We present a newly developed finite element program for direct current resistivity modelling, which can handle arbitrary 3-D electric anisotropy. For this purpose, it is of particular importance to construct appropriate grids because artificial anisotropy can be introduced through preferential directions associated with regular grid structures. Therefore, results from different kinds of grids (structured hexahedral, structured tetrahedral and unstructured tetrahedral) are checked for symmetry. After a series of comparisons, we conclude that unstructured tetrahedral grids generally perform best. In addition, this grid type allows for local refinement, which greatly reduces the number of nodes and, consequently, lowers the computational costs significantly. A singularity removal technique is applied, which improves the accuracy considerably. The resulting system of linear equations is solved by a conjugate gradient method with a symmetric successive overrelaxation pre-conditioner. Comparisons with analytical solutions prove the code to be highly accurate for both isotropic and anisotropic models. More complex models are investigated to analyse the response of anisotropic structures, for example, in form of the P₂ tensor invariant. Finally, we apply the code to a hot dry rock scenario and show that anisotropy reveals significant information on the hydraulically induced fracture system.

Summary. An accurate and efficient 3-D finite-element (FE) forward algorithm for DC resistivity modelling is developed. First, the total potential is decompose

The effects of sediments with different clay contents on the mechanical properties of hydrate deposits were studied using a high-pressure, low-temperature triaxial apparatus with in-situ synthesis, as well as the mechanical properties of self-developed hydrate sediments. Through multi-stage loading, triaxial compression tests were conducted by adding quartz sand with different clay contents as the sediment skeleton, and the stress–strain relationship of the shearing process and the strength of sediments with different clay contents were determined. Volumetric changes were also observed during shearing. The results show that the strength of hydrate sediments decreases with the increasing clay content of sediments; in the processes of depressurization and shearing, the hydrate samples exhibited obvious shear shrinkage, regardless of the sediment particle size.

We present a 3-D electrical resistivity modelling with long electrode sources using finite element method on unstructured tetrahedral grids. The radius and resistivity of the electrode is neglected and the electrode can be regarded as a line source with finite length in our modelling. An approximated equation for determining the current density along the electrode is derived for current density correction in heterogeneous media. Electrical fields excited by arbitrary long electrode sources in heterogeneous conductive media can be simulated. Both total- and secondary-potential methods are implemented in the simulation. The analytic electrical potentials of an arbitrary long electrode source in a conductive half-space is derived and used for accuracy analysis in the numerical simulations. The apparent resistivities for survey arrays with long electrode sources are proposed based on the analytic solution of the half-space model. The responses of electrical fields excited by point electrode and long electrode sources to resistivity anomalies are compared with ground and borehole simulations, which shows that the arrays with long electrode sources are more sensitive to the anomalies than those with point electrode sources. A time-lapsed monitoring for hydraulic fracturing in oil field using the production wells as long electrode sources is also simulated, which indicates that the borehole potential data of the long electrode source array can help assess the location of the fracture zone.