Certain aspects of the present disclosure generally relate to the field of geophysical surveying and may have particular applicability to electromagnetic (EM) and seismic surveying in marine or other settings.
In geophysical prospecting in a marine environment, sources and receivers are used to understand the geology of the earth below the water. In a particular surveying method, sources of electromagnetic or seismic fields are deployed according to a desired arrangement to direct energy toward the earth. The energy interacts with structures and materials in the earth, and the interaction changes the energy according to properties of the structures and materials. The changed energy is detected by receivers, which record data representing the changed energy. The data is then analyzed to understand the properties of the earth. The electromagnetic or seismic energy may take the form of a diffusive field and/or a wave field that propagates through the water and into the earth. The changed energy also typically propagates as a diffusive field and/or a wave field. The sources and receivers may be stand-alone devices, or may be arranged in elongated assemblies. The assemblies may be towed behind a vessel, or may be stationary in the water or on the sea floor. In particular, receiver assemblies that are towed behind a vessel are referred to as “towed streamers.” An electromagnetic survey employing towed streamers is referred to as a “towed streamer electromagnetic survey.”
In order to understand properties of the materials and structures in the earth, a model may be used to derive the properties from the recorded data. In the case of electromagnetic surveying, the recorded data are typically voltages, and these voltages, related to characteristics of the source energy and the geometry of the source and receiver arrangement, indicate the transformation of the energy by the structures and materials in the earth. The transformation, in turn, indicates physical properties of the materials, such as resistivity, magnetic permeability, density, and other physical properties. In the case of seismic surveying, the recorded data are typically pressures, which indicate the variations of physical properties of the subsurface materials, such as seismic pressure-wave or shear-wave velocities. Using a physical model that relates such physical properties to transformations in electromagnetic or seismic radiation, diffusion, or dispersion, the physical parameters can be iteratively determined by computing results from the model based on a representation of the known source energy, the geometry of the survey, and estimates of the physical properties. Agreement of the model results with the detected energy indicates the accuracy of the estimate, and if such accuracy is inadequate, the estimate is refined until a desired accuracy is reached. This iterative refining process seeking physical models that optimally fit the recorded data is typically referred to as “inversion.” In some circumstances, anisotropy, dependence of a geophysical property on direction, of the subsurface earth has to be taken into account. For instance, instead of inverting only one resistivity model, anisotropic electromagnetic inversions frequently aim at inverting two or more models, such as vertical resistivity and horizontal resistivity (also known as Vertical Transverse Isotropic or “VTI” modeling where bedding planes are modeled as horizontal) or a more general resistivity model incorporating a tilted axis of symmetry (also known as Tilted Transverse Isotropic or “TTI” modeling where bedding planes are not horizontal), to better approximate the recorded data. Inversions taking into account anisotropy are typically referred to as “anisotropic inversions.”
Various electromagnetic inversion techniques seeking the best resistivity model that optimally fits the recorded electromagnetic data are conventionally exploited. The inverted resistivity models are typically of low resolution, due to the inherent low frequency nature of the recorded electromagnetic data and the nature of the electromagnetic field diffusion. In contrast, broadband seismic data typically provides a high resolution structural image of the subsurface, namely, a seismic stratigraphic structure, after iterative seismic inversion processes, or even through direct imaging processes. Because both resistivity and seismic stratigraphic structure describe physical properties of the same subsurface region, their boundaries should roughly align with each other. In other words, the inverted resistivity model is expected to possess imprints identifiable in the seismic stratigraphic structure. Due to the contrasting resolution of electromagnetic inversion and seismic images or sub-surface properties determined from the seismic data, such matching is often lacking.
Therefore, techniques for integrating electromagnetic inversions and seismic stratigraphic structures are needed.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one aspect may be beneficially utilized on other aspects without specific recitation.
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