[1] We introduce a new approach to the joint inversion of multimodal geophysical data using Gramian spaces of model parameters and Gramian constraints, computed as determinants of the corresponding Gram matrices of the multimodal model parameters and/or their attributes. We demonstrate that this new approach is a generalized technique that can be applied to the simultaneous joint inversion of any number and combination of geophysical datasets. Our approach includes as special cases those extant methods based on correlations and/or structural constraints of the multimodal model parameters. As an illustration of this new approach, we present a model study relevant to exploration under cover for iron oxide copper-gold (IOCG) deposits, and demonstrate how joint inversion of gravity and magnetic data is able to recover alteration associated with IOCG mineralization. Citation: Zhdanov, M. S., A. Gribenko, and G. Wilson (2012), Generalized joint inversion of multimodal geophysical data using Gramian constraints, Geophys. Res. Lett., 39, L09301,

Marine controlled-source electromagnetic (MCSEM) surveys have become an important part of offshore petroleum exploration. However, due to enormous computational difficulties with full 3D inversion, practical interpretation of MCSEM data is still a very challenging problem. We present a new approach to 3D inversion of MCSEM data based on rigorous integral-equation (IE) forward modeling and a new IE representation of the sensitivity (Fréchet derivative matrix) of observed data to variations in sea-bottom conductivity. We develop a new form of the quasi-analytical approximation for models with variable background conductivity (QAVB) and apply this form for more efficient Fréchet derivative calculations. This approach requires just one forward modeling on every iteration of the regularized gradient-type inversion algorithm, which speeds up the computations significantly. We also use a regularized focusing inversion method, which provides a sharp boundary image of the petroleum reservoir. The methodology is tested on a 3D inversion of the synthetic EM data representing a typical MCSEM survey conducted for offshore petroleum exploration.

[1] Interpretation of the EarthScope MT (magnetotelluric) data requires the development of a large-scale inversion method which can address two common problems of 3D MT inversion: computational time and memory requirements. We have developed an efficient method of 3D MT inversion based on an IE (integral equation) formulation of the MT forward modeling problem and a receiver footprint approach, implemented as a massively parallel algorithm. This method is applied to the MT data collected in the western United States as a part of the EarthScope project. As a result, we present one of the first 3D geoelectrical images of the upper mantle beneath Yellowstone revealed by this large-scale 3D inversion of the EarthScope MT data. These images show a highly conductive body associated with the tomographically imaged mantle plume-like layer of hot material rising from the upper mantle toward the Yellowstone volcano. The conductive body identified in these images is west-dipping in a similar way to a P-wave low-velocity body. Citation: Zhdanov, M. S., R. B. Smith, A. Gribenko, M. Cuma, and M. Green (2011), Three-dimensional inversion of large-scale EarthScope magnetotelluric data based on the integral equation method: Geoelectrical imaging of the Yellowstone conductive mantle plume, Geophys. Res. Lett., 38, L08307,

We have studied the IP response of the multiphase porous systems by conducting complex resistivity (CR) frequency domain IP measurements for two different groups of samples: sands and sandstones containing salt water in pores and those whose unsaturated pores are filled with synthetic oil. We have observed the IP behavior in the imaginary parts of the analyzed complex resistivity curves. We have studied statistical aspects of CR measurements in HC-saturated samples using sand-cartridge-oil (SCO) and sandstone-oil (SSO) samples. A comparison of the complex resistivity of SCO and SSO samples with different saltwater pH values demonstrates a known shift of the IP peak to lower frequency with a decrease in pH. We used a GEMTIP model to analyze the IP parameters of the measured responses.

Marine controlled-source electromagnetic (MCSEM) surveys have become an important part of offshore petroleum exploration. In this paper we discuss new advances in the development of 3D inversion methods for the interpretation of MCSEM data. Our method is based on rigorous integral equation (IE) forward modeling and a new IE representation of the sensitivity (Fréchet derivative matrix) of observed data to variations in sea-bottom conductivity. We use quasi-analytical approximation for models with variable background conductivity (QAVB) for more efficient Fréchet derivative calculations. In our regularized focusing inversion algorithm we introduce a new stabilizing functional, a minimum vertical support stabilizer. This stabilizer helps generate a focused image of the relatively thin and flat resistive structure of a hydrocarbon (HC) reservoir. The methodology is tested on a 3D inversion of the synthetic EM data and the interpretation of an MCSEM survey conducted in the Troll West Gas Province (TWGP).