Marine Controlled-Source Electromagnetic Interferometry

In marine Controlled-Source Electromagnetics, a boat tows an electric source, whose signal is travelling on various paths to the receiver stations at the ocean bottom. Unfortunately, the signal does not only travel via the subsurface to the receivers, but also directly through the water and via the air-water interface. Signals travelling on the latter two travelpaths do not contain any information about the subsurface. On the contrary, they cover a possible response from a subsurface reservoir. Therefore, one aims to suppress the signal travelling along those paths. Interferometry by multidimensional deconvolution replaces the overburden by a homogeneous halfspace suppressing any interactions with the air-water interface. Furthermore, the direct field is removed and the source is redatumed to a receiver position. Since interferometry by multidimensional deconvolution is a data-driven method, no information about the ocean or the subsurface is needed, except the material parameters at the receiver level. This thesis investigates the benefits and limitations of interferometry by multidimensional deconvolution applied to marine Controlled-Source Electromagnetic data.Geoscience & EngineeringCivil Engineering and Geoscience

Modeling of GPR data in a stack of VTI-layers with an analytical code

Geoscience & EngineeringCivil Engineering and Geoscience

Considerations about the solution space of a VTI marine CSEM Inversion problem using vertical antennas

We exploit the randomness of a genetic inversion algorithm to map the global minimum of the solution space of Controlled-Source Electromagnetic inversion problems. In this study, we focus on the information content that vertical electric or magnetic receivers could add to solve for anisotropic conductivities of the subsurface. By analyzing the distribution of the found solutions by the genetic algorithm, we find that the vertical magnetic component adds complementary information to the horizontal components.Geoscience & EngineeringCivil Engineering and Geoscience

Probing the solution space of an EM inversion problem with a genetic algorithm

In an inversion for the subsurface conductivity distribution using frequency-domain Controlled-Source Electromagnetic data, various amounts of horizontal components may be included. We investigate which combination of components are best suited to invert for a vertical transverse isotropic (VTI) subsurface. We do this by probing the solutionspace using a genetic algorithm. We found, by studying a simple horizontally layered medium, that if only electric data are used, either the horizontal or the vertical conductivity of a layer can be estimated properly, but not both. Including the crossline electric field does not add additional information. In contrast, including the two horizontal magnetic components along with the two horizontal electric components allows to retrieve a better estimate of some of the VTI parameters. For an isotropic subsurface, the electric field is sufficient to invert for the subsurface conductivity.Geoscience & EngineeringCivil Engineering and Geoscience

The electromagnetic response in a layered vertical transverse isotropic medium: A new look at an old problem

We determined that the electromagnetic vertical transverse isotropic response in a layered earth can be obtained by solving two equivalent scalar equations, which were for the vertical electric field and for the vertical magnetic field, involving only a scalar global reflection coefficient. Besides the complete derivation of the full electromagnetic response, we also developed the corresponding computer code called EMmod, which models the full electromagnetic fields including internal multiples in the frequency-wavenumber domain and obtains the frequency-space domain solutions through a Hankel transformation by computing the Hankel integral using a 61-point Gauss-Kronrod integration routine. The code is able to model the 3D electromagnetic field in a 1D earth for diffusive methods such as controlled source electromagnetics as well as for wave methods such as ground penetrating radar. The user has complete freedom to place the source and the receivers in any layer. The modeling is illustrated with three examples, which aim to present the different capabilities of EMmod, while assessing its correctness.Geoscience & EngineeringCivil Engineering and Geoscience

Creating virtual vertical radar profiles from surface reflection ground penetrating radar data

Geoscience & EngineeringCivil Engineering and Geoscience

Seismoelectric wave propagation modeling for typical laboratory configurations: A numerical validation

The seismoelectric effect can be of importance for hydrocarbon exploration as it is complementary to conventional seismics. Besides enabling seismic resolution and electromagnetic sensitivity at the same time, the seismoelectric method can also provide us with additional, high-value information like porosity and permeability. However, very little is still understood of this complex physical phenomenon. Therefore, it is crucial to be able to perform numerical modeling experiments to carefully investigate the effect and the parameters that play a role. Over the last couple of years, several seismoelectric laboratory experiments have been carried out in an attempt to validate the underlying theory of the phenomenon and to better understand this complex physical phenomenon. We have recently extended our analytically based, numerical seismoelectric modeling code ’ESSEMOD’ to be able to model seismoelectric wave propagation in arbitrarily layered Earth geometries with fluid / porous medium / (fluid) interfaces. In this way, we are capable of effectively simulating full seismoelectric wave propagation, i. e. all existing seismoelectric and electroseismic source-receiver combinations, in typical laboratory configurations. We present the underlying theory that is required for the extension towards arbitrary fluid / porous medium / (fluid) geometries and an effective way to incorporate this in a general seismoelectric layered Earth modeling code. We then validate the underlying global reflection scheme by comparing it with an independently developed layered Earth modeling code for purely electromagnetic fields. The results show a perfect match in both amplitude and phase, indicating that ESSEMOD is correctly modeling the electromagnetic parts of the seismo-electric wave propagation in horizontally layered media with fluid / porous medium / fluid transitions. We finalize with a seismoelectric reciprocal modeling experiment, proving that also the full seismoelectric wave propagation through fluid / porous medium transitions is modeled consistently.Geoscience & EngineeringCivil Engineering and Geoscience

Effects of the airwave in time-domain marine controlled-source electromagnetics

In marine time-domain controlled-source electromagnetics (CSEM), there are two different acquisition methods: with horizontal sources for fast and simple data acquisition or with vertical sources for minimizing the effects of the airwave. Illustrations of the electric field as a function of space and time for various source antenna orientations, based on analytical formulation of the electric field in two half-spaces, provide insights into the properties of the airwave and the nature of diffuse electric fields. Observing the development of the electric field over time and space reveals that diffusive fields exhibit directionality. Therefore, techniques that have thus far mostly been applied to wavefields can be adapted for CSEM. Examples range from the well-known up-down decomposition to beam steering. Vertical sources have the advantage of not creating an airwave. On the other hand, it is quite difficult to achieve perfect verticality of the source antenna. Results, using a numerically modeled data set to analyze the impact of the airwave on a signal from a subsurface reservoir in the case of a slightly dipping vertical source, indicate that already for a dip of 0.05, the airwave contributes 20% to the complete electric field in our configuration of reservoir depth, water thickness, and conductivity values.Geoscience & EngineeringCivil Engineering and Geoscience

Green's tensors for the diffusive electric field in a VTI half-space

Geoscience & EngineeringCivil Engineering and Geoscience

Overview of marine controlled-source electromagnetic interferometry by multidimensional deconvolution

Interferometry by multidimensional deconvolution for marine Controlled-Source Electromagnetics can suppress the direct field and the airwave in order to increase the detectability of the reservoir. For monitoring, interferometry by multidimensional deconvolution can increase the source repeatability. We give an overview over the method and discuss a possible path of research for the future.Geoscience & EngineeringCivil Engineering and Geoscience