inversion of gravity data has been widely used to reconstruct the density distributions of ore bodies, basins, crust, lithosphere, and upper mantle. For global model of 3-D density structures of planetary interior, such as the Earth, the Moon, or Mars, it is necessary to use an inversion algorithm that operates in the spherical coordinates. We develop a 3-D inversion algorithm formulated with specially designed model objective function and radial weighting function in the spherical coordinates. We present regional and global synthetic examples to illustrate the capability of the algorithm. The inverted results show density distribution features consistent with the true models. We also apply the algorithm to a set of lunar Bouguer gravity anomaly derived from the Gravity Recovery and Interior Laboratory (GRAIL) gravity field and obtain a lunar 3-D density distribution. High-density anomalies are clearly identified underlying lunar basins, a wide region of the lateral density heterogeneities that exist beneath the South Pole-Aitken basin are found, and low-density anomalies are distributed beneath the Feldspathic Highlands Terrane on the lunar far-side. The consistency of these results with those obtained independently from other existing methods verifies the newly developed algorithm.

The fate of subducted oceanic slabs can provide important clues to plate reconstruction through Earth history. Since oceanic slabs in continental collision zones are typically not well preserved, ancient subduction zones have rarely been imaged by geophysical techniques. Here we present an exception from the Darbut belt in the Junggar accretionary collage in the southern Altaids of Asia. We deployed a 182 km long magnetotelluric (MT) profile including 60 broadband sounding sites across the belt. Quality off‐diagonal impedances were inverted by a three‐dimensional scheme to image resistivities beneath the profile. The resistivity model along with MT impedance phase ellipses and induction vectors were tested and interpreted in detail. Combining geological and geophysical observations, mineral physical experiment, and geodynamic modeling results, the MT transect suggests a fossil intraoceanic subduction zone during the Late Paleozoic in the western Junggar that has been well preserved due to lack of significant subsequent tecto‐thermal events.

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High-frequency (≥2 Hz) Rayleigh-wave data acquired with a multichannel recording system have been utilized to determine shear (S)-wave velocities in near-surface geophysics since the early 1980s. This overview article discusses the main research results of high-frequency surface-wave techniques achieved by research groups at the Kansas Geological Survey and China University of Geosciences in the last 15 years. The multichannel analysis of surface wave (MASW) method is a non-invasive acoustic approach to estimate near-surface S-wave velocity. The differences between MASW results and direct borehole measurements are approximately 15% or less and random. Studies show that simultaneous inversion with higher modes and the fundamental mode can increase model resolution and an investigation depth. The other important seismic property, quality factor (Q), can also be estimated with the MASW method by inverting attenuation coefficients of Rayleigh waves. An inverted model (S-wave velocity or Q) obtained using a damped least-squares method can be assessed by an optimal damping vector in a vicinity of the inverted model determined by an objective function, which is the trace of a weighted sum of model-resolution and model-covariance matrices. Current developments include modeling high-frequency Rayleigh-waves in near-surface media, which builds a foundation for shallow seismic or Rayleigh-wave inversion in the time-offset domain; imaging dispersive energy with high resolution in the frequency-velocity domain and possibly with data in an arbitrary acquisition geometry, which opens a door for 3D surface-wave techniques; and successfully separating surface-wave modes, which provides a valuable tool to perform S-wave velocity profiling with high-horizontal resolution.

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Rayleigh waves, surface waves that travel along a "free" surface such as the earth-air or the earth-water interface, are usually characterized by relatively low velocity, low frequency, and high amplitude. Rayleigh waves are the result of interfering P and SV waves. Particle motion of the fundamental mode of Rayleigh waves in a homogeneous medium moving from left to right is elliptical in a counterclockwise (retrograde) direction along the free surface. As depth increases, the particle motion becomes prograded and is still elliptical when reaching sufficient depth. The motion is constrained to a vertical plane consistent with the direction of wave propagation.In the case of a solid homogenous half-space, the Rayleigh wave is not dispersive and travels at a velocity of approximately 0.9194 v when Poisson's ratio is equal to 0.25 and where v is the Swave velocity in the half space. However, in the case of one layer over a solid homo-genous half-space, Rayleigh waves become dispersive when their wavelengths are in the range of 1-30 times the layer thickness.Longer wavelengths penetrate greater depths for a given mode, generally exhibit greater phase velocities, and are more sensitive to the elastic properties of the deeper layers. Conversely, shorter wavelengths are sensitive to the physical properties of near-surface layers. Therefore, a particular mode of surface wave will possess a unique phase velocity for each unique wavelength, leading to the dispersion of surface waves.Shear-wave velocities can be derived from inverting the dispersive phase velocity of the surface (Rayleigh and/or Love) wave. Near-surface S-wave velocity can be determined by inverting high-frequency Rayleigh waves using a process that is called multichannel analysis of surface waves (MASW). This process includes acquisition of highfrequency (>2 Hz) broad-band Rayleigh waves, efficient and accurate algorithms designed to extract Rayleigh-wave dispersion curves from Rayleigh waves, and stable and efficient inversion algorithms to obtain near-surface S-wave velocity profiles.Near-surface S-wave velocities. MASW estimates S-wave velocity from multichannel vertical component data and consists of three parts: data acquisition, dispersion-curve picking, and inversion. A 2D S-wave velocity section can be generated when surface wave data are acquired in a standard CMP rollalong acquisition format.1) Surface-wave data acquisition. Optimal recording of Rayleigh waves requires field configurations and acquisition parameters favorable to recording planar Rayleigh waves. Depending on investigation depth, Rayleigh waves of certain lengths need a specific amount of time to be developed into planar waves. Plane-wave propagation of surface waves does not occur in most cases until the near-offset (distance between the source and the first receiver) is greater than half the maximum desired wavelength. The maximum penetration depth in a homogeneous medium is about one wavelength. The currently accepted rule of thumb for the maximum penetration depth is approximat...