A new source location and attribute recognition method based on correlation analysis of gravity and magnetic anomaly

<p>The traditional gravity and magnetic correspondence analysis tends to have high correlation outside the field source area. In order to overcome the disadvantage, we propose a new method for identify the source position and attribute, which is based on similarity and vertical derivative of potential field. In this method, we put forward a new gravity and magnetic correlation parameter (GMCP), which can effectively reduce the range of potential field source and indicate the field intensity information. The distribution of the non-zero areas of GMCP reflects the size of the source. GMCP discriminant parameter values of positive and negative reflect the source attribute. When GMCP is greater than zero, it is a positive correlation indicating that there are high-density and high-magnetization or low-density and low-magnetization homologous bodies in this region; When GMCP is less than zero, it is negative correlation indicating that there are high-density and low-magnetic or low-density and high-magnetic density homologous bodies in this region. GMCP goes to zero, which means no gravity-magnetic homologous geological body. Complex models test results with different noise level and actual data processing of South China Sea Basin show the correctness and validity of identification of the proposed methods.</p>

The features of fault tectonics and deep structure of the seismoactive zones in Eastern Priamurye

The paper examines the spatial relationship between the seismoactive zones in eastern Priamurye (М ≥ 5) and the regional faults and hidden fault zones identified from the gravity and magnetic anomaly axes. The seismoactive zones where earthquakes with М ≥ 5 occurred are mostly confined to the regional faults, though such a relationship has not been validated in two cases. The seismoactive zones are detected both at the regional fault intersection and in areas where the regional faults intersect with the hidden faults of various ranks. According to the data obtained by deep seismic sounding (DSS), earthquake converted wave method (ECWM) and magnetotelluric sounding (MTS), the seismoactive zones are formed by deep inclined and subvertical faults. The indications of fluid saturation are found in the seismoactive zones from geophysical data which show that the seismoactive faults often control low-velocity and low-resistivity anomalies in the crust and upper mantle. In some cases, the Moho displacement and the dome-like flexures of the crustal and Moho boundaries are observed along these seismoactive faults and the abundance of the conversion boundaries in the crust is also noted. The deep pattern of the seismoactive faults and the revealed indications of fluid saturation allow us to consider the seismoactive zones in eastern Priamurye as the channels providing fluid supply from the mantle to the crust.

Three-dimensional numerical modeling of gravity and magnetic anomaly in a mixed space-wavenumber domain

Fast and accurate numerical modeling of gravity and magnetic anomalies is the basis of field-data inversion and quantitative interpretation. In gravity and magnetic prospecting, the computation and memory requirements of practical modeling is still a significant issue, which leads to the difficulty of using efficient and detailed inversions for large-scale complex models. A new 3D numerical modeling method for gravity and magnetic anomaly in a mixed space-wavenumber domain is proposed to mitigate the difficulties. By performing a 2D Fourier transform along two horizontal directions, 3D partial differential equations governing gravity and magnetic potentials in the spatial domain are transformed into a group of independent 1D differential equations wrapped with different wavenumbers. Importantly, the computation and memory requirements of modeling are greatly reduced by this method. A modeling example with 4,040,100 observations can be finished in approximately 28 s on a desktop using a single core, and the independent differential equations are highly parallel among different wavenumbers. The method preserves the vertical component in the space domain, and thus a mesh for modeling can be finer at a shallower depth and coarser at a deeper depth. In general, the new method takes into account the calculation accuracy and the efficiency. The finite-element algorithm combined with a chasing method is used to solve the transformed differential equations with different wavenumbers. In a synthetic test, a model with prism-shaped anomalies is used to verify the accuracy and efficiency of the proposed algorithm by comparing the analytical solution, our numerical solution, and a well-known numerical solution. Furthermore, we have studied the balance between computational accuracy and efficiency using a standard fast Fourier transform (FFT) method with grid expansion and the Gauss-FFT method. A model with topography is also used to explore the ability of modeling topography with our method. The results indicate that the proposed method using the Gauss-FFT method has characteristics of fast calculation speed and high accuracy.

Interpreting potential field anomaly of an isolated source of regular geometry revisited

I have developed an improved practical method for interpreting a symmetrical-shaped potential field anomaly due to an isolated source body of regular geometric configuration. The method uses the first-order horizontal derivative of the logarithmically transformed absolute value of the anomaly in estimating the source-body parameters, such as the location, depth of burial, and shape factor. To tackle noise in data, a regularization technique is designed, which ensures a robust estimate of the first-order derivative of logarithmically transformed data. The regularization technique uses an optimal value of regularization parameter that, although noise dependent, requires no a priori knowledge of the noise level in the data. A graphical method is designed to determine an optimal value of the regularization parameter from the position of the local minimum of a specially defined functional with respect to the regularization parameters. Numerical tests have been conducted on the noise-contaminated synthetic data to validate the proposed method. The successful application of the method on published field data for the gravity and magnetic anomaly suggests the applicability of the method.

Evidence for transform motion along the South Balearic margin and implications for the kinematics of opening of the Algerian basin

Two major types of kinematic models have been proposed to explain the opening of the western Mediterranean basins (Liguro-Provençal and Algerian basins, and Valencia trough). In one type of models, all continental blocks bounding the basins drift to the southeast, driven by the rollback of the Tethys subduction slab. In the other type of models, the Alboran domain drifts to the southwest, implying a westward rollback of the broken subducting slab and a NE-SW opening of the Algerian basin. In most models, however, the structure of the Balearic promontory was not taken into account, despite its key location at the boundary of the three major basins. We used the interpretation of a large seismic database coupled to gravity and magnetic anomaly analyses to characterize the nature and structure of the South Balearic margin. The constraints brought by the new analyses allow us to suggest a new scenario for the opening of the Algerian basin. Seismic profiles show that the South Balearic margin is composed of four segments with different morphologies and crustal structures. Two segments, the Mazarron and the Emile Baudot escarpments, are characterized by steep scarps and sharp crustal thinning. Two other segments, the South Ibiza and South Menorca margins, have a smoother bathymetry and crustal thinning. We interpret the former in terms of transform margins, and the latter as divergent margins. The distribution of faults on the passive margin segments suggests that they have recorded at least two phases of deformation. A first phase of opening, probably in a NW-SE direction, affected the south Balearic margin, and possibly created some oceanic floor. The existence of the transform margin segments and the prominent NW-SE orientation of the magnetic lineations in the eastern Algerian basin suggest that most of this basin opened in a NE-SW direction, in different oceanic corridors. The two eastern corridors formed by the southwestward drift of the Kabylies. The western corridor, bounded by the transform segments of the South Balearic margin and the Algerian margin, results from the southwestward drift of the Alboran domain, as suggested by previous studies.

Gravitational and magnetic anomaly inversion using a tree-based geometry representation

Gravitational and magnetic anomaly inversion of homogeneous 2D and 3D structures is treated using a geometric parameterization that can represent multiple, arbitrary polygons or polyhedra and a local-optimization scheme based on a hill-climbing method. This geometry representation uses a tree data structure, which defines a set of Boolean operations performed on convex polygons. A variable-length list of points, whose convex hull defines a convex polygon operand, resides in each leaf node of the tree. The overall optimization algorithm proceeds in two steps. Step one optimizes geometric transformations performed on different convex polygons. This step provides approximate size and location data. The second step optimizes the points located on all convex hulls simultaneously, giving a more accurate representation of the geometry. Though not an inherent restriction, only the geometry is optimized, not including material values such as density or susceptibility. Results based on synthetic and measured data show that the method accurately reconstructs various structures from gravity and magnetic anomaly data. In addition to purely homogeneous structures, a parabolic density distribution is inverted for 2D inversion.