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Potential field data form an important frontline exploration tool thanks to the modern-day airborne platforms capable of collecting large amounts of data over large areas. Interpretation of these data using a 3D inversion is still a challenging problem due to the large amount of data and the need to discretize the inversion volume into a huge number of elements.

We have developed a multi-level parallel magnetotelluric (MT) integral equation based inversion program which uses variable sensitivity domain. The limited sensitivity of the data, which decreases with increasing frequency, is exploited by a receiver sensitivity domain, which also varies with frequency. We assess the effect of inverting principal, full impedance tensor, and full tensor with magnetovariational data (tipper). We first apply this method to several models and then invert the EarthScope MT data. We recover well the prominent features in the area including resistive structure associated with the Juan de Fuca slab subducting beneath the northwestern United States, the conductive zone of partially melted material above the subducting slab at the Cascade volcanic arc, conductive features in the Great Basin resulting from prior subduction and in the area of Yellowstone associated with the hot spot, and resistive areas to the east corresponding to the older and more stable cratons. Our results agree well with other publications which furthers the confidence in the inversion of the EarthScope MT project.

The ability to monitor reservoir behavior over the course of production is necessary for optimization of production performance and the development of production strategies. This goal can be achieved by geophysical monitoring of the fluid propagation within the reservoir. Electromagnetic methods represent an important technique for geophysical monitoring of reservoirs, because they can distinguish between hydrocarbons and saline water based on their differing resistivities. The induced polarization (IP) method can be used to detect additional electrical property contrasts between the fluids and thus can increase the method’s sensitivity to the differing fluids. This chapter considers an application of nanoparticles for reservoir monitoring in order to enhance the electrical conductivity contrast and the IP responses associated with the oil–water interface within the reservoir.

We consider an application of the ideas of electromagnetic (EM) holography/migration to the interpretation of a typical marine controlled-source EM (MCSEM) survey, which consists of a set of sea-bottom receivers and a moving electrical bipole transmitter. The three-dimensional interpretation of MCSEM data is a very challenging problem because of the enormous amount of computations required in the case of the multitransmitter and multireceiver data acquisition systems used in these surveys. At the same time, we show that the MCSEM surveys with their dense system of transmitters and receivers are extremely well suited for application of the holography/migration method. The combined EM signal in the receivers forms a broadband EM “hologram” of the sea-bottom geological target. As in optical and radiowave holography, we can reconstruct the volume image of the geological target by “illuminating” this EM hologram with the reference signal. The principles of holography/migration imaging formulated in this chapter are tested on typical models of a sea-bottom petroleum reservoir. We also apply this new technique to the interpretation of an MCSEM survey conducted in the Troll West gas province, offshore from Norway.

The 3D inversion of potential field data constitutes an increasingly important method of interpretation of geophysical data. A generalized inversion method first discretizes the 3D earth models into cells of constant density, susceptibility, or magnetization vector. In the case of continental-scale geophysical data collected by a combination of land, airborne and satellite measurements, the survey area may cover thousands and even millions of square kilometres, which makes the size of the inversion domain and the number of inverse model parameters extremely large. It is well known that for potential field data the computational complexity increases linearly with the size of the problem. Even a small-sized 3D inversion of huge amounts of data to 3D earth models with hundreds of thousands of cells can exceed the memory available on a desktop computer. In the case of several millions of discretization cells, the memory requirements may exceed the capacity even of the PC clusters. The second obstacle is the amount of CPU time required to apply a huge, dense matrix of the forward modelling operator to the data and model vectors, even using parallel computing.

One of the critical problems in the interpretation of marine controlled-source electromagnetic geophysical data is taking into account the anisotropy of the rock formations. We evaluated a 3D anisotropic inversion method based on the integral equation method. We applied this method to the full 3D anisotropic inversion of towed-streamer electromagnetic (EM) data. The towed-streamer EM system makes it possible to collect EM data with a high production rate and over very large survey areas. At the same time, 3D inversion of towed-streamer EM data has become a very challenging problem because of the huge number of transmitter positions of the moving towed-streamer EM system, and, correspondingly, the huge number of forward and inverse problems needed to be solved for every transmitter position over the large areas of the survey. We overcame this problem by exploiting the fact that a towed-streamer EM system’s sensitivity domain is significantly smaller than the area of the towed-streamer EM survey. This approach makes it possible to invert entire towed-streamer EM surveys with no approximations into high-resolution 3D geoelectrical sea-bottom models. We present an actual case study for the 3D anisotropic inversion of towed-streamer EM data from the Troll field in the North Sea.

A towed streamer electromagnetic system capable of simultaneous seismic and electromagnetic data acquisition has recently been developed and tested in the North Sea. We introduce a 3D inversion methodology for towed streamer electromagnetic data that includes a moving sensitivity domain. Our implementation is based on the 3D integral equation method for computing responses and Frechet derivatives and uses the re‐weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. We present two model studies relevant to hydrocarbon exploration in the North Sea. First, we demonstrate the ability of a towed electromagnetic system to detect and characterize the Harding field, a medium‐sized North Sea hydrocarbon target. We compare our 3D inversion of towed streamer electromagnetic data with 3D inversion of conventional marine controlled‐source electromagnetic data and observe few differences between the recovered models. Second, we demonstrate the ability of a towed streamer electromagnetic system to detect and characterize the Peon discovery, which is representative of an infrastructure‐led shallow gas play in the North Sea. We also present an actual case study for the 3D inversion of towed streamer electromagnetic data from the Troll field in the North Sea and demonstrate our ability to image all the Troll West Oil and Gas Provinces and the Troll East Gas Province. We conclude that 3D inversion of data from the current generation of towed streamer electromagnetic systems can adequately recover hydrocarbon‐bearing formations to depths of approximately 2 km. We note that by obviating the need for ocean‐bottom receivers, the towed streamer electromagnetic system enables electromagnetic data to be acquired over very large areas in frontier and mature basins for higher acquisition rates and relatively lower cost than conventional marine controlled‐source electromagnetic methods.

We have recently introduced a massively parallel regularized 3D inversion of potential fields data. This program takes as an input gravity or magnetic vector, tensor and Total Magnetic Intensity (TMI) measurements and produces 3D volume of density, susceptibility, or three dimensional magnetization vector, the latest also including magnetic remanence information. The code uses combined MPI and OpenMP approach that maps well onto current multiprocessor multicore clusters and exhibits nearly linear strong and weak parallel scaling. It has been used to invert regional to continental size data sets with up to billion cells of the 3D Earth's volume on large clusters for interpretation of large airborne gravity and magnetics surveys. In this paper we explain the features that made this massive parallelization feasible and extend the code to add GPU support in the form of the OpenACC directives. This implementation resulted in up to a 22x speedup as compared to the scalar multithreaded implementation on a 12 core Intel CPU based computer node. Furthermore, we also introduce a mixed single-double precision approach, which allows us to perform most of the calculation at a single floating point number precision while keeping the result as precise as if the double precision had been used. This approach provides an additional 40% speedup on the GPUs, as compared to the pure double precision implementation. It also has about half of the memory footprint of the fully double precision version. We describe implementation of scalable massively parallel potential fields modeling and inversion program.The code is capable of inverting gravity data for density and magnetics data for susceptibility or magnetization vector.Key features that allow scalability and good performance are use of the moving sensitivity domain around each data receiver and on-demand calculation of the sensitivity.Further improvement in performance is gained by use of mixed single-double precision arithmetic.The code is parallelized with MPI and OpenMP and alternatively the computation heavy kernels can be offloaded to GPUs using OpenACC.

This paper presents a fast and efficient large-scale anisotropic inversion technique for towed streamer electromagnetic (EM) data, which incorporates seismic constraints. The inversion algorithm is based on the 3D contraction integral equation method and utilizes a reweighted regularized conjugate gradient technique to minimize the objective functional (Zhdanov et al. 2014a, b). We have also introduced the concept of a moving sensitivity domain for seismically guided EM inversion, originally developed for airborne EM surveys (Zhdanov and Cox, 2013), which makes it possible to invert the entire large-scale towed streamer EM survey data while keeping the accuracy of the computation of the EM fields. The developed algorithm and software can take into account the constraints based on seismic and well-log data, and provide the inversion guided by these constraints. To demonstrate the practical effectiveness of this approach for large-scale inversion of marine EM data, as well as integration with seismic data, we apply the method to the inversion of about 2000 line-km of towed streamer EM data. The data form part of a larger survey in the Norwegian Barents Sea (McKay et al., 2016). We show that the technique produces a single resistivity model that is consistent with both the measured EM data and the main seismically defined structures. Thus, the resistivity model is ready to be interpreted and used in further quantitative interpretation studies.

Summary Airborne electromagnetic (AEM) methods are widely used for mineral exploration. However, until recently AEM data acquired for geophysical exploration have been interpreted using 1D layered-earth models calculated for each transmitter-receiver pairs. These 1D resistivity models are often stitched or interpolated in order to produce a pseudo- 3D model over the survey area. However, the geological structures of particular interest, such as mineral deposits, are poorly resolved with these various 1D methods. Exploiting the fact that an AEM system’s sensitivity domain is significantly smaller than the area of an AEM survey, we have developed a practical 3D inversion methodology based on the moving sensitivity domain approach. This approach has resulted in a paradigm change for AEM data interpretation by making it possible to invert entire AEM surveys with no approximations into high-resolution 3D geoelectrical earth models.

This case study compares 3D inversion results from Spectrem Air?s SPECTREM 2000 fixed-wing timedomain airborne electromagnetic (AEM) system, and Geotech?s Z-axis Tipper Electromagnetic (ZTEM) airborne audio-frequency magnetics (AFMAG) system flown over the Pebble Cu-Au-Mo deposit in Alaska. Within the commonality of their physics, 3D inversions of both SPECTREM and ZTEM recover conductivity models consistent with each other and with the known geology. Both 3D inversions recover conductors coincident with alteration associated with both Pebble East and Pebble West. The 3D interpretation of both surveys has yielded improved the understanding of the geology, alteration and mineralization of the Pebble system. There are distinct practical advantages to the use of both SPECTREM and ZTEM, so we draw no recommendation on one system over the other. We do conclude however, that 3D inversions of AEM and ZTEM surveys add significant value to exploration.

To improve mineral exploration success, there is an accepted need to increase the ?discovery space? by exploring under cover and to greater depths using 3D geological modelling supported by multiple 3D geophysical inversions. To facilitate this approach, multi-sensor airborne platforms capable of simultaneously measuring electromagnetic, gravity, and magnetic data are now being deployed. The availability of data from such systems poses a significant challenge to the exploration geophysicist: How do you generate a shared earth model that satisfies all data? We address this with a case study from the Reid-Mahaffy test site in Ontario, demonstrating how multiple 3D inversions of MEGATEM time-domain electromagnetic, FALCON gravity gradiometry and TMI data can be analysed by the self-organizing maps (SOM) data mining approach to produce 3D pseudo-lithological models. The results of our analyses are shown to be in agreement with the known geology of the Reid-Mahaffy test site.

Geological, geochemical, and geophysical surveys have been conducted in the area of the Pebble Cu–Au–Mo porphyry deposit in south-west Alaska since 1985. This case study compares three-dimensional (3D) inversion results from Anglo American’s proprietary SPECTREM 2000 fixed-wing time-domain airborne electromagnetic (AEM) and Geotech’s ZTEM airborne audio-frequency magnetics (AFMAG) systems flown over the Pebble deposit. Within the commonality of their physics, 3D inversions of both SPECTREM and ZTEM recover conductivity models consistent with each other and the known geology. Both 3D inversions recover conductors coincident with alteration associated with both Pebble East and Pebble West. The high grade CuEqn 0.6% ore shell is not consistently following the high conductive trend, suggesting that the SPECTREM and ZTEM responses correspond in part to the sulphide distribution, but not directly with the ore mineralization. As in any exploration project, interpretation of both surveys has yielded an improved understanding of the geology, alteration and mineralization of the Pebble system and this will serve well for on-going exploration activities. There are distinct practical advantages to the use of both SPECTREM and ZTEM, so we draw no recommendation for either system. We can conclude however, that 3D inversion of both AEM and ZTEM surveys is now a practical consideration and that it has added value to exploration at Pebble.

Today’s mineral exploration is driven by the simple fact that discovery rates have not kept pace with the depletion of existing reserves. To improve discovery rates, there is an industry-wide consensus on the need to increase the “discovery space” by exploring under cover and to greater depths. This attracts increased risks which may be mitigated by improved targeting. To do this, mining geophysics needs to shift toward 3D geological models founded upon improved petrophysical understanding and geophysical inversion. Regardless of the inversion methodology used, all geological constraints manifest themselves in the user’s prejudice of an a priori model, upper and lower bounds, and choice of regularization. However, the practice of geologically constrained inversion is not the major problem needing to be addressed. It is known (and accepted) that geology is inherently 3D, and is a result of complex, overlapping processes related to genesis, metamorphism, deformation, alteration and/or weathering. Yet, the mining geophysics community to date has not fully accepted that geophysics should also be 3D, and most often relies on qualitative analysis, 1D inversion, and depositscale 2D or 3D inversion. There are many reasons for this unfortunate deficiency, not the least of which has been the lack of capacity of existing 3D inversion algorithms. To date, these have not been able to invert entire surveys with sufficient resolution in sufficient time to practically affect exploration decisions. This problem is most critical for airborne geophysical sur

Inversion of gravity and/or magnetic data attempts to recover the density and/or magnetic susceptibility distribution in a 3D earth model for subsequent geological interpretation. This is a challenging problem for a number of reasons. First, airborne gravity and magnetic surveys are characterized by very large data volumes. Second, the 3D modelling of data from large-scale surveys is a computationally challenging problem. Third, gravity and magnetic data are finite and noisy and their inversion is ill posed so regularization must be introduced for the recovery of the most geologically plausible solutions from an infinite number of mathematically equivalent solutions. These difficulties and how they can be addressed in terms of large-scale 3D potential field inversion are discussed in this paper. Since potential fields are linear, they lend themselves to full parallelization with near-linear scaling on modern parallel computers. Moreover, we exploit the fact that an instrument’s sensitivity (or footprint) is considerably smaller than the survey area. As multiple footprints superimpose themselves over the same 3D earth model, the sensitivity matrix for the entire earth model is constructed. We use the re-weighted regularized conjugate gradient method for minimizing the objective functional and incorporate a wide variety of regularization options. We demonstrate our approach with the 3D inversion of 1743 line km of FALCON gravity gradiometry and magnetic data acquired over the Timmins district in Ontario, Canada. Our results are shown to be in good agreement with independent interpretations of the same data.

The key to deriving a reliable quantitative interpretation from marine controlledsource electromagnetic data is through the integration of shared earth modeling and robust 3D electromagnetic inversion. Subsurface uncertainty is minimized through efficient workflows that use all available subsurface data as a priori information and which permit multiple resistivity models to explain the same observed data. To this end, we present our implementation of an iterative migration method for controlledsource electromagnetic data that is equivalent to rigorous 3D inversion. Our iterative migration method is based on the 3D integral equation method with inhomogeneous background conductivity and focusing regularization with a priori terms. We will show that focusing stabilizers recover more geologically realistic models with sharper resistivity contrasts and boundaries than traditional smooth stabilizers. Additionally, focusing stabilizers have better convergence properties than smooth stabilizers. Finally, inhomogeneous background informationdescribed as a prioriresistivitymodels can improve the fidelity of the final models. Our method is implemented in a fully parallelized code. This makes it practical to run large-scale 3D iterative migration on multicomponent, multifrequency and multiline controlled-source electromagnetic surveys for 3D models with millions of cells. We present a suite of interpretations obtained from different iterative migration scenarios for a 3D controlled-source electromagnetic feasibility study computed from a detailed model of stacked anticline structures and reservoir units of the Shtokman gasfield in the Russian sector of the Barents Sea.

Reliance on desktop computers limits the scale of 3D inversion of gravity and gravity gradiometry surveys, making it impractical to achieve an appropriate level of resolution and detail for geological interpretation. To begin with, airborne surveys are characterised by very large data volumes. They typically contain hundreds to thousands of line kilometres of data with measurement locations every few metres. Often, surveys cover thousands of square kilometres in area with tens of thousands of line kilometres of data. Regional surveys may be even larger and denser as the result of merging multiple and/or historic surveys. Secondly, 3D modelling of large-scale surveys exceeds the capacity of desktop computing resources. And finally, gravity data are finite and noisy, and their inversion is ill posed. Regularisation must be introduced in order to recover the most geologically plausible solutions from the infinite number of mathematically equivalent solutions. Various strategies for 3D inversion have been previously proposed but few lend themselves to truly large-scale 3D inversion. In this paper, we describe how gravity and gravity gradiometry surveys can be inverted to 3D earth models of unprecedented scale (i.e., hundreds of millions of cells) within hours using cluster computers.

Three-dimensional magnetotelluric (MT) inversion is an emerging technique for offshore hydrocarbon exploration. We have developed a new approach to the 3D inversion of MT data, based on the integral equation method. The Tikhonov regularization and physical constraint have been used to obtain a stable and reasonable solution of the inverse problem. The method is implemented in a fully parallel computer code. We have applied the developed method and software for the inversion of marine MT data collected by the Scripps Institution of Oceanography (SIO) in the Gemini prospect, Gulf of Mexico. The inversion domain was discretized into 1.6 million cells. It took nine hours to complete 51 iterations on the 832-processor cluster with a final misfit between the observed and predicted data of 6.2%. The inversion results reveal a resistive salt structure, which is confirmed by a comparison with the seismic data. These inversion results demonstrate that resistive geoelectrical structures like salt domes can be mapped with reasonable accuracy using the 3D inversion of marine MT data.

In this paper we apply 3D inversion to MT data collected in the Northwestern United States as a part of the EarthScope project. By the end of 2009 MT data had been collected from 262 stations located throughout Oregon, Washington, Idaho, and most of Montana and Wyoming. We used data from 139 MT stations in this analysis. We developed fully parallelized rigorous 3D MT inversion software based on the integral equation method with variable background conductivity. We also implemented a receiver footprint approach which considerably reduced the computational resources needed to invert the large volumes of data covering vast areas. The data set used in the inversion was obtained through the Incorporated Research Institutions for Seismology (IRIS). The inversion domain was divided into 2.7 M cells. The inverted electrical conductivity distribution agrees reasonably well with geological features of the region.

We present a multigrid integral equation (IE) method for three-dimensional (3D) electromagnetic (EM) field computations in large-scale models with inhomogeneous background conductivity (IBC). This method combines the advantages of the iterative IBC IE method and the multigrid quasi-linear (MGQL) approximation. The new EM modelling method solves the corresponding systems of linear equations within the domains of anomalous conductivity, Da, and inhomogeneous background conductivity, Db, separately on coarse grids. The observed EM fields in the receivers are computed using grids with fine discretization. The developed MGQL IBC IE method can also be applied iteratively by taking into account the return effect of the anomalous field inside the domain of the background inhomogeneity Db, and vice versa. The iterative process described above is continued until we reach the required accuracy of the EM field calculations in both domains, Da and Db. The method was tested for modelling the marine controlled-source electromagnetic field for complex geoelectrical structures with hydrocarbon petroleum reservoirs and a rough sea-bottom bathymetry.

Conventional 3D magnetic inversion methods are based on the assumption that there is no remanent magnetization, and the inversion is run for magnetic susceptibility only. However, this approach ignores the situation where the direction of magnetization of the rocks is different from the direction of the induced magnetic field. This situation happens in a case of remanent magnetization, typical for geological structures such as kimberlites, dykes, iron-rich ultramafic pegmatitoids (IRUP), platinum group element (PGE) reefs, and banded iron formations (BIF). This paper presents a novel method of inversion of magnetic data for the scalar components of the magnetization vector. The method is based on a new magnetic forward modeling algorithm, which uses triangular prisms of arbitrary shape in order to achieve a more accurate approximation of the topography and complex geological structures. The inversion also includes Gramian constraints in order to obtain a robust solution of otherwise ill-posed magnetic inverse problems. The method was successfully tested on a number of synthetic models of the magnetized bodies. The results of inversion of airborne TMI data demonstrate how inversion for the magnetization vector with Gramian constraints can improve the subsurface imaging of kimberlites.

Airborne gravity or gravity gradiometry surveys typically contain hundreds to thousands of line kilometers of data with measurement locations every few meters. The goal of 3D inversion of airborne gravity and/or gravity gradiometry (AGG) data is to recover the density distribution in a 3D earth model at an appropriate level of resolution and detail for subsequent use in geological interpretation. This is a challenging problem for a number of reasons. First, as mentioned, airborne surveys are characterized by very large data volumes. Second, the 3D modeling of large-scale surveys is a computationally challenging problem. In this paper, we discuss how these difficulties can be overcome by using a number of innovations, more specifically, the moving sensitivity domain approach, focusing regularization, and massively parallel computer implementation. We illustrate our method with an example of 3D inversion of AGG data collected by Bell Geospace over the Bathurst Mining Camp in New Brunswick, Canada.

The thermal discrete dipole approximation (T-DDA) is a numerical approach for modeling near-field radiative heat transfer in complex three-dimensional geometries. In this work, the convergence of the T-DDA is investigated by comparison against the exact results for two spheres separated by a vacuum gap. The error associated with the T-DDA is reported for various sphere sizes, refractive indices and vacuum gap thicknesses. The results reveal that for a fixed number of subvolumes, the accuracy of the T-DDA degrades as the refractive index and the sphere diameter to gap ratio increase. A converging trend is observed as the number of subvolumes increases. The large computational requirements associated with increasing the number of subvolumes, and the shape error induced by large sphere diameter to gap ratios, are mitigated by using a nonuniform discretization scheme. Nonuniform discretization is shown to significantly accelerate the convergence of the T-DDA, and is thus recommended for near-field thermal radiation simulations. Errors less than 5% are obtained in 74% of the cases studied by using up to 82712 subvolumes. Additionally, the convergence analysis demonstrates that the T-DDA is very accurate when dealing with surface polariton resonant modes dominating radiative heat transfer in the near field., 46 pages, 12 figures, 3 tables

Surface energies of Si(0 0 1), (1 1 0), (1 1 1), and (1 1 3) surfaces with different reconstructions are calculated systematically using first-principles total-energy method. In order to quantitatively compare their relative stability, the surface energies of different surface orientations and their respective theoretical bulk atom energies are determined simultaneously by linear fitting slab supercell total energy as a function of the atom number in the slab. Equivalent computational parameters and convergence criteria are used for all calculations. Without considering entropy contribution, the relative stability of these Si surfaces with given reconstructions is shown, in decreasing order, from (1 1 1) to (0 0 1) and (1 1 3) at low temperature, and from (0 0 1), (1 1 3), (1 1 0), to (1 1 1) at high temperature.

This paper presents a parallelized version of the edge-based finite element method with a novel post-processing approach for numerical modeling of an electromagnetic field in complex media. The method uses an unstructured tetrahedral mesh which can reduce the number of degrees of freedom significantly. The linear system of finite element equations is solved using parallel direct solvers which are robust for ill-conditioned systems and efficient for multiple source electromagnetic (EM) modeling. We also introduce a novel approach to compute the scalar components of the electric field from the tangential components along each edge based on field redatuming. The method can produce a more accurate result as compared to conventional approach. We have applied the developed algorithm to compute the EM response for a typical 3D anisotropic geoelectrical model of the off-shore HC reservoir with complex seafloor bathymetry. The numerical study demonstrates that the modeling algorithm is capable of simulating the complex topography and bathymetry that is commonly encountered in controlled source electromagnetic problems.

Building on the previously developed multistate empirical valence bond model [U. W. Schmitt and G. A. Voth, J. Chem. Phys 111, 9361 (1999)] for the dynamics and energetics of an excess proton in bulk phase water, a second generation model is described. This model is shown to produce similar dynamic and structural properties to the previous model, while allowing for the use of the full hydronium charge. This characteristic of the model is required for its implementation in a host of realistic applications beyond bulk water. An improved state selection algorithm is also presented, resulting in a significantly reduced energy drift during microcanonical molecular dynamics simulations. The unusually high self diffusion constant of an excess proton in water due to the proton hopping (Grotthuss) process is observed in the simulation data and is found to be quantitatively in the same range as the experimental value if a quantum correction is taken into consideration. Importantly, a more complete analysis of proto...

The development and application of a multistate empirical valence bond (MS-EVB) model for a weak acid dissociation and subsequent proton transport in aqueous solution is described. The weak acid dissociation step is modeled by the inclusion of an additional EVB state describing the case when proton is bound to the acid's conjugate base. The model was parametrized for the imidazolium cation deprotonation. Classical molecular dynamics simulation methodology was used to study both equilibrium and dynamic properties of this system. Free energy profiles of the deprotonation reaction, studied using a novel center of excess charge reaction coordinate, reveal the need to include several solvation shells around the weak acid in order to stabilize the hydronium species formed upon the weak acid deprotonation. The solvent atomic density plots examined at selected points along the proton transfer coordinate display a relatively large reorganization of the solvent around the weak acid molecule, caused by the shift in ...

The dimer and trimer of BC2N are studied at the HF and MPn levels using 3-21G, 6-31G∗ and 6-31+G∗ basis sets. It is found that stability is enhanced by maintaining –BN– units together in the cyclic form. Both molecules have cyclic ground state structures with cumulenic-type equal bond lengths in the –BNBN– and polyacetylenic –CCCC– fragments. The linear–cyclic energy difference increases with the size of the cluster. Electron correlation substantially favors the cyclic structures over the linear forms. No general trend of atomic combination has been found in the linear isomers. Diffuse functions have negligible effects on the relative energies and geometries of the dimers.

Classical molecular dynamics simulations in conjunction with a multi-state empirical valence bond (MS-EVB) model are used to study proton transport in strong and weak acid aqueous solutions. The strong acid, HCl, is modeled in its ionized state by inserting a chloride counter ion into the protonated water solution. Both equilibrium and dynamical properties differ only slightly from the previously studied isolated excess proton in water. The free-energy profile as a function of the separation between the excess charge and chlorine atom reveal minimal barrier for the anion-excess charge separation. To model the weak acid, protonated imidazole, the MS-EVB model was extended to include the protonated form of the acid in the EVB description, so that the dissociation step can be studied. Free energy profiles for the weak acid deprotonation show that several solvation shells around the weak acid molecule need to be included in the EVB model to correctly describe the stabilization of the solvated species. Structurally, one water molecule is coordinated to the proton donor in the protonated acid case, while two water molecules coordination is likely when the acid is deprotonated.

The ground and first few excited states of o-hydroxybenzaldehyde (oHBA) are computed at the CIS and MP2/CIS levels with a 6-31+G** basis set, with emphasis on its intramolecular H-bond. These results are compared with those for malonaldehyde, which differs from oHBA in that it lacks an adjoining benzene ring. In most respects, the addition of the latter aromatic system exerts surprisingly little influence upon the properties of malonaldehyde. With the exception of the 1ππ* state, electronic excitation weakens the H-bond and simultaneously raises the barrier to proton transfer in either system. Unlike the symmetric transfer potential in malonaldehyde, the enol and keto tautomers of oHBA are chemically distinct. π→π* excitation reverses the preference for the enol tautomer in the ground state. This reversal is connected with the changing degree of aromaticity in the benzene ring of oHBA. The asymmetric transfer potential in oHBA leads to forward and reverse barriers of different magnitude. When this factor is accounted for by an averaging procedure, the transfer barriers in oHBA are remarkably similar to those of the corresponding states of malonaldehyde.

Tetra-atomic BC2N, the building block of the mixed carbon and BN clusters, has been studied at the HF, MP2, and CCSD(T) levels using both double- and triple-ζ basis sets with polarization and diffuse functions. In contrast to the parents C4 and (BN)2, the linear triplet (3Π) BCCN is found to be the most stable and the linear−cyclic energy difference is about 28 kcal/mol. In the cyclic structure, isomers with adjacent B and N atoms are more stable, whereas no general trend of atomic combination has been found in the linear isomers. The preferred sequence of atoms in the linear form depends on the cluster size. The nature of bonding and atomization energies of the parent and hybrid molecules are compared and discussed.

The proton transfer from one oxygen atom to the other within the intramolecular H-bond in a molecule like o-hydroxybenzaldehyde (oHBA) would be precluded by a prior rotational isomerism that breaks this H-bond. The likelihood of such rotamerization in the ground and several excited electronic states is investigated by ab initio calculations at the CIS and MP2 levels with a 6-31+G** basis set. In the ground state, the energetics of proton transfer and rotamerization are competitive with one another; both processes are endothermic and must surmount an energy barrier. Excitation to the singlet or triplet ππ* states presents a situation where tautomerization to the keto is exothermic, with a small barrier. In contrast, rotamerization is endothermic with high intervening barriers, so excited-state proton transfer is favored. The opposite situation is encountered in the nπ* states, where rotations of the hydroxyl and carbonyl groups are facile and lead energetically downhill, in contrast to the high barriers op...

The central C atom of the OCCCO skeleton of the malonaldehyde molecule is replaced by N, and the effects upon the intramolecular H-bond and the proton transfer are monitored by ab initio calculations in the ground and excited electronic states. The H-bond is weakened in the singlet and triplet states arising from nπ* excitation in both molecules, which is accompanied by a heightened barrier to proton transfer.3ππ* behaves in the same manner, but the singlet ππ* state has a stronger H-bond and lower barrier. Replacement of the central C atom by N strengthens the intramolecular H-bond. Although the proton transfer barrier in the ground state of formimidol is lower than in malonaldehyde, the barriers in all four excited states are higher in the N-analog. The latter substitution also dampens the effect of the nπ* excitation upon the H-bond and increases the excitation energies of the various states, particularly ππ*. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 129–138, 1998

We introduce a 3D inversion methodology for towed streamer EM data that takes into account anisotropy and includes a moving sensitivity domain. Our implementation is based on 3D IE method for computing the responses and Frechet derivatives, and uses re-weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. Interpretation of the towed streamer EM data is a difficult problem because the data are acquired over large areas with huge number of moving towed streamer EM system positions. We overcome this problem by exploiting the concept of moving sensitivity domain, which is implemented using the IE method. In the framework of this concept, for a given transmitter-receiver pair, the responses and Frechet derivatives are computed from a 3D earth model that encapsulates the towed EM system’s sensitivity domain. The Frechet matrix for the entire 3D earth model is then constructed as the superposition of Frechet derivatives from all transmitter-receiver pairs over the entire 3D earth model. This makes large-scale 3D inversion a tractable problem with moderate cluster resources. We present a case study of 3D anisotropic inversion of towed streamer EM data from the Troll West Oil Province.

The transfer of a proton in malonaldehyde takes place within an intramolecular H-bond involving a five-membered ring. This process is compared via ab initio methods with the transfer in analogous systems in which the size of the ring is altered to four and to six and in which the system bears an overall negative charge. In addition to the ground state, calculations are applied to the singlet and triplet ππ* states, as well as to 1nπ* and 3nπ*. The barriers to proton transfer are found to correlate strongly with various geometric and energetic markers of the strength of the H-bond. The H-bond is weakened by n → π* excitation, particularly for the neutral molecule, resulting in a higher transfer barrier. In the case of the two anions, excitation to 3ππ* strengthens the H-bond, while the result is more ambiguous for the 1ππ* state. This trend is reversed in malonaldehyde where the singlet is strengthened by the excitation and the triplet weakened. Some of these patterns are traced directly to the nature of t...

Although the electronic contribution to the strength of a H–bond is unaffected by isotopic substitution, the heavier mass of deuterium compared with protium lowers some of the vibrational frequencies in the complex. The binding energy of the complex, which includes zero–point and thermal vibrational energies, can thus be altered by several tenths of a kcal mol−1 by H/D substitution. Ab initio calculations are used to analyze this phenomenon in a number of common organic functional groups that are prone to form H–bonds: hydroxyl, carboxyl and amide, both self–complexing as homodimers and with water molecules as partners. It is found that any site of D–substitution increases the complexation energy; however, the bridging sites show a stronger preference for D over H than do the non–bridging, or terminal, sites. Hence D–bonding can be considered to be stronger than H–bonding in these functional groups. Of the groups considered, the energetic preference for D over H is greater in the hydroxyl group, so deuterium would be expected to gravitate toward solvent water molecules in isotopic scrambling experiments. The increments in H–bonding energy resulting from each site of substitution are addititve in cases of multiple substitution. © 1997 John Wiley & Sons, Ltd.

Summa ry We apply Gramian constraints for the joint inversion of airbo rne gravity gradiometry and magnetic data. The method does not require any a priori knowledge about the types of relationships between the different model parameters, but instead determines the form of these relationships in the process of the inversion. The Gramian constraints make it possible to consider both linear and nonlinear relationships between the different physical parameters of a geological model. As an illustration, we consider in this paper polynomial relationships between different model parameters. The case study includes joint inversion of airborne gravity gradiometer (AGG) and magnetic data collected by Fugro Airborne Surveys in the area of McFaulds Lake located in northwestern Ontario. This case study demonstrates how joint inversion may enhance the produced subsurface images of a deposit.

Moving sensitivity domain that varies with frequency was implemented and tested in a parallel magnetotelluric (MT) integral equation inversion. This approach reduces computation time and memory requirements. We assess the robustness of the approach by model tests and apply it to the inversion of EarthScope MT data collected over the Northwestern US. Prominent features obtained by this inversion include resistive structure associated with the Juan de Fuca slab subducting beneath the northwestern United States and the conductive zone of partially melted material above the subducting slab corresponding to the Cascade volcanic arc. We also observe extensive areas of moderate-to-high conductive asthenosphere below 100 to 200 km and high-conductive body associated with the Yellowstone mantle plume.

The relative energies of H and D bonds are due to differences in zero-point vibrational energy (ZPVE). Ab initio calculations are used to assess the changes in this quantity that accompany all possible substitutions of protium by deuterium in a number of complexes. The ZPVE of the D bond is lower than that of the H bond in the neutral dimer and trimer of water. This difference can be traced to one particular vibrational mode, the one which displaces the bridging atom away from the O···O axis. The heavier mass of D lowers the frequency, and hence the ZPVE associated with it. The situation reverses itself in ionic H bonds. The total ZPVE of the (H2O··H··OH2)+ complex is higher when a D occupies the bridging position, as compared to a terminal site. This difference is attributed to the intramolecular modes. Although replacement of the central H by D reduces the intermolecular ZPVE, the reduction of the intramolecular ZPVE is even larger when the substitution is made at a peripheral atom, so a D would tend to...

We have recently introduced a massively parallel regularized 3D inversion of potential fields data. The code uses MPI/OpenMP approach that maps well on the current HPC clusters and exhibits nearly linear parallel scaling. It has been used to invert regional to continental size data sets with up to billion of cells of the 3D Earth volume on large clusters and aided to interpretation of large airborne gravity and magnetics surveys. In this presentation we explain the features that made this massive parallelization feasible and extend the code to add GPU support in the form of the OpenACC directives. Thanks to similarity of OpenACC with OpenMP, this implementation was very straightforward and resulted in up to 22x speedup as compared to a 12 core Intel CPU based compute node. Furthermore, we also introduce a mixed single-double precision version of this code, which allows to perform most of the calculation at the single floating point number precision while keeping the result as accurate as the double precision. This provides an additional 40% speedup on the GPUs, as compared to pure double precision implementation. It also has about half of the memory footprint and makes the inversion efficient on low cost graphics cards.

During 2010, PGS conducted one of the first field trials of their towed streamer electromagnetic (EM) system at the Troll field in the Norwegian North Sea. Obviating the need for ocean bottom receivers, the towed-streamer EM system enables CSEM data to be acquired simultaneously with seismic over very large areas in frontier and mature basins for higher production rates and relatively lower cost than conventional CSEM. The towed streamer EM data are currently processed and delivered as a spectrum of frequency-domain responses. In this paper, we review the large-scale 3D inversion of towed streamer EM data using a moving sensitivity domain. We present a case study for the 3D inversion of towed streamer EM data from the Troll field, and demonstrate our ability to image the Troll West Gas Province and Troll East Gas Province.

Following recent advances in superconducting quantum interference devices (SQUIDs), airborne full tensor magnetic gradiometry (FTMG) is emerging as a practical geophysical exploration method that is intended to recover information about remanent magnetization. In this paper, we introduce 3D regularized inversion of FTMG data that recovers the total magnetization vector in each cell of the 3D earth model. If a priori information about the susceptibility or remanent magnetization is available, the 3D inversion can be constrained to recover the remanent magnetization vector. If a priori information is not available, it is possible to recover attributes such as the amplitude, components, and angle of the magnetization vector relative to the inducing field. We present a case study for data acquired over a dike swarm in South Africa that compares our 3D FTMG inversion for magnetization with a 3D total magnetic intensity (TMI) inversion for a positively-constrained susceptibility distribution. Given the significant remanent magnetization present, the 3D FTMG inversion for magnetization recovers results that are most consistent with the known geology.

Following recent advances in SQUID technology, airborne full tensor magnetic gradiometry (FTMG) is emerging as a practical mineral exploration method that is intended to recover information about remanent magnetization. In this paper, we introduce 3D regularized inversion of FTMG data that recovers the total magnetization vector in each cell of the 3D earth model. If a priori information about the susceptibility or remanent magnetization is available, the 3D inversion can be constrained to recover the remanent magnetization vector. If a priori information is not available, it is possible to recover attributes of remanent magnetization such as the amplitude and angle of the magnetization vector relative to the inducing field. We present a case study for data acquired over a dyke swarm in South Africa that compares our 3D FTMG inversion for magnetization with a 3D total magnetic intensity (TMI) inversion for a positively-constrained susceptibility distribution. Given the significant remanent magnetization present, the 3D FTMG inversion for magnetization recovers results that are most consistent with the known geology.