We simulated and analyzed short-offset transient electric-field measurements excited by a vertical electric dipole (VED) source over complex 3D offshore models. A finite-element time-domain modeling algorithm was used to efficiently model complex offshore structures. Using a series of cross-sectional snapshots of transient electric fields in the complex offshore models, we examined the characteristics of the short-offset seafloor electricfield measurements. The numerical modeling analysis indicated that the short-offset horizontal electric-field (E x) measurements are very sensitive to subtle multidimensional seafloor topography near a VED source and can show a sign reversal at late times. The sign reversal occurs because the VED source is no longer normal to the seafloor. The occurrence of the sign reversal limits the application of the 1D inversion to the E x measurements, even at a short source-receiver offset. In contrast, the short-offset vertical electric-field (E z) measurements are robust to subtle seafloor topography around the source, and can be interpreted using the 1D inversion. The 1D inversion of the shortoffset E z measurements over the complex 3D offshore models shows that the measurements lack the resolution of the thickness and the resistivity of a hydrocarbon reservoir and a salt dome, but can provide useful insights into their lateral extent.

Stable dispersions of superparamagnetic nanoparticles that are already in use in biomedicine as image-enhancing agents also have potential use in subsurface applications. Surface-coated nanoparticles are capable of flowing through micron-sized pores across long distances in a reservoir, with modest retention in rock. These particles change the magnetic permeability of the flooded region, and thus one can use them to enhance images of the flood. In this paper, we model the propagation of a "ferrofluid" slug in a reservoir and its response to a crosswell magnetic tomography system.This approach to monitoring fluid movement within a reservoir is built on established electromagnetic (EM) conductivity-monitoring technology. In this work, however, we investigate the contrast between injected and resident fluids when they have different magnetic permeabilities. Specifically, we highlight the magnetic response at low frequency to the magnetic excitations generated by a vertical magnetic dipole source positioned at the injection well. At these frequencies, the induction effect is small, the casing effect is manageable, the crosswell response originates purely from the magnetic contrast in the formation, and changes in fluid conductivities are irrelevant.The sensitivity of the measurements to the magnetic slug is highest when the slug is closest to the source or receivers and lower when the slug is midway in the interwell region. At low frequencies, the magnetic response of the ferrofluid slug is largely independent of frequency. As expected for the conductive slug, the sensitivity of the inductive measurements is negligible at low frequencies whereas significant levels of detectability result at higher frequencies. We demonstrate sensitivity to the vertical boundaries of the slug by shifting the vertical position of the excitation source relative to the magnetic slug. The slug geometry plays a key role in determining the magnetic response. With a fixed volume of ferrofluid, there is an optimum slug geometry that results in the maximum magnetic response. Hydrodynamic dispersion of the slug has negligible effect on the magnetic response during early stages of the waterflood. As the slug travels farther into the formation, however, dispersion reduces the concentration of nanoparticles, and the spatial contributions of the magnetic measurements are more diffuse. We illustrate how these low-frequency excitation behaviors are consistent with the quasistatic magnetic dipole physics. The fact that the progress of the magnetic slug can be detected at very early stages of the flood, that the traveling slug's vertical boundaries can be identified at low frequencies, and that the magnetic nanoparticles can be sensed well before the actual arrival of the slug at the observer well provide significant value of the use of the magnetic-contrast agents in crosswell EM tomography.

Summary We develop an algorithm to model the magnetometric resistivity (MMR) response over an arbitrary 3‐D conductivity structure and a method for inverting surface MMR data to recover a 3‐D distribution of conductivity contrast. In the forward modelling algorithm, the second‐order partial differential equations for the scalar and vector potentials are discretized on a staggered‐grid using the finite‐volume technique. The resulting matrix equations are consequently solved using the bi‐conjugate gradient stabilizing (BiCGSTAB), combined with symmetric successive over relaxation (SSOR) pre‐conditioning. In the inversion method, we discretize the 3‐D model into a large number of rectangular cells of constant conductivity, and the final solution is obtained by minimizing a global objective function composed of the model objective function and data misfit. Since 1‐D conductivity variations are an annihilator for surface MMR data, the model objective function is formulated in terms of relative conductivity with respect to a reference model. A depth weighting that counteracts the natural decay of the kernels is shown to be essential in typical problems. All minimizations are carried out with the Gauss–Newton algorithm and model perturbations at each iteration are obtained by a conjugate gradient least‐squares method (CGLS), in which only the sensitivity matrix and its transpose multiplying a vector are required. For surface MMR data, there are two forms of fundamental ambiguities for recovery of the conductivity. First, magnetic field data can determine electrical conductivity only to within a multiplicative constant. Thus for a body buried in a uniform host medium, we can find only the relative conductivity contrast, not the absolute values. The choice of a constant reference model has no effect on the reconstruction of the relative conductivity. The second ambiguity arises from the fact that surface MMR cannot distinguish between a homogeneous half‐space and a 1‐D conductive medium. For a 3‐D body in a 1‐D layered medium, it is still difficult to obtain information concerning the general background 1‐D medium, if sources and receivers are at the surface. Overall, the surface MMR technique is useful so long as significant current flows through the body. This happens when the overburden is thin and moderately conductive (less than 10 times the conductivity of the underlying basement) and if the current sources are placed so there is good coupling with the body. Our inversion method is applied to synthetic examples and to a field data set. The low‐resolution image obtained from using traditional MMR data, involving one source and one magnetic component, illustrates the need for acquiring data from multiple sources if 3‐D structure of complex geometries are sought.

In the past several years, marine controlled-source electromagnetic (MCSEM) techniques have been applied successfully in deep water (depth > 1 km) for oil and gas exploration. The application of this technology in shallow water is challenged, however, because of “airwaves” that mask the signal from the target reservoir at depth. Based upon the understanding that an airwave is a lateral wave, which can be analytically expressed in a dual-half-space resistivity model, we propose three airwave-mitigation approaches to reduce the effects of these airwaves on MCSEM data. In the EM “x-bucking” approach, the effect of the airwaves can be “bucked” out from two measurements by using the analytic expression of the airwave. The frequency derivative (dE/dFreq) approach takes advantages of the unique characteristics of the airwaves in frequency domain, enhancing the reservoir signals while suppressing the airwave. The magnetotelluric (MT) stripping method uses the plane-wave feature of the airwaves and subtraction of the lateral wave electric component, which is obtained from measured marine MT impedance and controlled-source electromagnetics (CSEM) data, to generate a new data set in which the effects of the airwaves are removed substantially. By comparing the detectability, which is defined as the ratio of inline Ex fields between a reservoir model and a corresponding baseline model, for a reservoir target in deep water versus shallow water with a moderate 2D bathymetry, we show that the effects of the airwaves in shallow water can be reduced in the data, leading to greater reservoir detectability. In addition, these approaches have been applied successfully to a real shallow water MCSEM data set in which the detectability to the deeper resistive basement is enhanced.

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