Directional borehole radar (DBR) is a powerful tool for constructing the 3-D morphology of subsurface geological bodies. Direction of arrival (DOA) estimation is a key step in 3-D imaging. However, the existing DOA methods are constrained by factors such as DBR aperture, and can only identify one signal source within a time window. Consequently, when faced with multiple targets, DBR encounters challenges in effectively distinguishing them, especially when their echoes nearly overlap. In addition, the presence of interference waves makes echo overlap more likely to occur. So we propose a 3-D imaging method based on echo separation to solve this problem. This method first separates echoes of different targets through echo separation methods such as correlation method, deconvolution method, τ-ptransformation method, and moving window method. Subsequently, the azimuth is obtained through the multiple signal classification (MUSIC) algorithm, while the depth and radial distance are obtained through the inverse bi-static boundary scattering transform (IBBST), finally achieving the 3-D imaging of multiple geological targets. The effectiveness of the proposed method is demonstrated through synthetic data of both simple and complex fracture models, and its feasibility in practical applications is demonstrated through field data examination. This method improves the detection capability of DBR, opening up new possibilities for accurately mapping subsurface geological features.

The emergence of directional borehole radar (DBR) has made it possible to obtain the 3-D morphology of fractures through a single borehole. The use of a uniform circular array (UCA) as the array receiving antenna is an important way to achieve directional detection; however, the direction of arrival (DOA) method and 3-D imaging method with DBR still have limitations in the imaging capability of rough fractures. We, therefore, propose a method of performing migration first and then DOA estimation next, combined with the moving window method (MWM), to achieve multitarget 3-D imaging, where the DOA estimation adopts the idea of applying the multiple signal classification (MUSIC) algorithm directly. We use the multigrid finite difference time domain (FDTD) method for numerical simulation to verify the effectiveness of the proposed 3-D imaging method. Subsequently, the response of DBR to rough fractures was studied, and the results showed that the roughness characteristics of fractures would significantly change the imaging results but also bring more fracture feature information; we can even detect the rough fracture under unfavorable dip angle. The research provides a theoretical basis for the detection of fractures under complex conditions and has practical application value in the future.

The flight altitude has a large effect on the airborne electromagnetic (AEM) responses. Due to the dynamic environment of the aircraft, the recorded sensor altitudes may contain errors. Research demonstrates that the AEM responses caused by a several meters altitude errors can be larger than caused by some anomalous body. Ignoring these errors will create erroneous results in AEM data interpretation. Considering that there is not yet a published 3D AEM inversion method that takes into account the flight altitude, we develop in this paper a 3D inversion algorithm for AEM with the flight height treated as an inversion parameter. For the forward modeling we use the finite element method, while for the inversion we use the Gauss-Newton optimization method. To make our inversion works for variable flight altitudes, we propose a scheme of 3D Jacobean matrix calculation for both the resistivities and flight altitudes without much increasing the computational cost. The numerical simulation result confirms that the flight altitude really has a large effect on the AEM responses. The inversions of synthetic data show that our 3D inversion method can both recover the resistivity distribution in the underground and decrease the altitude errors recorded, while the field data inversion demonstrates that our method can deliver a better inversion model with a smaller data misfit.

The 3-D airborne electromagnetic (AEM) inversions have been restricted by the modeling efficiency resulting from the complex geology in exploration areas and massive amount of data collected by AEM systems. In order to improve the modeling efficiency, we develop an algorithm that combines the hexahedral vector finite element (FE) with octree meshes, in which the boundary conditions are imposed via an algebraic constraint to ensure the continuity of the FE solution. This makes the division with hexahedral meshes more flexible for complex geology such as rugged topography or underground structures so that we can reduce the number of elements while maintaining the accuracy. After formulating the forward problem, we check the accuracy of our algorithm by taking a homogeneous half-space model and comparing the results of our octree method with the semianalytical solutions. Furthermore, we demonstrate the efficiency of our octree method by comparing with the traditional FE method using tetrahedral meshes. Finally, we subdivide a complex topography constructed using the 2-D Gaussian rough surface and calculate the EM responses with and without anomaly embedded. The results show that the EM responses are overwhelmed by the Earth topography. We carry out the topographic correction by taking a method based on the ratio of EM responses with and without anomaly. The experiments show that after topographic correction to AEM data, the response of anomaly becomes more distinguishable so that the anomaly can be clearly identified. Furthermore, we also calculate the EM response for a realistic model—the Ovoid Zone ore body located at Voisey’s Bay, Labrador, Canada, to verify the flexibility and practicality of our algorithm.

Borehole radar is a powerful tool for detecting subsurface fractures. Fracture modeling and numerical simulation are essential means to study fracture detectability. In this article, we first propose a method to construct a single fracture model, which combines the Baecher disk model and random rough surface and includes features, such as roughness, pinch-out, and irregularity. As the precise description of the fracture raises the requirement for accuracy of the simulation algorithm, the 3-D conformal finite-difference time-domain (CFDTD) method is used in this work. Numerical simulations of a sphere and rough fractures show that the CFDTD has higher calculation accuracy than conventional FDTD. Then, we analyzed how the fractures with different roughness affect the electromagnetic wave response. It is found that as the fracture surface becomes rougher, the wave scattered by it becomes stronger, more fracture contour-related information is obtained, and the fracture morphology is recovered better. Combined with accurate fracture modeling and high-precision numerical simulation, the electromagnetic response characteristics of different forms of fractures are obtained, which provides a basis for the accurate detection and interpretation of fractures in the future.