Search

The airborne gravimetry measurement can better highlight the anomalous response of deeper resources than ground gravity data, so an integrated survey of airborne and ground gravity data is an effective strategy to obtain the obvious response of shallow and deep mineral resources simultaneously. In order to obtain a more reliable and higher resolution 3-D density distribution model through the joint inversion of airborne and ground gravity data to support deeper source exploration, we propose a high-resolution joint inversion method with a cross-constraint technique. We use structural constraints to establish an objective function for joint inversion of airborne and surface gravity data, and add density-weighting constraints through cross-calculation to make full use of the sensitivity of different data and geological targets. Synthetic model tests prove that the cross-constraint joint inversion method of airborne and ground data improves the model resolution effectively, and can delineate the distribution of deeper targets more clearly compared to the existing joint inversion way. In order to predict the spatial distribution of deep ore-bearing rock bodies in a north Qinling area of Shaanxi province, we obtained regional measured airborne and surface gravity data. High-resolution density joint cross-constraint inversion method was used for real data interpretation, and the distribution feature of possible deep ore-forming rock bodies.

The effect of groundwater level on dynamic compaction is significant but remains poorly understood. Model tests of dynamic compaction on sand with different groundwater levels were conducted to investigate the effect of groundwater depth in dynamic compaction. The crater depth, the dynamic stresses, and the pore water pressures induced by dynamic compaction were recorded and analyzed. The soil movements caused by the impacts were monitored using a high-speed camera, and the corresponding shear strain fields were generated employing the digital photography technology. For cases where the groundwater level is below the surface, the crater depth, the dynamic stresses, the normalized peak porewater pressures, and the shear strain response all increase as the water level becomes lower. Dewatering, as expected, is beneficial for ground improvement. Besides, the shear strain distributions indicate that the unsaturated sand above the water table experiences compression-induced compaction, previously observed in dynamic compaction on dry sand. Whether a compaction zone and the associated shear bands can be formed or not depends on the thickness of unsaturated soils above groundwater. By comparison, the saturated sand immersed in water displays the liquefaction-induced compaction. The densification of saturated sand in dynamic compaction is less efficient than that in dry or unsaturated sand in terms of energy utilization.

Three-dimensional (3D) gravitational and magnetic exploration is performed using aerial measurement tools, however, this has difficulties with measuring-height design and the construction of a joint-interpretation scheme. At present, the height in such experiments is set according to the measurement scale, and the distribution characteristics of anomalies are not fully considered. Here, we present the idea of using the attenuation characteristics of a singular-value spectrum to evaluate the contributions of various measurement heights and multi-height combinations for inversion to correctly and reasonably design appropriate measuring heights and the number of various measurement heights to be set. The joint-gradient Euler-deconvolution method can accurately obtain the distribution of geological bodies from 3D gravitational and magnetic data at an improved resolution, and experimental tests confirm these findings. Therefore, an actual 3D aeromagnetic-data-acquisition and inversion test were carried out in the vicinity of the Zhurihe Iron Mine in Inner Mongolia. The flight-height difference was set to 60 m, and the specific distribution of lodes was obtained by the joint-gradient Euler-deconvolution method. This provides a reliable basis for future detailed exploration and proves that the methods presented in this paper have good practical-application effects and prospects.

The Shuangjianzishan deposit in Inner Mongolia is a typical Ag-Pb-Zn deposit of the southern Great Xing’an Range. Proven reserves of Ag, Pb, and Zn in this deposit have reached the scale of super-large deposits, with favorable metallogenic conditions, strong prospecting signs, and high metallogenic potential. This paper reports a study involving integrated geophysical methods, including controlled-source audio-frequency magnetotelluric, gravity, magnetic, and shallow-seismic-reflection methods, to determine the spatial distribution of ore-controlling structures and subsurface intrusive rock for a depth range of

The creation and evolution of the Songliao Basin is closely related to the closure of the Paleo-Asian Ocean and the Mongolia-Okhotsk Ocean, and the subduction of the Paleo-Pacific Ocean. In an atte...

Since polymetallic ores show higher anomalies in gravity exploration methods, we usually obtain the position and range of ore bodies by density inversion of gravity data. The three-dimensional (3D) gravity focusing density inversion is a common interpretation method in mineral exploration, which can directly and quantitatively obtain the density distribution of subsurface targets. However, in actual cases, it is computation inefficient. We proposed the preconditioned Jacobian-free Newton-Krylov (JFNK) method to accomplish the focusing inversion. The JFNK method is an efficient algorithm in solving large sparse systems of nonlinear equations, and we further accelerate the inversion process by the preconditioned technique. In the actual area, the gravity anomalies are distributed on the naturally undulating surface. Nowadays, the gravity inversion under undulating terrain was mainly achieved by discretizing the ground into unstructured meshes, but it is complicated and time-consuming. To improve the practicality, we presented an equivalent-dimensional method that incorporates unstructured meshes with structured meshes in gravity inversion, and the horizontal size is determined by the gradient of observed gravity and terrain data. The small size meshes are adopted at the position where the terrain or gravity gradient is large. We used synthetic data with undulating-terrain to test our new method. The results indicated that the recovered model obtained by this method was similar to the inversion method of unstructured meshes, and the new method computes faster. We also applied the method to field data in Huayangchuan, Shaanxi Province. The survey area has complicated terrain conditions and contains multiple polymetallic ores. Based on the high-density characteristics of polymetallic ore bodies in the area, we calculate the field data into 3D density models of the subsurface by the preconditioned JFNK method and infer six polymetallic ores.

Gravity migration is a fast imaging technique based on the migration concept to obtain subsurface density distribution. For higher resolution of migration imaging results, we propose a 3D regularized focusing migration method that implements migration imaging of an entire gravity survey with a focusing stabilizer based on regularization theory. When determining the model parameters, the iterative direction is chosen as the conjugate migration direction, and the step size is selected on the basis of the Wolfe&ndash, Powell conditions. The model tests demonstrate that the proposed method can improve the resolution and precision of imaging results, especially for blocky structures. At the same time, the method has high computational efficiency, which allows rapid imaging for large-scale gravity data. It also has high stability in noisy conditions. The developed novel method is applied to interpret gravity data collected from the skarn-type iron deposits in Yucheng, Shandong province. Migration results show that the depth of the buried iron ore in this area is 750&ndash, 1500 m, which is consistent with the drilling data. We also provide recommendations for further mineral exploration in the survey area. This method can be used to complete rapid global imaging of large mining areas and it provides important technical support for exploration of deep, concealed deposits.

The middle part of the Da hinggan mountains is an important mineral resource base in China. Since the Mesozoic, due to the uplift of the mantle, the extension of the crust, the strong intraplate volcanic eruption and magmatic intrusion, the Mesozoic magmatic tectonic belt has attracted worldwide attention. For further exploring the deep structure of this area, the final geoelectricity structure was established based on a 50-kilometer magnetotelluric sounding profile in this area. Through data acquisition, phase tensor decomposition processing, and nonlinear conjugate gradient inversion, the model reveals the main electrical structural characteristics of this area: 1. The whole has a characteristic of “longitudinal layering and horizontal block”, and the shallow electrical structure is consistent with known geological conditions; 2. There is a distribution inferred that a low-resistance anomaly appears due to the fragmentation of the surface rock layer caused by the uplift of deep intrusive rocks on the surface; 3. There are three more obvious electrical gradient zones or low-resistance anomalies in the deep part, which are presumed to be fault zones or structural erosion variable zone; 4. Three electrical gradient zones in the section divide the deep structure into four large-scale and high-resistance bodies, which reflects the spatial distribution of deep concealed granite, thereby restoring the complex anticline, secondary grade anticline, oblique uplift and depression. In addition, given the electrical structural characteristics and known the mine locations and the field exploration conditions, we conclude that the location of the prospecting target should be on a belt with Shuangjianzishan Pb-Zn-Ag mine, Nailinba Cu-Ag mine, Nailinba Fe mine, and Baiyinnuoer Pb-Zn mine. Moreover, this target is further divided into the middle area of Nailinba Cu-Ag mine and Baiyinnuoer Pb-Zn mine in combination with field surveys.

Potential field migration of gravity data is a kind of method using the migration concept to efficiently obtain the density distributions subsurface. To get migration imaging results with the high resolution, we propose the regularized focusing migration method that adopts the regularization theory to realize focusing migration imaging and the conjugate migration direction method to solve the optimal solution of model parameters, which effectively improves the divergence of the conventional migration imaging. Synthetic model tests demonstrate that the proposed advanced method improves the imaging resolution significantly while guaranteeing the computational efficiency, and can bring about the relatively convergent results in a random Gaussian noise environment. This method is applied to the interpretation of the gravity anomaly from Huayangchuan, Shaanxi Province, which predicts the spatial distribution of uranium deposits.

A derivative formula for interface inversion using gravity anomalies, combining the Parker–Oldenburg method for calculating and inverting gravity anomalies with Xu’s iteration method for continuing potential fields, leads to a convergent inversion algorithm and an optimally located density interface geometry. In this algorithm, no filtering or any other convergence control techniques are needed during iteration. The method readily iterates the variable depth of the gravity interface by means of upward continuation in a form equivalent to inversion iteration in the Fourier domain instead of the divergent, downward continuation term. This iteration algorithm not only efficiently solves the divergence problem in the inversion iteration procedure but also validly obtains an excellent result for the density interface. A numerical example is presented to illustrate perfect execution of this approach in gravity exploration, and a real geophysical example of inversion of the Moho depth by means of this approach using a set of measured gravity anomalies over the Qinghai–Tibet Plateau in China is offered.

Many of the existing balanced edge detection filters of potential field data only use the feature that the vertical derivative is zero above the source edges to recognise the source edges. This will produce spurious edges in the interpretation of potential field data. In order to solve this disadvantage, a new format of the edge detection filter is presented, which produces maximum values only when the horizontal derivative is a maximum and the vertical derivative is zero, so the new filters will not produce spurious edges, and will provide more accurate results. The proposed filters are tested on synthetic potential field data. The recognised edges are shown to be consistent with the true edges, and do not produce additional edges. Moreover, the proposed filter suppresses the effect of noise, and displays the edges more clearly compared to previous edge detection filters. It is also applied to real magnetic data to obtain the horizontal locations of iron ore.

Balanced edge detection filters can recognize the edges of the shallow and deep bodies simultaneously, and are commonly used in the edge detection of potential field data. In this paper, we present using the balanced edge detection filters to estimate source locations, and derive two linear equations based on the balanced edge detection filters that can estimate the locations of the source without any priori information about the nature (structural index) of the source. The proposed methods are demonstrated on synthetic gravity anomalies, and the inversion results show that the proposed methods can successfully estimate location parameters of the sources. I also apply the proposed methods to real magnetic data, and the inversion results estimated by the proposed methods are consistent with the results estimated by the other similar method.

Three alternative local wavenumber methods are proposed to estimate the depth and the nature (structural index) of the 2D magnetic source simultaneously using various combinations of different forms of the local wavenumbers to compute the source parameters without any prior information about the source. A clustering method is also provided to get more accurate results. The proposed local wavenumber methods are demonstrated on synthetic noise-free and noise-corrupted magnetic data, and they successfully estimate the location parameters and structural index of the causative sources. The actual application of the proposed methods is demonstrated on a magnetic anomaly from southern Illinois.

Edge detection is a requisite task in the interpretation of potential field data. There are many high-pass filters based on horizontal and vertical derivatives in use, such as total horizontal derivative, tilt angle, theta map, et al. In this paper, we present a new edge detection filter, which uses the combination of the different order horizontal derivatives to delineate the edges of the sources, called improved local phase (ILP) filter. The new filter is computationally stable, as it does not need the computation of the vertical derivatives of potential field data. The new filter is tested on synthetic and real potential field data. The resolving power of the ILP filter is tested by comparing the results with those obtained by the other filters. The advantage of the ILP filter in the edge detection of potential field data is due to the fact that it can display the edges of the causative sources more precisely and clearly, and can bring out more subtle details.

We present two new potential-inversion methods for estimating the depth and the nature (structural index) of the source, which use various combinations of different forms of local wavenumbers and the information about the horizontal location to estimate individually the depth and the nature of a magnetic source. The improved local wavenumber methods only use the horizontal offset and vertical offset of local wavenumbers to estimate the depth and the structural index of the source, so they yield more stable results compared with the results obtained by current methods that require the derivatives of local wavenumbers. Tests conducted with synthetic noise-free and noise-corrupted magnetic data show that the proposed methods can successfully estimate the depth and the nature of the geologic body. However, our methods are sensitive to high-wavenumber noise present in the data, and we reduced the noise effect by upward continuing the noise-corrupted magnetic data. The practical application of the new methods is tested on a real magnetic anomaly over a dike whose source parameters are known and the inversion results are consistent with the true values.