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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.

In time-lapse controlled-source electromagnetics, it is crucial that the source and the receivers are positioned at exactly the same location at all times of measurement. We use interferometry by multidimensional deconvolution (MDD) to overcome problems in repeatability of the source location. Interferometry by MDD redatums the source to a receiver location and replaces the medium above the receivers with a homogeneous half-space. In this way, changes in the source position and changes of the conductivity in the water-layer become irrelevant. The only remaining critical parameter to ensure a good repeatability of a controlled-source electro-magnetic measurement is the receiver position.

Interferometry by multidimensional deconvolution applied to Controlled-Source Electromagnetic data replaces the medium above the receivers by a homogeneous halfspace, suppresses the direct field and redatums the source positions to the receiver locations. In that sense, the airwave and any other interactions of the signal with the air-water interface and the water layer are suppressed and the source uncertainty is reduced. Interferometry requires grid data and cannot be applied to line data unless the source is infinitely long in the crossline direction. To create such a source, a set of source lines is required. We use an iterative algorithm to determine the optimal locations of these source lines and show that more source lines are required if the source is towed closer to the sea bottom and closer to the receivers.

Controlled‐source electromagnetics (CSEM) has been used as a de‐risking tool in the hydrocarbon exploration industry. Although there have been successful applications of CSEM, this technique is still not widely used in the industry because the limited types of hydrocarbon reservoirs CSEM can detect. In this paper, we apply the concept of synthetic aperture to CSEM data. Synthetic aperture allows us to design sources with specific radiation patterns for different purposes. The ability to detect reservoirs is dramatically increased after forming an appropriate synthetic aperture antenna. Consequently, the types of hydrocarbon reservoirs that CSEM can detect are significantly extended. Because synthetic apertures are constructed as a data processing step, there is no additional cost for the CSEM acquisition. Synthetic aperture has potential for simplifying and reducing the cost of CSEM acquisition. We show a data example that illustrates the increased sensitivity obtained by applying synthetic aperture CSEM source.

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.

Crosswell reflection method is a high-resolution seismic imaging method that uses recordings between boreholes. The need for downhole sources is a restrictive factor in its application, for example, to time-lapse surveys. An alternative is to use surface sources in combination with seismic interferometry. Seismic interferometry (SI) could retrieve the reflection response at one of the boreholes as if from a source inside the other borehole. We investigate the applicability of SI for the retrieval of the reflection response between two boreholes using numerically modeled field data. We compare two SI approaches — crosscorrelation (CC) and multidimensional deconvolution (MDD). SI by MDD is less sensitive to underillumination from the source distribution, but requires inversion of the recordings at one of the receiver arrays from all the available sources. We find that the inversion problem is ill-posed, and propose to stabilize it using singular-value decomposition. The results show that the reflections from deep boundaries are retrieved very well using both the CC and MDD methods. Furthermore, the MDD results exhibit more realistic amplitudes than those from the CC method for downgoing reflections from shallow boundaries. We find that the results retrieved from the application of both methods to field data agree well with crosswell seismic-reflection data using borehole sources and with the logged P-wave velocity.

Controlled-source electromagnetics (CSEM) has been used as a derisking tool in the hydrocarbon exploration industry. We apply the concept of synthetic aperture to the lowfrequency electromagnetic field in CSEM. Synthetic aperture sources have been used in radar imaging for many years. Using the synthetic aperture concept, big synthetic sources can be constructed by adding the response to small sources (building blocks) in different ways, and consequently, big sources with different radiation patterns can be created. We show that the detectability of hydrocarbons is significantly enhanced by applying synthetic aperture to CSEM data. More challenging targets such as deep reservoirs (4km belowsea floor) can be detected. The synthetic aperture technique also increases the sensitivity of the field to subsurface targets in the towing streamer acquisition.We also show that a pseudovertical source (orthogonally distributed dipole pairs) can be constructed synthetically, and that the detection capability of this pseudovertical source is increased by applying field steering. The synthetic aperture concept opens a new line of research in CSEM, with the freedom to design suitable synthetic aperture sources for a given purpose.