关键词: 呼和浩特-包头盆地, 初至波, 反射波, 联合层析成像, 有限差分, 精细结构

Abstract:

This study utilizes deep seismic reflection data from the Hohhot-Baotou Basin and applies a joint tomographic imaging approach that integrates first-arrival and reflection waves to derive a high-resolution P-wave velocity structure of the shallow subsurface. The primary objective is to elucidate the geological structural characteristics of the basin, with particular emphasis on fault activity, deep fault relationships, and fine-scale lithospheric architecture. The results are of substantial scientific and practical significance for mineral resource assessment, geological exploration planning, and earthquake hazard mitigation.
The joint tomographic imaging framework employs the Fast Sweeping Method(FSM) for forward modeling, which is based on a grid equation solver and effectively circumvents the nonlinear issues of conventional rays in complex media. In the inversion stage, a memory-efficient quasi-Newton method(L-BFGS)combined with adjoint techniques is adopted. By approximating the Hessian matrix, this approach substantially reduces computational cost and is well suited for large-scale tomographic problems. The resulting velocity models reveal pronounced lateral velocity contrasts at distances of approximately 45km and 53km, extending downward to the base of the basin.
The imaging results provide a detailed depiction of the shallow P-wave velocity structure of the Hohhot-Baotou Basin. The identified velocity contrasts at 45km and 53km correlate closely with the Ordos North Marginal fault zone, thereby supporting the reliability of the tomographic results. Further validation is achieved through integration with regional geological information, previous studies, and Bouguer gravity anomaly data. The joint inversion of first-arrival and reflection waves preserves stratigraphic continuity while restoring intralayer velocity variations, resulting in improved imaging resolution and reduced noise and inversion artifacts.
Laterally, clear stratigraphic layering and refined velocity variations are observed between 28.92km and 45km. Vertically, zones of abrupt velocity change extend to the base of the Paleogene strata(T_E, approximately 2km depth), which is disrupted by fault F₆ and approaches the T_g interface. This configuration corresponds well with the fault-controlled stratigraphic block defined by Faults F6 and the Ordos North Marginal Fault(F₇). These findings enhance the understanding of the basin’s subsurface architecture and provide new constraints on regional crustal evolution.
In conclusion, the joint tomographic imaging of first-arrival and reflection waves successfully resolves the shallow P-wave velocity structure of the Hohhot-Baotou Basin, offering a robust basis for interpreting its tectonic framework and fault systems. By combining the high-frequency sensitivity of first-arrival waves with the high-energy characteristics of reflection waves, the method achieves higher-resolution subsurface imaging, facilitates the identification of small-scale geological bodies, suppresses seismic artifacts, and improves overall imaging quality. The velocity anomalies associated with the Ordos North Marginal fault zone further highlight the close relationship between subsurface velocity structure, regional tectonics, and seismic activity. Overall, this study advances the understanding of the basin’s subsurface structure and provides valuable data support for future crustal evolution research, geological exploration, and seismic hazard assessment.

Key words: Hohhot-Baotou Basin, first arrival wave, reflection wave, joint tomography imaging, finite differences, high-resolution P-wave velocity structure