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Identification of time spectrum is one of the core issues of the time-domain induced polarization (IP) method, which can be considered as one of the bases for distinguishing various polarized rocks. Fitting the IP data based on the spectrum forward model and obtaining optimal solution of the model parameters is a key step in spectrum identification. However, the suitable forward model should consider the observation conditions, such as the charging time and the time window. The time spectrum identification may be difficult to implement stably due to the lack of objective reference for the optimal solution. Therefore, our purpose is to improve the forward model and implement the spectrum identification for IP data. First, using the Weibull (WB) distribution function as the basis, a time spectrum forward model considering the charging time and observation time window is provided according to the typical measurement mode. Then, based on the WB spectrum model and Barzilai-Borwein gradient optimization, a method for solution of apparent spectral model parameters for spectrum identification is developed. Finally, this method is used to process the IP data from a mine in which the anomalies related to ore-bearing beds are identified based on the processing results. Results obtained demonstrate that the spectrum forward model based on the WB function is feasible in describing the IP data. The limited charging time and a wide observed time window should be considered to realize accurate simulations and description of the time-domain IP data. The essence of the time spectrum identification is to comprehensively reflect the time-varying state of the whole time channel through the spectral model parameters, whereas the decay field ratio of the adjacent time channel can be used as an objective reference. The parameters of the spectrum model characterizing the time-varying state are independent of polarization and resistivity and thus can be directly used for identification of IP anomalies.

The dispersive characteristics of high-frequency (≥2 Hz) Rayleigh waves have been widely applied to determine subsurface shear velocity ([Formula: see text]). However, the [Formula: see text] of the high-velocity surface layers (e.g., rigid roads in urban areas) and underlying layers is poorly constrained in the Rayleigh-wave dispersion inversion. Compared to the phase velocity, the Rayleigh-wave horizontal-to-vertical (H/V) ratios of particle motion (i.e., ellipticity) are particularly sensitive to shallow [Formula: see text] structures. Therefore, we have proposed a robust method for obtaining near-surface [Formula: see text] structures by inverting Rayleigh-wave dispersion and H/V data. Initially, we separate the fundamental-mode Rayleigh waves from the active-seismic records using a high-resolution linear Radon transformation method. Then, we extract the high-frequency H/V and interstation phase traveltimes from the separated fundamental mode using deconvolution interferometry and the frequency-time analysis method. Based on the phase traveltimes, we calculate the phase velocity dispersion using ray-based tomography. Finally, we perform the joint inversion of dispersion and H/V data to obtain the [Formula: see text] structure. According to the inversion results of synthetic and field data, the accuracy of the [Formula: see text] in the rigid surface layer and underlying layers is considerably improved. We find that the joint inversion of Rayleigh-wave phase velocity dispersion and H/V data from surface-wave surveys is a useful tool for accurately determining the complex shallow [Formula: see text] structures.

Marine magnetic surveys over oceanic ridge regions are of great interest for investigations of structure and evolution of oceanic crust, and have played a key role in developing the theory of plate tectonics (Dyment, 1993; Maus et al, 2007; Vine and Matthews, 1963). In this study, we propose an interpolation approach based on the dual-layer equivalent source model for the generation of a magnetic anomaly map based on sparse survey line data over oceanic ridge areas. In this approach, information from an ocean crust age model is utilized as constraint for the inversion procedure. The constraints can affect the magnetization distribution of equivalent sources following crust age. The results of synthetic tests show that the obtained magnetic anomalies have higher accuracy than those obtained by other interpolation methods. Meanwhile, considering the unclear on the true magnetization directions of sources and the background field in the synthetic model, well interpolation result can still be obtained. We applied the approach to magnetic data obtained from five survey lines east of the Southeast Indian Ridge. This prediction result is useful to improve the lithospheric magnetic field models WDMAMv2 and EMAG2v3, in the terms of spatial resolution and the consistency with observed data.

We have developed a case study on the use of constrained inversion of magnetic data for recovering ore bodies quantitatively in the Macheng iron deposit, China. The inversion is constrained by the structural orientation and the borehole lithology in the presence of high magnetic susceptibility and strong remanent magnetization. Either the self-demagnetization effect caused by high susceptibility or strong remanent magnetization would lead to an unknown total magnetization direction. Here, we chose inversion of amplitude data that indicate low sensitivity to the direction of magnetization of the sources when constructing the underground model of effective susceptibility. To reduce the errors that arise when treating the total-field anomaly as the projection of an anomalous field vector in the direction of the geomagnetic reference field, we develop an equivalent source technique to calculate the amplitude data from the total-field anomaly. This equivalent source technique is based on the acquisition of the total-field anomaly, which uses the total-field intensity minus the magnitude of the reference field. We first design a synthetic model from a simplified real case to test the new approach, involving the amplitude data calculation and the constrained amplitude inversion. Then, we apply this approach to the real data. The results indicate that the structural orientation and borehole susceptibility bounds are compatible with each other and are able to improve the quality of the recovered model to obtain the distribution of ore bodies quantitatively and effectively.

Non-uniqueness is a problem encountered while imaging subsurface susceptibility by inverting magnetic field surveys. We propose a novel method to construct velocity-susceptibility initial models using Vp together with the Vp/Vs ratio from seismic tomography to improve the model which satisfies more information and better identifies locations of high-susceptibility materials. For comparison, two recovered susceptibility structures are derived from two distinct initials (homogeneous and velocity-susceptibility) models through the standard inversion process. Two profiles with intense undulation of magnetic anomalies over sedimentary areas in central-west Taiwan and complex geological structures at the rim of the subduction zone in northeast Taiwan are used as examples to compare the results of the initial models. The inversion results suggest that the susceptibility structures from both models agree in terms of the location of the fault zones, specifically at depth 10 km. Consequently, the initial model with velocity constraints lowers the non-uniqueness associated with the inversion of magnetic anomalies.

In surface-wave methods, horizontal resolution can be defined as the ability to distinguish anomalous objects that are laterally displaced from each other. The horizontal length of a recognizable geological anomalous body is measured by the lateral variation of shear (S)-wave velocity. Multichannel analysis of surface waves (MASW) is an efficient tool to determine near-surface S-wave velocities. The acquisition of the MASW method involves the same source-receiver configuration moved successively by a fixed distance interval (a few to several stations) along a linear survey line, which is called a roll-along acquisition geometry. A pseudo-2D S-wave velocity section is constructed by aligning 1D models, and each inverted 1D S-wave velocity model reflects the vertical S-wave velocity variation at the midpoint of each geophone spread. Although the MASW method can improve the horizontal resolution of S-wave velocity sections to some degree, the amount of fieldwork is increased by the roll-along acquisition geometry. We propose surface-wave tomography method to investigate horizontal resolution of surface-wave exploration. Phase-velocity dispersion curves are calculated by a pair of traces within a multichannel record through cross-correlation combined with a phase-shift scanning method. Then with the utilization of travel-time tomography, we can obtain high resolution pure-path dispersion curves with diverse sizes of grids at different frequencies. Finally, the pseudo-2D S-wave velocity structure is reconstructed by inverting the pure-path dispersion curves. Travel-time tomography of surface waves can extract accurate dispersion curves from a record with a short receiver spacing, and it can effectively enhance the ability of random noise immunity. Synthetic tests and a real-world example have indicated that travel-time tomography has a great potential for improving the horizontal resolution of surface waves using multi-channel analysis.