Complex Subsurface Geology Research Articles

Spatial and seasonal fluctuations in fresh submarine groundwater discharge revealed by marine continuous resistivity profiling

Submarine Groundwater Discharge (SGD) is a major pathway for the discharge of fresh and saline groundwater and associated dissolved compounds into marine environments. However, assessing SGD processes in coastal aquifers is challenging due to inaccessibility, dynamic conditions, complex subsurface geology, and the need for long-term monitoring to capture temporal and spatial variations in SGD rates accurately. This study employs marine continuous resistivity profiling (MCRP) as a main method to assess the presence of freshwater or brackish SGD offshore and to examine its potential seasonal variations. The method has been applied in the coastal alluvial aquifer of Maresme (Spain) and validated with other methods to trace SGD, including salinity profiles, Ra isotopes, and piezometric levels. Several MCRP transects of 700 m long, perpendicular to the coastline, were performed in a coastal marine area to obtain electrical resistivity data of the seabed covering an area of 3 km2. The data was acquired in two field campaigns with contrasting hydrological conditions (dry and wet seasons). The MCRP results allow the identification of areas of fresh SGD in marine sediments, with a clear seasonal variability that indicates a higher discharge of fresh groundwater in the wet season.

Borehole‐Based Interval Kriging for 3D Lithofacies Modeling

AbstractDeveloping a three‐dimensional (3D) lithofacies model from boreholes is critical for providing a coherent understanding of complex subsurface geology, which is essential for groundwater studies. This study aims to introduce a new geostatistical method—interval kriging—to efficiently conduct 3D borehole‐based lithological modeling with sand/non‐sand binary indicators. Interval kriging is a best linear unbiased estimator for irregular interval supports. Interval kriging considers 3D anisotropies between two orthogonal components—a horizontal plane and a vertical axis. A new 3D interval semivariogram is developed. To cope with the nonconvexity of estimation variance, the minimization of estimation variance is regulated with an additional regularization term. The minimization problem is solved by a global‐local genetic algorithm embedded with quadratic programming and Brent's method to obtain kriging weights and kriging length. Four numerical and real‐world case studies demonstrate that interval kriging is more computationally efficient than 3D kriging because the covariance matrix is largely reduced without sacrificing borehole data. Moreover, interval kriging produces more realistic geologic characteristics than 2.5D kriging, while conditional to spatial borehole data. Compared to the multiple‐point statistics (MPS) algorithm—SNESIM, interval kriging can reproduce the geological architecture and spatial connectivity of channel‐type features, meanwhile producing tabular‐type features with better connectivity. Because the regularization term constrains kriged value toward 0 or 1, interval kriging produces more certainty in sand/non‐sand classification than 2.5D kriging, 3D kriging, and SNESIM. In conclusion, interval kriging is an effective and efficient 3D geostatistical algorithm that can capture the 3D structural complexity while significantly reducing computational time.

Geophysical assessment of seawater intrusion: the Volturno Coastal Plain case study

In coastal alluvial plains, the variability of sedimentary inputs, tectonic and eustatism causes a complex subsurface geology which influences the position of fresh/saltwater interface. Furthermore, in these areas densely populated, the over-pumping of freshwater, coupled with the climate change events, promotes the landward migration of freshwater/saltwater boundary. This research illustrates the ability of geophysical tools to recognize the presence of salt/brackish water at Volturno Coastal Plain, Southern Italy. This area is characterized by a peculiar geological setting, due to the proximity at Somma–Vesuvio and Campi Flegrei volcanic areas, which profoundly influences the circulation of groundwater. The subsurface is mainly characterized by: (i) two denser layers located at − 10 m and − 20 m depth which in part prevents the vertical migration of groundwater, (ii) facies heteropy that facilitates the hydraulic connection between the different geological bodies, (iii) a discontinuous Campanian Ignimbrite deposits which favor the hydraulic connection between deeper and shallower aquifers. In this geological framework, 2D-ERT and 3D-ERT integrated with Downhole, Multichannel Analysis of Surface Waves and boreholes made possible to recognize the presence of two main zones with salt and brackish waters, respectively. The first zone, characterized by very low resistivity (≤ 1 Ωm) typical of salt water, stretches 1.5 km inland from the coast. The second zone, with a resistivity between 2 and 5 Ωm typical of brackish water, continues for other 3 km inland. This knowledge is useful for the engagement of all stakeholders (farmers, ranchers and policy makers) in the sustainable use of fresh water and for making water management plan operational tools.

2D Joint Inversion of Gravity and Magnetic Data to Evaluate a Geological Structure in Tanimbar Island

The need for hydrocarbon energy, especially oil and gas, requires continuous exploration of hydrocarbon reserves, especially in eastern Indonesia. Tanimbar Islands, Maluku was chosen to be the research area because of its complex subsurface geology and is located on the fold thrust belt resulting from tectonic evolution. In this research, an innovation is carried out using joint inversion modeling by correlating physical parameters from two different geophysical data, which are gravity and geomagnetic. The purpose of this research is to determine the subsurface mapping of the research area, determine the density and susceptibility values of rocks, and determine the structural patterns that exist in the subsurface. The results of subsurface mapping of the Tanimbar Islands based on the results of Joint Inversion modelling have found a geological structure pattern in the form of a fold thrust belt due to the Banda Arc subduction zone. In section A-A', there is a sequence of fold thrust belt with 10 structural patterns, including 6 structures in the northwest-southeast direction and 4 structures in the northeast-southwest direction. In section B-B' there are 9 structural patterns in the northeast-southwest direction. High anomalies obtained density values ranging from 2.77 - 2.81 gr/cm3 and susceptibility ranging from 0.00125 - 0.0013 SI are thought to be caused by high basement in the form of volcanic rocks, while low anomalies obtained density values ranging from 2.45 - 2.49 gr/cm3 and susceptibility ranging from 0.0008 - 0.00085 SI are thought to be caused by thick sedimentary layers of the Batimafudi Formation.

2.5D multifocusing imaging of crooked-line seismic surveys

Two-dimensional seismic surveys often are conducted along crooked-line traverses due to the inaccessibility of rugged terrains, logistical and environmental restrictions, and budget limitations. The crookedness of line traverses, irregular topography, and complex subsurface geology with steeply dipping and curved interfaces could adversely affect the signal-to-noise-ratio of the data. The crooked-line geometry violates the assumption of a straight-line survey that is a basic principle behind the 2D multifocusing (MF) method and leads to crossline spread of midpoints. In addition, the crooked-line geometry can give rise to potential pitfalls and artifacts and thus leads to difficulties in imaging and velocity-depth model estimation. We develop a novel MF algorithm for crooked-line seismic data and revise the traveltime equation accordingly to achieve better signal alignment before stacking. Specifically, we have developed a 2.5D MF reflection traveltime equation that explicitly takes into account the midpoint dispersion and cross-dip effects. The new formulation corrects for normal, inline, and crossline dip moveouts (DMOs) simultaneously, which is significantly more accurate than removing these effects sequentially. Applying normal moveout, DMO, and cross-DMO separately tends to result in significant errors, especially for large offsets. The 2.5D MF method can perform automatically with a coherence-based global optimization search on data. We investigated the accuracy of the new formulation by testing it on different synthetic models and a real seismic data set. Applying our approach to the real data led to a high-resolution seismic image with a significant quality improvement compared to the conventional method. Numerical tests indicate that the new formula can accurately focus the primary reflections at their correct location, remove anomalous dip-dependent velocities, and extract true dips from seismic data for structural interpretation. Our method efficiently projects and extracts valuable 3D structural information when applied to crooked-line seismic surveys.

Exploring conceptual models of infiltration and groundwater recharge on an intermittent river: The role of geologic controls

Study regionThis study is along an intermittent reach of the Alamosa River in the San Luis Valley of south-central Colorado, a river that is typical of the semi-arid southwestern United States with respect to climate, land use, and the impacts of upstream dam regulation. Study focusWe use conceptual steady-state models to identify geologic factors that may control water loss through infiltration. These conceptual models are parameterized according to a range of conditions observed from stream discharge, topographic data, geologic data, and drone magnetometer data. New hydrological insightsThe introduction of a fault, variation in the alluvial aquifer hydraulic conductivity, and presence or absence of a confining unit in the numerical models were the primary geologic controls that affected infiltration across the study reach. Conversely, variation in the thickness of the streambed had little impact. This information may help determine future data collection within this and similar semi-arid regions where rivers are controlled by a combination of surface water availability (e.g. through dam regulation) and complex subsurface geology, which are often not well constrained.

Reprocessing of Regional 2D Marine Seismic Data of Part of Taranaki Basin, New Zealand Using Latest Processing Techniques

This study employed the use of various newly developed seismic data processing techniques which were unavailable as at the time (1986) of acquisition of the regional 2D marine seismic data (TRV 434) of part of Taranaki Basin, New Zealand, to reprocess the data in order to improve the volume as well as the quality of subsurface information derivable from the data which remain one of the vital sources of information for preliminary insight for petroleum prospect evaluation of the basin. The reprocessing operations attenuated various unwanted signals associated with the seismic data, F – K transform filter filtered out low frequency noise including swell noise while other noise types embedded in the seismic data were attenuated using Time Variant Omsby-Bandpass filters. Predictive deconvolution attenuated water bottom multiples as well as other periodic unwanted signals. True amplitude recovery technique restored lost reflection energies and made deeper reflections visible. Post and Pre-Stack Time Kirchhoff migration (PSTM) techniques appropriately repositioned dipping reflection events to their appropriate locations in time and space. Diffraction curves were collapsed to improve data resolution of both the shallow and deep reflection events. The reprocessing activities generally increased the illuminating strength of the TRV 434 marine seismic data to image the subsurface of the surveyed part of Taranaki Basin which presented complex subsurface geology in terms of structures and rock association.

Reprocessing of Regional 2D Marine Seismic Data of Part of Taranaki Basin, New Zealand Using Latest Processing Techniques

This study employed the use of various newly developed seismic data processing techniques, which had not been available as at the time (1986) of acquisition of the regional 2D marine seismic data (TRV 434) of part of Taranaki Basin, New Zealand, to reprocess the data in order to improve the volume as well as the quality of subsurface information derivable from the data which remain one of the vital sources of information for the preliminary insight for petroleum prospect evaluation of the basin. The reprocessing operations attenuated various unwanted signals associated with the seismic data, F – K transform filter filtered out low frequency noise including swell noise, while other noise types embedded in the seismic data were attenuated using Time Variant Omsby-Bandpass filters. Predictive deconvolution attenuated water bottom multiples as well as other periodic unwanted signals. True amplitude recovery technique restored lost reflection energies and made deeper reflections visible. Post and Pre-Stack Time Kirchhoff migration (PSTM) techniques appropriately repositioned dipping reflection events to their appropriate locations in time and space. Diffraction curves were collapsed to improve data resolution of both the shallow and deep reflection events. The reprocessing activities generally increased the illuminating strength of the TRV 434 marine seismic data to image the subsurface of the surveyed part of Taranaki Basin, which presented complex subsurface geology in terms of structures and rock association. Keywords: Kirchhoff migration; Omsby-Bandpass filters; Swell noise; Taranaki Basin; Velocity analysis. DOI 10.35180/gse-2020-0035

Comparative study of different risk measures for robust optimization of oil production under the market uncertainty: a regret-based insight

Model-based production optimization relies on a dynamic model that simulates the fluid flow in the oil reservoirs, and an economic objective function that assigns an economic measure to the recoverable oil reserves. An optimization algorithm utilizes the dynamic model to find the production scenario which maximizes the economic measure of profit. However, due to incompleteness and doubtfulness of available data, the reservoir model describing the complex subsurface geology is quite uncertain. Moreover, the definition of the economic objective functions such as net present value (NPV) requires economic variables such as oil price, interest rate, and production costs which unpredictably vary with time. In recent years, robust optimization (RO) has been widely used as an appropriate tool for handling the uncertainties in production optimization problems. However, previous works on robust optimization paid less attention to economic uncertainties arising from market volatility. Instead, they are mostly focused on geological uncertainties. This paper is devoted to production optimization under oil market uncertainty. To narrow down the range of economic uncertainties, a Bayesian framework for oil price history matching and forecasting has been developed which allows generating more reliable realizations of oil price future trend. It is common to include a measure of risk-averse in the objective function of RO problems. However, the quality of the solutions depends directly on the used risk measure. In the oil industry, risk measures such as worst-case scenario and CVaR (Conditional Value at Risk) have been used to mitigate the risk of low-profit realizations. These risk measures are appropriate in many cases for measuring the robustness. Though, they are inadequate in evaluating robustness in a relative sense in cases where the worst-case realizations have an undue effect on the final decisions. The risk measure defined based on the minimax regret approach takes into account all realizations instead of just considering the worst-case realizations. In this research, RO has been performed to maximize NPV using the minimax regret approach. In addition, the results are compared with the common risk measures used in the oil industry including expected profit, CVaR, and worst-case. Results show that while worst-case scenario and CVaR perform better than other risk measures in lower-profit realizations, they give inappropriate results for other scenarios. In contrast, regret-based approach and expected profit give nearly optimum solutions for all realizations. In this paper, the minimax regret approach was compared with other risk measures in the presence of oil price uncertainty. However, the results might be extended to optimization under geological uncertainty.

A new approach to improve reservoir modeling via machine learning

In highly heterogeneous basins with complex subsurface geology, such as the Nile Delta Basin, accurate prediction of reservoir modeling has been a challenge. Reservoir modeling is a continuous process that begins with field discovery and ends with the last phases of production and abandonment. Currently, the stochastic reservoir modeling method is widely used instead of the traditional deterministic modeling method to consider spatial statistics and uncertainties. However, the modeling workflow is demanding and slow, typically requiring months from the initial model concept to flow simulation. In addition, errors from early model stages become cumulative and are difficult to change retroactively. To overcome these limitations, a new workflow is proposed that implements probabilistic neural network inversion to predict reservoir properties. First, well-log data were conditioned properly to match the seismic data scale. Then, the networks were trained and validated, using the conditioned well-log data and seismic internal/external attributes, to predict water saturation and effective porosity 3D volumes. The resulting volumes were sampled in simulation 3D grids and tested using a blind well test. Subsequently, the permeability was calculated from a porosity-permeability relationship inside the reservoir. Finally, a dynamic simulation project of the field was performed in which the historical field production and pressures were compared to the predicted values. One of the Pliocene deepwater turbidite reservoirs in the offshore Nile Delta was used to demonstrate the proposed approach. The results proved the accuracy of the model in predicting the reservoir properties and honoring the heterogeneity of the reservoir. The new approach represents a shortcut for the seismic-to-simulation process, providing a reliable and fast way of constructing a reservoir model and making the seismic-to-simulation process easier.

Imaging of Hidden Structures from the North Apuseni Mts, Romania, Using Narrow-Angle Seismic Reflection Data

I present results of processing and structural interpretation of narrow-angle seismic reflection data recorded over an area of 30 × 50 km located in the southern part of the North Apuseni Mts, Romania. The investigated area is characterized by complex subsurface geology and rough topography. The seismic measurements were performed along five linear profiles, P1-P5, using an active spread of 96 geophones for each shot point; geophone spacing was 25 m. The length of each acquisition line is greater than 10 km. The signal-to-noise ratio of these data varies along the lines and its variation is considered to be an effect of rough topography, complex subsurface geology and varying surface conditions encountered during seismic data acquisition. The data processing was performed using a standard processing flow but with different processing parameters from one data set to another. I obtained five depth-converted migrated seismic sections after data processing. The accuracy of the depth values depends on that of the stacking velocities obtained from the velocity analyses performed on the filtered seismic data. Borehole information is not available, the investigated area belonging to the areas investigated for hydrocarbons. Each seismic section shows a different structural image of the subsurface and provides useful information about the tectonic and stratigraphic evolution of the investigated area. I obtained various structural images of the subsurface after the interpretation of the depth-converted migrated seismic sections, from a simple one with undeformed and inclined reflectors to a complex one with folded and faulted reflectors, especially the older ones. I interpreted intrusive bodies piercing through the overlying sediments, which are in good agreement with the results of older geophysical studies.

Revealing new hydrocarbon potential through Q-compensated prestack depth imaging at Wenchang Field, South China Sea

Wenchang Field in the South China Sea contains a well-developed fault system, resulting in complex subsurface geology. Imaging the complex fault system plays an important role in hydrocarbon exploration in this area since the fault system forms a link between the source rocks and reservoirs. However, it is difficult to obtain a high-quality depth image of the fault system due to the effects of complex velocity and seismic absorption. Inaccurate depth velocities lead to fault shadows and structure distortions at the target zone. Absorption effects further deteriorate seismic imaging as they cause amplitude attenuation, phase distortion, and resolution reduction. We demonstrate how a combination of high-resolution depth velocity modeling and Q imaging work together to resolve these challenges. This workflow provides a step change in image quality of the complex fault system and targeted source rocks at Wenchang Field, significantly enhancing structure interpretation and reservoir delineation. A couple of commercial discoveries have been made, and several other potential hydrocarbon reservoirs have been identified based on the reprocessed data, which reveal new hydrocarbon potential in this region.

TTEM — A towed transient electromagnetic system for detailed 3D imaging of the top 70 m of the subsurface

There is a growing need for detailed investigation of the top 30–50 m of the subsurface, which is critical for infrastructure, water supply, aquifer storage and recovery, farming, waste deposits, and construction. Existing geophysical methods are capable of imaging this zone; however, they have limited efficiency when it comes to creating full 3D images with high resolution over dozens to hundreds of hectares. We have developed a new and highly efficient towed transient electromagnetic (tTEM) system, which is capable of imaging the subsurface up to depth of 70 m at a high resolution, horizontally and vertically. Towed by an all-terrain vehicle, the system uses a [Formula: see text] transmitter coil and has a [Formula: see text]-component receiver placed at 9 m offset from the transmitter. The tTEM uses dual transmitter moment (low and high moment) measurement sequence to obtain the early and late time gates corresponding to shallow and deep information about the subsurface layers. The first bias-free gate is as early as [Formula: see text] from beginning of the ramp ([Formula: see text] after end of ramp). Data are processed and inverted using methods directly adopted from airborne electromagnetics. The system has been successfully used in Denmark for various purposes, e.g., mapping raw materials, investigating contaminated sites, and assessing aquifer vulnerability. We have also used the tTEM system in the Central Valley of California (United States) for locating artificial recharge sites and in the Mississippi Delta region, to map complex subsurface geology in great detail for building hydrogeologic models.

Technology Focus: Drilling Technology and Rigs (February 2018)

Technology Focus When considering the most significant aspects of new drilling technology to highlight for SPE readers, the latest downhole tools, rig design, and operational procedures often make the headlines. This year, however, a case can be made that the most profound advancements exceed anything achievable with new tools, procedures, or machines. On 29 November, the San Antonio Business Journal reported a major oil company had sold a nearby data center to a West Coast cloud-computing giant in a deal estimated at $100 million. The next day, the companies executed a strategic contract to partner and further implement digitization of upstream assets in a cloud-computing environment. The oil company executive involved said, “We have started digitizing our oil fields but want to accelerate deployment of new technologies that position us to increase revenues, lower costs, and improve safety and reliability of our operations.” Far from an isolated event, this case is but one example of an overall trend revolutionizing our industry. Perhaps this is the dominant technology evolution transforming our landscape today. Because drilling and well construction typically involve the majority of the cost and risk for upstream projects, this new revolution is inevitably the game changer for overall health, safety, and environment; efficiency; and financial performance on wells. Phrases such as “dawn of the new age of the oil and gas industry” and “the fourth industrial revolution” echoed through halls of SPE events in 2017 and into the new year, with rapid advancements in big-data management, digital connectivity, and high-performance computing (paper OTC 27638). Exciting developments in new downhole tools, fluids, and rig design continue to advance, with gains in safety and efficiency being multiplied by this transformation. The new revolution is also occurring in parallel with accelerated applications of managed-pressure- and underbalanced-drilling technology. The authors of highlighted paper SPE 185283 suggest the benefits of the technique collectively categorized as closed-loop drilling and the new drilling convention are so numerous that, indeed, all future rig operations should be configured as such. Concurrent with these trends toward increased data acquisition, data analysis, and remote control, automation of rig functionality continues as a primary focus and area of great potential across operator, rig-contractor, and equipment-supplier boundaries. The 3-year industry downturn persisting through 2017 brought an unprecedented purging of paradigms. But perhaps the extreme pressure to reduce costs and add value has indeed forced the innovation and acceleration being realized today during this new revolution. Recommended additional reading at OnePetro: www.onepetro.org. OTC 27638 The Dawn of the New Age of the Industrial Internet and How It Can Radically Transform the Offshore Oil and Gas Industry by Partha Sharma, DNV GL, et al. SPE/IADC 184695 Development to Delivery—A Collaborative Approach to Implementing Drilling Automation by Riaz Israel, BP, et al. SPE 187477 Latest Drilling Techniques Applied to Coring Operations of a Complex Subsurface Geology in WCSB Led to Operational Success and Cost Savings While Setting a Record in North America by Ali Hooshmandkoochi, Seven Generations Energy, et al. SPE/IADC 184650 The Floating Factory Concept: Engineering Efficiencies Up Front To Reduce Deepwater-Well-Delivery Cost by James Hebert, Diamond Offshore

Investigation of the effects of sediments inhomogeneity and nonlinearity on aggravation factors for sedimentary basins

The prediction of the seismic response of sedimentary basins and its incorporation into seismic building codes is of prime interest to the engineering community, since highly populated areas and infrastructures are often located within such geological configurations. The additional effect of the two-dimensional (2D) basin response above the corresponding one-dimensional (1D) response, expressed as the ratio between 2D and 1D acceleration response spectra at the basin surface and commonly referred to as “aggravation factor”, might be an engineering feasible way to incorporate such effects into building codes. The majority of the studies are based on numerical simulations for modeling the seismic behaviour of complex subsurface geology and thus for the estimation of aggravation factors, usually taking place under the assumption of homogeneous viscoelastic sediments, ignoring the fact that sedimentary deposits may be very heterogeneous in both the horizontal and the vertical directions, as well as the effect of soil nonlinearity, which for strong seismic excitation may be crucial. The present work is an extension of our work [1], in which extensive numerical analyses of the linear viscoelastic response of homogeneous sedimentary basins were performed, in order to explore the potential additional effects of sediments inhomogeneity and nonlinearity on aggravation factors. To this end, a generic relationship between shear wave velocity and depth was developed based on experimental shear wave velocity data from well-documented sites worldwide. To further model the nonlinear behaviour of sediments material, a nonlinear kinematic hardening constitutive model was used, combined with the Von Mises failure criterion and an associated plastic flow rule. All analyses were conducted for trapezoidal sedimentary basins using the generic finite element code ABAQUS. The results of the analyses revealed that the replacement of an inhomogeneous soil with an equivalent homogeneous might lead to underestimation of aggravation in the vicinity of the edges and overestimation of aggravation at the flat part of the basin, while consideration of soil nonlinearity may result in a decrease of aggravation factors for the flat part of the basin. These outcomes may be useful for the proposal of simple recommendations to the engineering community for the introduction of basin effects in the seismic design of structures.

The impact of multifocusing in the processing of land 3D seismic data in a fold and thrust belt setting: Ranquil Norte Block, Neuquén Basin, Argentina

A total of 270 km2 of 3D seismic data was acquired in the southeast corner of the Ranquil Norte Block in the north of the fold and thrust belt area of the Neuquén Basin, west of Argentina. The survey was acquired in an area with hard topography, disturbing anthropomorphic conditions, and very complex subsurface geology. All of these situations negatively contributed to the signal-to-noise ratio of the resulting data. Therefore, and considering a structural target only, the main objective of the seismic processing sequence was the mitigation of the impact of such geologic and anthropomorphic noise on the data. Initially, a tailored processing sequence was performed, including noise suppression in the cross-spread domain. This first processing approach included Kirchhoff prestack time migration, structural seismic interpretation, velocity modeling, tomography, and Kirchhoff prestack depth migration. Given that multifocusing can deliver high-resolution subsurface images in areas with similar challenging conditions, this processing technique was then included in the processing workflow. Naturally, this second approach necessarily involved a reinterpretation of the structural model, a rebuilding of the velocity model, and a new tomographic computation. A significant improvement of the results was achieved in this manner.

Inversion gradients for acoustic VTI wavefield tomography

Wavefield tomography can handle complex subsurface geology better than ray-based techniques and, ultimately, provide a higher resolution. Here, we implement forward and adjoint wavefield extrapolation for VTI (transversely isotropic with a vertical symmetry axis) media using a generalized pseudospectral operator based on a separable approximation for the P-wave dispersion relation. This operator is employed to derive the gradients of the differential semblance optimization (DSO) and modified image-power objective functions. We also obtain the gradient expressions for a data-domain objective function that can more easily incorporate borehole information necessary for stable VTI velocity analysis. These gradients are similar to the ones obtained with a space-time finite-difference (FD) scheme for a system of coupled wave equations but the pseudospectral method is not hampered by the imprint of the shear-wave artifact. Numerical examples also show the potential advantages of the modified image-power objective function in estimating the anellipticity parameter [Formula: see text].

The use of seismic modeling for the geologic interpretation of deep seismic reflection data with low signal-to-noise ratios

We have performed a deep seismic reflection study, DACIA-PLAN, based on the data recorded along a crooked line across the southeastern Romanian Carpathians. The signal-to-noise ratio (S/N) of these data varies along the seismic profile, and its variation is considered to be an effect of the rough topography, complex subsurface geology, and varying surface conditions encountered during seismic data acquisition. The migrated time section that covers the mountainous area is clear, without visible reflections, making the geologic interpretation very difficult. We used a seismic modeling technique to explain the poor S/N of the recorded data and to generate synthetic seismic sections that can be useful for the geologic interpretation of the field seismic section (migrated time section). We used ray-tracing modeling to obtain the expected seismic expression of horizons of interest. Subsurface illumination modeling indicates that the complex subsurface geology and irregularly deployed sources and receivers are responsible for the incomplete and/or uneven illumination of the subsurface and can lead to strong amplitude variations. We then used 2.5D acoustic finite-difference modeling to analyze the effect of a crooked line on seismic wave propagation. The synthetic shot gathers prove that crooked line arrival times for reflected and head waves contain static time shifts relative to a straight line regular sampling geometry. Some geologic interfaces of interest are not well-imaged on the synthetic seismic section, and this is considered to be an effect of poor positioning during seismic data acquisition. We used the velocity model from the tomographic inversion of first-arrival traveltimes and synthetic and field crooked line deep seismic reflection data to create a structural image for the southeastern Romanian Carpathians and the Focsani Basin, which tie well with the geologic model built for this area on the basis of geologic and well data only.

Separation of diffracted waves in transversely isotropic media

Imaging diffracted waves can provide useful information about complex subsurface geology and fracture networks. Separation of diffractions from typically more intensive reflected events can be done based on specularity, which measures deviation from Snell’s law. Here, we analyze two formulations of specularity and their applicability to diffraction processing in the presence of anisotropy. We show that the most common definition of specularity, originally introduced for pure modes in isotropic media, remains valid for both pure and converted waves in arbitrarily anisotropic models. The other formulation operates directly with the difference between the slowness projections onto the reflector for the incident and reflected waves. Testing on a VTI (transversely isotropic with a vertical symmetry axis) diffraction ramp model demonstrates that both formulations produce satisfactory results for anisotropic media with appropriate tapering of the specularity gathers. Then separation and imaging of diffractions is performed for the structurally complex VTI Marmousi model. We also analyze the sensitivity of diffractions in the specularity gathers to errors in the symmetry-direction velocity and anellipticity parameter η.

A deep electrical conductivity structure of the southern Barberton Greenstone Belt, South Africa, derived from magnetotelluric measurements

The Barberton Greenstone Belt (BGB) in South Africa is one of the few well-preserved, albeit deformed and complex volcano-sedimentary remnants from the Paleoarchean, and thus an excellent locality to study the formation and evolution of the early Earth’s crust. Due to the significant amounts of resources, especially gold in shear zones, the BGB has been extensively studied by geologists for almost 100 years. While the surface geology is well known, only a few geophysical studies have been conducted to investigate the deeper architecture of the BGB and its granitoid surroundings. Here we describe the results of a Magnetotelluric (MT) survey that was conducted over two field seasons to image the subsurface electrical conductivity distribution of geological units of the southern BGB, and to locate dykes, faults and shear zones that are imprints of subsequent tectonic processes. Specifically, mineralization along the shear zones is predicted to reveal high electrical conductivities, in contrast with highly resistive adjacent mafic to ultramafic rocks. The MT station layout of our survey was planned to allow for 2D and 3D interpretation, although it was expected that the 2D inversion models might not be adequate to reveal the expected complex subsurface geology of the BGB and its surrounding region. However, both 2D and 3D inversion results show electrically conductive structures that appear to correlate well with surface traces of known fault zones such as the Inyoka-Saddleback fault system. High resolution 2D conductivity images along selected profiles suggest that some faults might continue further south into the granitoids of the Mpuluzi batholith, implying that the batholith was emplaced along faults (Inyoka-Saddleback fault system and/or Komati Fault), or that a younger fault cuts across the pre-existing batholith. This is contrasted by 3D models that reveal deep-reaching (>10 km) resistive structures beneath the intrusive bodies within the BGB and surrounding batholiths. These results suggest that the granitoids are not disrupted by shear zones, and may imply that episodes of predominant magmatic emplacement have affected the BGB in large parts. A network of conductive faults, especially in the central part of the BGB, suggests that tectonic processes along shear planes have also shaped the BGB, and may have provided pathways for fluids creating zones of gold mineralization.