Full-waveform inversion (FWI) is increasingly used in land seismic exploration to achieve high-resolution near-surface models. In complex near-surface environments, however, FWI is challenged by noisy data and inaccurate initial models, raising the need for an effective regularization strategy to mitigate the inherent ill-posedness of FWI. Incorporating geological information, such as structural dips, into the regularization operator, has proven effective in stabilizing FWI and promoting geologically meaningful results. Obtaining clear seismic images that provide structural insights is, nonetheless often hindered in complex near-surface surveys due to noisy reflection data and gradient weathering velocity. Structural regularization treats imaged reflectors as velocity contours and penalizes velocity variations along the dips. We propose a novel approach involving deformable-layer tomography (DLT) to directly invert for velocity contour distributions. DLT is well suited for near-surface applications as it accommodates both gradient velocity variations and abrupt velocity contrasts. In order to avoid erroneous structural regularization, we evaluate the accuracy of the DLT-estimated dips and assign weights accordingly. The weighted dip field is used to construct directional total variation (TV) in regularizing FWI, using the edge-preserving smoothing of TV regularization as well as the dip constraint. Using a realistic near-surface model, we demonstrate that FWI with the DLT-guided directional TV regularization outperforms conventional TV regularization. Our findings underscore the advantages of incorporating geologic structural constraint into FWI under complex near-surface conditions, highlighting the improved fidelity and resolution in near-surface velocity model building.

In complex near-surface geologic settings, first-arrival traveltime tomography suffers from strong data noise and uneven raypath coverage. To reduce the inversion nonuniqueness, model smoothing is necessary but often without incorporating structural constraints based on seismic images. The reasons include a lack of shallow reflections, strong noise, and gradient velocity field with strong lateral variation. We propose to estimate the near-surface structural trend from the velocity solution of deformable-layer tomography (DLT), that inverts for velocity contours directly. Then the structural dips derived from the DLT solution are taken as the structural regularization in the subsequent traveltime tomographic inversion. Tests for the 2011 Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP) workshop model illustrate that this DLT-guided structural regularization steers the tomographic inversion toward more accurate near-surface models. The application of the DLT-guided structural regularization to field data reveals the near-surface geology of a beach area in Qingdao, China. The DLT solution could complement seismic images or other structural information in regularizing tomographic inversion.

The inadequate resolution of cavity karst reservoir characterization is a key factor affecting the hydrocarbon production efficiency of ultradeep marine carbonates. Full waveform inversion (FWI) using high-frequency seismic data can provide a higher resolution than conventional methods. However, computational efficiency limits its application. This letter proposed a target-oriented local FWI method for cavity karst reservoir characterization. Based on the wavefield injection and Marchenko redatuming methods, a local wavefield forward modeling operator is derived. It can obtain the local wavefield corresponding to the physical source at the surface. Based on this local wavefield reconstruction, an objective function of the local FWI is established. To enhance the reservoir boundaries in the inversion results, a stabilizing strategy that combines total variation (TV) and minimum support (MS) regularization is proposed. A synthetic data test demonstrates that the obtained model can well describe the cavity karst reservoir structures.

Seismic migrations are generally formulated as the adjoint operators of linear forward modeling and often lead to images with degraded resolution, unbalanced illumination, and migration artifacts, especially in surveys with geologic complexity and irregular acquisition geometry. Least-squares migration (LSM) is able to mitigate these problems and produce better resolved images that are suitable for subsequent AVO/AVA inversion. However, no matter what domain LSM is implemented in, the computational cost is still several times or even one order of magnitude more than that of the traditional migration. In this article, we present an efficient image-domain least-square Kirchhoff depth migration (LSKDM), in which the Hessian matrix is approximated by a grid of point-spread-functions (PSFs). Traditional PSF computing algorithm requires a nonnegligible cost caused by a successive operation of modeling and migration and has to satisfy a sampling restriction to avoid interference between nearby PSFs. We present in this article that, by using the ray-based Green’s functions and the linear traveltime approximation, the PSFs can be constructed explicitly at a significantly reduced computational cost and are able to adapt flexible spatial sampling that is fine enough to detect small-scale illumination variation. With the constructed PSFs, we formulate an image-domain LSKDM to iteratively solve for the optimal reflectivities. Numerical tests on synthetic and field data examples demonstrate that the proposed LSKDM is highly efficient and is capable of producing images with enhanced spatial resolution and amplitude fidelity when compared with the Kirchhoff depth migration (KDM) image.

The pre-/sub-salt fractured networks provide key clues to understanding the tectonic history and the subsurface reservoir evolution. Yet, they are among the most complicated targets for geophysical investigations. We develop a structure-oriented mapping technique to delineate the high-resolution dipping layers across the borehole region using the zero-offset vertical-seismic-profiling (ZVSP) survey, which is conventionally used to provide one-dimensional (1-D) wave propagation information. The key information for the single-shot VSP mapping, which is the structural dip across the borehole, is unreliable from the poor sub-salt surface seismic image. Alternatively, we obtain the structural dips via reflection layer tomography. Taking advantage of the accurate interval velocities and the high-fidelity identifications from the reflection events, we manage to invert for the geometry of the main reflectors identified across the wellbore. Based on such information, we propose an effective processing strategy and achieve a high-resolution image of the dipping structures around the borehole region from the ZVSP. The current result compares reasonably well with the corresponding surface seismic profile but supplies higher-resolution details of the dipping structures and fault networks below the thick evaporite caprock where the surface seismic image degrades sharply. The enhanced subsurface image encourages better structural evaluation, geologic interpretation, and future 2-D/3-D VSP survey design.

Accurate delineation of the salt flank is important for oil and gas exploration in areas with salt intrusion. We describe the application of deformable-layer tomography (DLT) to invert for the geometry of salt flank using travel times of transmitted P- and S-waves from surface sources to downhole receivers. The DLT allows us to take advantage of a common situation that the salt velocity is known and generally invariant, but we need to determine the variable geometry of salt flank. We demonstrate our new method using a physical model experiment mimicking the setup of a walkaway vertical-seismic-profiling (VSP) survey. We first use picked P-wave arrivals to invert for the salt flank geometry; the sinusoid shape of the salt flank is estimated fairly by the DLT due to the uneven P-wave raypath coverage. As an improvement, we further incorporate the S-wave arrivals in the DLT. The picking of S-wave arrivals is assisted with modeled S-wave arrivals based on the P-wave DLT model. The DLT solution using both P- and S-wave arrivals delineates the salt flank geometry more accurately than that using P-wave arrivals alone. The salt flank delineation using DLT can complement with the conventional salt proximity and migration methods. It could also serve as constraints or the initial model for the full-waveform inversion of salt geometry.