Adaption and GPU based parallelization of the code TEMDDD for the 3D modelling of CSEM data

The finite difference time domain code TEMDDD was modified for the 3D forward modeling of marine CSEM data. After changes in the code, which make it possible to create model geometries typically encountered in marine CSEM experiments, parts of the code have been parallelized using massive parallelization on graphic cards. Parts of the singular value decomposition, which is the most time consuming part of the code, have been successfully ported with massive speed-ups (8-12x faster) observed as compared to the standard code. The full parallelization of the code is still work in progress

Die Anwendung von Krylov Unterraum Methoden zur Berechnung von Forwärts Lösungen und Model Sensitivitäten von 3D mariner, aktiver elektromagnetischer Probleme im Zeitbereich

To reduce the run-times of 3D modeling and inversion software for the interpretation of marine controlled source electromagnetics (CSEM) in time domain, the implementation of efficient algorithms on massive parallel hardware is presented. Two forward modeling implementations as well as an implementation for sensitivity calculation are illustrated. The first forward code is an implementation of the spectral Lanczos decomposition method on a graphics processing unit (GPU). The applicability of the code for a CSEM system, how it is used at GEOMAR, is demonstrated. In the second forward code, the SLDM is replaced by the more efficient Rational Krylov Subspace Method (RKSM). This reduces the dimension and run-time of the problem drastically. The accuracy of the code is investigated for different models and conductivity contrasts. The run-times of SLDM and RKSM are compared on different architectures. The sensitivities are computed with the MOR-method (Model Order Reduction). It is shown that the method works and the applicability to a real data set is shown.Zur Reduzierung der Laufzeiten von 3D Modellierungs- und Inversions-Software für die Interpretation von mariner, aktiver Elektromagnetik (engl. CSEM, controlled source electro magnetics) im Zeitbereich, werden effiziente Algorithmen und Implementierungen auf massiv-paralleler Hardware vorgestellt. Zwei Implementierungen zur Berechnung der Vorwärts Modellierung, sowie eine Implementierung zur Berechnung der Sensitivitäten werden dargestellt. Bei dem ersten Vorwärts Code handelt es sich um eine Implementierung der Spektralen Lanczos Zerlegung (engl. SLDM, Spectral Lanczos Decomposition Method) auf dem Prozessor von Graphik Karten (engl. GPU, Graphics Processing Unit). Die Anwendbarkeit des Codes wird für ein CSEM System demonstriert, wie es am GEOMAR im Einsatz ist. Bei dem Zweiten Vorwärts Code wird die SLDM durch das effektivere Rationale Krylov Unterraum Verfahren (engl. RKSM, Rational Krylov Subspace Method) ersetzt. Die Genauigkeit des Codes wird für verschiedene Modelle und Kontraste des elektrischen Leitwertes untersucht. Ein Laufzeitvergleich von SLDM und RKSM wird gegeben.Die Sensitivitäten werden mit dem MOR-Verfahren (engl. Model Order Reduction) berechnet. Es wird gezeigt, dass die Methode funktioniert und seine Anwendbarkeit auf einen echten Datensatz demonstriert

The application of Krylov subspace methods for the calculation of forward solutions and model sensitivities of 3D time domain marine controlled source electromagnetic problems

To reduce the run-times of 3D modeling and inversion software for the interpretation of marine controlled source electromagnetics (CSEM) in time domain, the implementation of efficient algorithms on massive parallel hardware is presented. Two forward modeling implementations as well as an implementation for sensitivity calculation are illustrated. The first forward code is an implementation of the spectral Lanczos decomposition method on a graphics processing unit (GPU). The applicability of the code for a CSEM system, how it is used at GEOMAR, is demonstrated. In the second forward code, the SLDM is replaced by the more efficient Rational Krylov Subspace Method (RKSM). This reduces the dimension and run-time of the problem drastically. The accuracy of the code is investigated for different models and conductivity contrasts. The run-times of SLDM and RKSM are compared on different architectures. The sensitivities are computed with the MOR-method (Model Order Reduction). It is shown that the method works and the applicability to a real data set is shown

The Use of Rotational Invariants for the Interpretation of Marine CSEM Data with a Case Study from the North Alex Mud Volcano, West Nile Delta

Submarine mud volcanos at the seafloor are surface expressions of fluid flow systems within the seafloor. Since the electrical resistivity of the seafloor is mainly determined by the amount and characteristics of fluids contained within the sediment's pore space, electromagnetic methods offer a promising approach to gain insight into a mud volcano's internal resistivity structure. To investigate this structure, we conducted a controlled source electromagnetic experiment, which was novel in the sense that the source was deployed and operated with a remotely operated vehicle, which allowed for a flexible placement of the transmitter dipole with two polarization directions at each transmitter location. For the interpretation of the experiment, we have adapted the concept of rotational invariants from land-based electromagnetics to the marine case by considering the source normalized tensor of horizontal electric field components. We analyse the sensitivity of these rotational invariants in terms of 1-D models and measurement geometries and associated measurement errors, which resemble the experiment at the mud volcano. The analysis shows that any combination of rotational invariants has an improved parameter resolution as compared to the sensitivity of the pure radial or azimuthal component alone. For the data set, which was acquired at the ‘North Alex’ mud volcano, we interpret rotational invariants in terms of 1-D inversions on a common midpoint grid. The resulting resistivity models show a general increase of resistivities with depth. The most prominent feature in the stitched 1-D sections is a lens-shaped interface, which can similarly be found in a section from seismic reflection data. Beneath this interface bulk resistivities frequently fall in a range between 2.0 and 2.5 Ωm towards the maximum penetration depths. We interpret the lens-shaped interface as the surface of a collapse structure, which was formed at the end of a phase of activity of an older mud volcano generation and subsequently refilled with new mud volcano sediments during a later stage of activity. Increased resistivities at depth cannot be explained by compaction alone, but instead require a combination of compaction and increased cementation of the older sediments, possibly in connection to trapped, cooled down mud volcano fluids, which have a depleted chlorinity. At shallow depths (≤50 m) bulk resistivities generally decrease and for locations around the mud volcano's centre 1-D models show bulk resistivities in a range between 0.5 and 0.7 Ωm, which we interpret in terms of gas saturation levels by means of Archie's Law. After a detailed analysis of the material parameters contained in Archie's Law we derive saturation levels between 0 and 25 per cent, which is in accordance with observations of active degassing and a reflector with negative polarity in the seismics section just beneath the seafloor, which is indicative of free gas

Electromagnetic and Seismic Investigation of Methane Hydrates Offshore Taiwan – The Taiflux Experiment

We report on an electromagnetic and seismic experiments carried out offshore Taiwan in April 2013 in the quest to quantify methane hydrate saturation (TAIFLUX project). The quantification of hydrates is essential to efficiently and cost effectively plan a drilling campaign and for a resource assessment of methane hydrates as an energy source to Taiwan. The work has been carried out in a collaboration of Taiwanese and German scientists and funding organizations. Preliminary and qualitative processing of seismic and electromagnetic data acquired on the Four-Way-Closure accretionary ridge situated on the active margin south-west of Taiwan shows the presence of hydrates and indication that hydrate concentrations are relatively high. We furthermore show that the derivation of quantitative hydrate concentration models from the data is difficult to achieve using either seismic or electromagnetic data alone. Best resolution and certainties in a hydrate concentration model may be achieved if both type of data sets are inverted together to a common model, since both types of data sets carry complementary information

Implementation of the Rational Krylov subspace method for marine tCSEM forward modeling and sensitivity calculation on GPU

We implemented the Rational-Krylov-Subspace (RKS) Algorithm on graphics cards for marine 3D time domain CSEM forward modeling as well as sensitivity calculation. We present a comparison between the run-time of a) the new code and the older Polynomial-Krylov-Subspace method code and b) with run-times on old and new graphics cards (to quantify the improvements by hardware and by algorithms). We show, that both together improve the performance significantly by a factor of 20 in comparison with an old GPU parallelized code. For sensitivity computation, we expanded the implementation of the RKS algorithm to block spaces and implemented a model reduction framework, instead of the commonly used adjoint method. Comparisons with brute-forced Jacobians validate the approach

Footprint and detectability of a well leaking CO2 in the Central North Sea: Implications from a field experiment and numerical modelling

Highlights • CO2 gas bubbles are completely dissolved within 2 m above the seabed. • CO2 is not emitted into the atmosphere but retained in the North Sea. • Dissolved CO2 is rapidly dispersed by tidal currents in the North Sea. • Harmful effects on benthic biota occur in the direct vicinity of the leak. • Monitoring has to be performed at the seabed and close to the leak. Abstract Existing wells pose a risk for the loss of carbon dioxide (CO2) from storage sites, which might compromise the suitability of carbon dioxide removal (CDR) and carbon capture and storage (CCS) technologies as climate change mitigation options. Here, we show results of a controlled CO2 release experiment at the Sleipner CO2 storage site and numerical simulations that evaluate the detectability and environmental consequences of a well leaking CO2 into the Central North Sea (CNS). Our field measurements and numerical results demonstrate that the detectability and impact of a leakage of <55 t yr−1 of CO2 would be limited to bottom waters and a small area around the leak, due to rapid CO2 bubble dissolution in seawater within the lower 2 m of the water column and quick dispersion of the dissolved CO2 plume by strong tidal currents. As such, the consequences of a single well leaking CO2 are found to be insignificant in terms of storage performance. Only prolonged leakage along numerous wells might compromise long-term CO2 storage and may adversely affect the local marine ecosystem. Since many abandoned wells leak natural gas into the marine environment, hydrocarbon provinces with a high density of wells may not always be the most suitable areas for CO2 storage

Tectonic Controls on Gas Hydrate Distribution off SW Taiwan

The northern part of the South China Sea is characterized by widespread occurrence of bottom simulating reflectors (BSR) indicating the presence of marine gas hydrate. Because the area covers both a tectonically inactive passive margin and the termination of a subduction zone, the influence of tectonism on the dynamics of gas hydrate systems can be studied in this region. Geophysical data show that there are multiple thrust faults on the active margin while much fewer and smaller faults exist in the passive margin. This tectonic difference matches with a difference in the geophysical characteristics of the gas hydrate systems. High hydrate saturation derived from ocean bottom seismometer data and controlled source electromagnetic data and conspicuous high‐amplitude reflections in P‐Cable 3D seismic data above the BSR are found in the anticlinal ridges of the active margin. In contrast all geophysical evidence for the passive margin points to normal to low hydrate saturations. Geochemical analyses of gas samples collected at seep sites on the active margin show methane with heavy δ13C isotope composition, while gas collected at the passive margin shows light carbon isotope composition. Thus, we interpret the passive margin as a typical gas hydrate province fuelled by biogenic production of methane and the active margin gas hydrate system as a system that is fuelled not only by biogenic gas production but also by additional advection of thermogenic methane from the subduction system