anomalies, which seem to continue offshore, in the southwestern and the northeastern sides, respectively. The detected anomalies were analyzed by a quadratic programming (QP) algorithm to obtain a 3D subsurface magnetization distribution. The volcano magnetization model reveals the presence of intensely magnetized volumes, interpreted as the feeding systems of the main eruptive centers of the island, which roughly follow the trend of the main regional structural lineaments. These findings highlight how regional tectonics has strongly affected the structural and magmatic evolution of the Ustica volcanic complex producing preferential ways for magma ascent.     

Robust methods for the decomposition and interpretation of compound dunes applied to a complex hydromorphological setting

Underwater dunes are a morphological feature that are explored by marine scientists and coastal engineers alike. This study presents new methodologies in order to simplify bedform identification and morphodynamic analyses. Specifically, subaqueous compound dunes are decomposed with a simple yet extensive tracking algorithm, which relies on a repeated evaluation of unfiltered bed elevation profiles according to five predefined length classes. In a second step, morphological trends are assessed in the form of bed migration rates, bed slope asymmetries and net sediment changes, in which all parameters are referred to equidistant sections of the examined fairway stretch. This integrated approach not only avoids the challenges in weighting the varying size and abundance of dunes of different scales but also ensures comparability between dune-specific and areal parameters, which significantly improves the interpretation of the morphological setting as a whole. The developed methods are applied to the Outer Jade fairway, an anthropogenically influenced and regularly maintained waterway in the German Bight, and allow scrutiny of spatio-temporal trends in this region. Based on a unique data set of 100 sequential high-quality echo-sounding surveys, various types of bedforms are identified, comprising large-scale primary as well as superimposing secondary dunes that are assumed to interfere with each other. Temporal trends show a long-term rise of the troughs of major bedforms and constant maximum crest elevations near the official maintenance depth, which matches the observed long-term aggradation of sediments. The spatial distribution of integrated morphodynamic parameters reflects a previously described zone of primary dune convergence and facilitates the precise localization of this geophysical singularity. The presented findings both confirm the robustness of the proposed methodologies and, in return, enhance the understanding of morphological processes in the Outer Jade. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

The strong ground motion in Mexico City: Analysis of data recorded by a 3D array

In 2001, we installed 11 new accelerometers in the lake-bed zone of Mexico City, which, together with the existing stations, form a 3D array of small aperture, with 15 surface and six borehole stations. This new array recorded the 10/08/2001 Guerrero event (M=6.1) and the 01/22/2003 Colima (M=7.6) event. The analysis of these two events showed that, for periods longer than 2.5 s, the dominant wavefield is composed of a fundamental mode of surface waves arriving from the epicenter and diffracted surface waves generated at the southern boundary of the Mexican Volcanic Belt. For periods shorter than 2.5 s, the resonance of the superficial clay layer becomes predominant. The wavefield consists in a superposition of diffracted waves propagating from the western part of the valley. Our results show that the ground motion is strongly amplified by the soft clay layer but its duration is controlled by the incident wavefield.     

Effective solutions to a global 3D visual system in networking environments

Today, with the proposal of digital earth, digital city and digital community, and rapid advances in informa-tion technologies and collection of spatial data, to a great degree, the traditional 2D GIS is promoted to Web and 3D GIS, which greatly challenges the capaci-ties of the system in transmitting and handling massive datasets. A great number of 3D GIS prototype systems have been developed and applied in such areas as mine resource management, digital cities. However, appli-cation sys…     

High resolution profiles for arsenic in the Seine Estuary. Seasonal variations and net fluxes to the English Channel

Three sampling cruises were conducted in the Seine Estuary from 1993 to 1995 in varying hydrological and seasonal conditions. The site included all of the lower part of the river under the influence of tidal dynamics and the dilution plume in the Baie de Seine. Chemical speciation of arsenic showed high seasonal variations, especially in September when AsIII represented around 50% of dissolved arsenic. The inclusion of organoarsenic compounds not accessible to direct analysis by hydride generation required preliminary mineralisation of the samples. The ratio of dissolved to particulate arsenic distribution was controlled mainly by the iron content of particles. Biological activity had an influence on chemical speciation and thus on the partition coefficient (K_D 10⁻³=6±1 in September and 12±0.9 in February). The zone of conservative mixing used for Seine River flow calculations was limited to a salinity range of approximately 10–30. Dissolved arsenic concentrations extrapolated to null salinities were lower during the high-water period because of dilution (17.6±1.1 nM), and maximal during the low-water period in summer (35.7±0.9 nM). Mean arsenic export to the English Channel was estimated at 33.2±6 T yr⁻¹ for dissolved arsenic. Observation of an arsenic output greater than the upstream input, as well as a simultaneous increase in dissolved and particulate arsenic concentrations during the mixing of freshwater with seawater, strongly suggested the existence of an important intra-estuarine source of arsenic, either of industrial origin or related to the transport and diagenesis of marine sediments.     

A multifractal formalism for vector-valued random fields based on wavelet analysis: application to turbulent velocity and vorticity 3D numerical data

Extreme atmospheric events are intimately related to the statistics of atmospheric turbulent velocities. These, in turn, exhibit multifractal scaling, which is determining the nature of the asymptotic behavior of velocities, and whose parameter evaluation is therefore of great interest currently. We combine singular value decomposition techniques and wavelet transform analysis to generalize the multifractal formalism to vector-valued random fields. The so-called Tensorial Wavelet Transform Modulus Maxima (TWTMM) method is calibrated on synthetic self-similar 2D vector-valued multifractal measures and monofractal 3D vector-valued fractional Brownian fields. We report the results of some application of the TWTMM method to turbulent velocity and vorticity fields generated by direct numerical simulations of the incompressible Navier–Stokes equations. This study reveals the existence of an intimate relationship between the singularity spectra of these two vector fields which are found significantly more intermittent than previously estimated from longitudinal and transverse velocity increment statistics.

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A methodology to estimate the maximum offset and reflection imaging radius for a 2D/3D vertical seismic profiling survey

We have developed a straightforward and ray based methodology to estimate both the maximum offset and reflection imaging radius for multi‐layered velocity models, which can be used for a 2D/3D VSP survey design. Through numerical examples, we demonstrate that the presence of a high‐velocity layer above a target zone significantly reduces the maximum offset and reflection imaging radius. Our numerical examples also show that including in a migration VSP data acquired beyond a recommended maximum offset, radically degrades the quality of the final VSP image. In addition, unlike the conventional straight‐line based approximation that often produces an incorrect large reflection imaging radius, our methodology predicts the VSP imaging radius with more accuracy than does the conventional approximation.     

基于抛物线密度模型的频率域三维界面反演及其在川滇地区的应用

密度界面反演作为了解地球内部结构的一种重要方法,长期以来都是重力学研究的主要内容.本文结合抛物线密度模型及频率域算法的优点,将抛物线密度函数应用于Parker-Oldenburg算法,经过理论推导得到了抛物线密度模型的频率域公式,从而建立了基于抛物线密度模型的三维密度界面重力异常正反演的算法和流程.理论模型数据试验表明本方法快速、有效,适用于大多数浅部比深部增加更快的实际地壳密度.研究中还利用该方法对川滇地区重力异常进行了反演,获得了该区的莫霍面深度分布,并与接收函数研究结果进行对比分析,进一步验证了本文方法的正确性和有效性.     

A 3D gravity data interpretation of the Matagami mining camp, Abitibi Subprovince, Superior Province, Québec, Canada: Application to VMS deposit exploration

Two inversions, unconstrained and constrained, of a gravity survey of the Matagami mining camp (Abitibi Archaean Subprovince, Canada) have been performed in order to identify the downward extension of a rhyolitic horizon hosting VMS-type base metals deposit and the morphologies of the major felsic plutons. A comparative study exhibits the similarities between measured and calculated densities from chemical compositions of the Matagami lithologies. This allows building an initial 3D geodensity model which integrates densities and available structural and geological surface mapping data. This model is integrated during the iteration process of the constrained inversion in the objective function. The resulting true density model and two derived cross-sections upgrade the 3D imaging of this area. Also, the model gives new insight for regional geological interpretation exposing possible shapes of the main geological units at depth and suggests the potential existence of deep fertile geological bodies.