Andrei Swidinsky

The marine controlled source electromagnetic response of a steel borehole casing: applications for the NEPTUNE Canada gas hydrate observatory

Gas hydrates are a potential energy resource, a possible factor in climate change and an exploration geohazard. The University of Toronto has deployed a permanent seafloor time-domain controlled source electromagnetic (CSEM) system offshore Vancouver Island, within the framework of the NEPTUNE Canada underwater cabled observatory. Hydrates are known to be present in the area and due to their electrically resistive nature can be monitored by 5 permanent electric field receivers. However, two cased boreholes may be drilled near the CSEM site in the near future. To understand any potential distortions of the electric fields due to the metal, we model the marine electromagnetic response of a conductive steel borehole casing. First, we consider the commonly used canonical model consisting of a 100 m, 100 m thick resistive hydrocarbon layer embedded at a depth of 1000 m in a 1 m conductive host medium, with the addition of a typical steel production casing extending from the seafloor to the resistive zone. Results show that in both the frequency and time domains the distortion produced by the casing occurs at smaller transmitter-receiver offsets than the offsets required to detect the resistive layer. Second, we consider the experimentally determined model of the offshore Vancouver Island hydrate zone, consisting of a 5.5 m, 36 m thick hydrate layer overlying a 0.7 m sedimentary half-space, with the addition of two borehole casings extending 300 m into the seafloor. In this case, results show that the distortion produced by casings located within a 100 m safety zone of the CSEM system will be measured at 4 of the 5 receivers. We conclude that the boreholes must be positioned at least 200 m away from the CSEM array so as to minimize the effects of the casings.

Three-dimensional controlled-source electromagnetic modelling with a well casing as a grounded source: a hybrid method of moments and finite element scheme

Steel well casings in or near a hydrocarbon reservoir can be used as source electrodes in time-lapse monitoring using grounded line electromagnetic methods. A requisite component of carrying out such monitoring is the capability to numerically model the electromagnetic response of a set of source electrodes of finite length. We present a modelling algorithm using the finite-element method for calculating the electromagnetic response of a three-dimensional conductivity model excited using a vertical steel-cased borehole as a source. The method is based on a combination of the method of moments and the Coulomb-gauged primary–secondary potential formulation. Using the method of moments, we obtain the primary field in a half-space due to an energized vertical steel casing by dividing the casing into a set of segments, each assumed to carry a piecewise constant alternating current density. The primary field is then substituted into the primary–secondary potential finite-element formulation of the three-dimensional problem to obtain the secondary field. To validate the algorithm, we compare our numerical results with: (i) the analytical solution for an infinite length casing in a whole space, excited by a line source, and (ii) a three-layered Earth model without a casing. The agreement between the numerical and analytical solutions demonstrates the effectiveness of our algorithm. As an illustration, we also present the time-lapse electromagnetic response of a synthetic model representing a gas reservoir undergoing water flooding.

Time causal operational estimation of electric fields induced in the Earth's lithosphere during magnetic storms

In support of projects for monitoring geomagnetic hazards for electric power grids, we develop a simple mathematical formalism, consistent with the time causality of deterministic physics, for estimating electric fields that are induced in the Earths lithosphere during magnetic storms. For an idealized model of the lithosphere, an infinite half-space having uniform electrical conductivity properties described by a galvanic tensor, we work in the Laplace-transformed frequency domain to obtain a transfer function which, when convolved with measured magnetic field time series, gives an estimated electric field time series. Using data collected at the Kakioka, Japan observatory, we optimize lithospheric conductivity parameters by minimizing the discrepancy between model-estimated electric field variation and that actually measured. With our simple model, we can estimate 87% of the variance in storm time Kakioka electric field data; a more complicated model of lithospheric conductivity would be required to estimate the remaining 13% of the variance. We discuss how our estimation formalism might be implemented for geographically coordinated real-time monitoring of geoelectric fields.

GPU parallelization of a three dimensional marine CSEM code

One of the main problem of 3D time domain controlled source electromagnetic (CSEM) inversion is the high runtimes of forward modeling codes. We reduced the runtime of the 3D time domain finite difference CSEM code TEMDDD by the GPU-parallelization of expensive algorithms. The code solves the electromagnetic diffusion equation by discretization of spatial operators and subsequent calculation of eigenpairs. These eigenpairs are found by approximation of the eigenspace in a Krylov subspace using the spectral Lanczos decomposition Method. This algorithm was in its original form not parallelizable due to implementation of the upper boundary condition at the air-water interface. We show for the marine case that replacing the original boundary condition at the air-water surface by a discretized air layer allows GPU parallelization of every time consuming algorithm of the code in the marine case. Speedups between 20 and 60 have been achieved compared to the original code for a larger 3-D model. In this model the bathymetry from a survey area offshore Egypt is used as an example demonstrating that the parallelized version of the code is applicable to real survey scenarios.

Observatory geoelectric fields induced in a two-layer lithosphere during magnetic storms

We report on the development and validation of an algorithm for estimating geoelectric fields induced in the lithosphere beneath an observatory during a magnetic storm. To accommodate induction in three-dimensional lithospheric electrical conductivity, we analyze a simple nine-parameter model: two horizontal layers, each with uniform electrical conductivity properties given by independent distortion tensors. With Laplace transformation of the induction equations into the complex frequency domain, we obtain a transfer function describing induction of observatory geoelectric fields having frequency-dependent polarization. Upon inverse transformation back to the time domain, the convolution of the corresponding impulse-response function with a geomagnetic time series yields an estimated geoelectric time series. We obtain an optimized set of conductivity parameters using 1-s resolution geomagnetic and geoelectric field data collected at the Kakioka, Japan, observatory for five different intense magnetic storms, including the October 2003 Halloween storm; our estimated geoelectric field accounts for 93% of that measured during the Halloween storm. This work demonstrates the need for detailed modeling of the Earth’s lithospheric conductivity structure and the utility of co-located geomagnetic and geoelectric monitoring.

Rapid resistivity imaging for marine CSEM surveys with two transmitter polarizations: An application to the North Alex mud volcano

To image the internal resistivity structure of the North Alex mud volcano offshore Egypt, the marine electromagnetics group at the Helmholtz Centre for Ocean Research Kiel (GEOMAR) developed and conducted a novel transient marine controlled-source electromagnetic experiment. The system, which was specifically developed to image the mud volcano, is also generally suitable for surveys of other small seafloor targets, such as gas-hydrate reservoirs, fluid-flow features, and submarine massive-sulfide deposits. An electric bipole antenna is set down by a remotely operated vehicle on the seafloor sequentially in two perpendicular polarizations at each transmission station. Two orthogonal horizontal electric field components are recorded on the seabed by an array of independently deployed nodal receivers (RXs). With two transmitter polarizations, the unique acquisition geometry of the system provides a very rich data set. However, for this geometric setup, conventional marine electromagnetic interpretation schemes (such as normalized magnitude variation with offset plots) have been difficult to implement. We have developed a simple imaging technique, which can be used for a first-step mapping of seafloor apparent resistivity with the GEOMAR system. Images can be produced in just a few minutes on a regular laptop computer, and the robustness of the approach was demonstrated using two synthetic data sets from simple seafloor models. The method was then applied to the real data acquired at the North Alex mud volcano in 2008. Results found increased apparent sediment resistivities of up to 4 Omega m near the center of the mud volcano occurring at source-RX offsets greater than 500 m, which mapped to apparent depths of greater than 150 m. This may be caused by large quantities of free gas or freshwater in the sediment pore space.

The electromagnetic response of a horizontal electric dipole buried in a multi-layered earth: Horizontal electric dipole tutorial

The electromagnetic response of a horizontal electric dipole transmitter in the presence of a conductive, layered earth is important in a number of geophysical applications, ranging from controlled-source audio-frequency magnetotellurics to borehole geophysics to marine electromagnetics. The problem has been thoroughly studied for more than a century, starting from a dipole resting on the surface of a halfspace and subsequently advancing all the way to a transmitter buried within a stack of anisotropic layers. The solution is still relevant today. For example, it is useful for one-dimensional modelling and interpretation, as well as to provide background fields for twoand three-dimensional modelling methods such as integral equation or primary–secondary field formulations. This tutorial borrows elements from the many texts and papers on the topic and combines them into what we believe is a helpful guide to performing layered earth electromagnetic field calculations. It is not intended to replace any of the existing work on the subject. However, we have found that this combination of elements is particularly effective in teaching electromagnetic theory and providing a basis for algorithmic development. Readers will be able to calculate electric and magnetic fields at any point in or above the earth, produced by a transmitter at any location. As an illustrative example, we calculate the fields of a dipole buried in a multi-layered anisotropic earth to demonstrate how the theory that developed in this tutorial can be implemented in practice; we then use the example to examine the diffusion of volume charge density within anisotropic media—a rarely visualised process. The algorithm is internally validated by comparing the response of many thin layers with alternating high and low conductivity values to the theoretically equivalent (yet algorithmically simpler) anisotropic solution, as well as externally validated against an independent algorithm.

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.

Investigations on Small Scale Targets with Sputnik, A Two Polarization Transmitter System

SUMMARY Sputnik is a novel CSEM transmitter system, which is used for small scale investigations. The system is special in the sense that it can excite two perpendicular TX polarizations at each TX location. It can be shown that this generally increases the sensitivity of measurements to the resistivity structure of the seafloor. This style of experiment requires new approaches for a first pass data interpretation, for which we have adapted the concept of rotational invariants to the marine case. Rotational invariants allow a display of measured data in terms of apparent resistivity sections. They can also be used in the inversion of data. Additionally - using the skew invariant - a dimensionality analysis of the underlying resistivity structure is possible directly from the measured data. Within the past two years the system has been used in three successful experiments for the investigation of methane hydrates and free gas. First results prove that the system is a useful tool for investigations on small targets.

Quantification of marine sulfide deposits using marine electromagnetic methods

Electromagnetic methods are commonly employed in exploration for land based mineral deposits. A suite of airborne, land and borehole electromagnetic techniques consisting of different coil and dipole configurations have been developed over the last few decades for this purpose. In contrast, because the commercial value of marine mineral deposits has only recently been recognized, the development of suitable marine electromagnetic methods for mineral exploration at sea is still in its infancy. We examine conventional marine electromagnetic techniques (i.e. the electric dipole-dipole configuration) for their use in seafloor mineral exploration. Small scale targets such as sulfide deposits require short transmitter-receiver offsets to be properly imaged. Modeling and sensitivity analyses show that to obtain responses that are sensitive to sulfide structure at these small offsets, the transmitter must be positioned on or very close to the seafloor. This implies that conventional marine electromagnetic systems employing a source flown in the seawater column are not suitable for small scale exploration. One particularly interesting method which could be used to image a mineral deposit on the ocean floor is the central loop configuration. Central loop systems consist of a concentric transmitting loop of wire and a receiving coil. While these types of systems are frequently used in land-based or airborne surveys, to our knowledge neither system has been used for marine mineral exploration. The advantages of using central loop systems at sea are twofold: (1) simplified navigation, since the transmitter and receiver are concentric and (2) simplified operation, since only one compact unit must be deployed. In this paper we produce layered seafloor type curves for two particular types of central loop methods: the inloop and coincident loop configurations. In particular, we consider models inspired by real marine sulfide exploration scenarios consisting of overburdens 0m to 5m thick overlying a conductive sulfide body 5m to 30m thick. Modeling analysis shows that using a 50m2 transmitting loop, these two configurations are useful tools to determine both the overburden and conductor thickness. In this case, absolute voltage errors on the order of ten nanovolts are required to resolve the base of a 30m thick sulfide deposit, while the noise floor may be as much as five orders of magnitude higher to determine the sedimentary overburden thickness to the conductor.