Three-dimensional Least-squares Migration of Teleseismic Receiver Function and Its Application to the Qaidam Basin

The development of dense seismic arrays offers new opportunities to develop advanced array imaging techniques to improve subsurface characterization. We present a high-resolution receiver function imaging method implemented through a three-dimensional wave-equation-based least-squares migration (LSM) framework, specifically optimized for the teleseismic wavefields recorded by dense nodal arrays. The proposed 3D algorithm is adaptive to arbitrary array geometry and enables incorporating earthquakes from any propagation direction. We employ the split‑step Fourier algorithm to efficiently solve the acoustic wave equation, thereby constructing the forward (de-migration) and adjoint (migration) operators for wavefield extrapolation. The key concept of the proposed migration technique is to formulate the migration process as a regularized least‑squares optimization problem, which allows us to readily incorporate a priori constraints of the model (e.g., lateral smoothness) to suppress strong acquisition footprints and compensate for inadequate illumination. We conduct synthetic experiments by constructing a two-layer model with a 3D undulating Moho interface and numerically simulating teleseismic wavefields using the SPECFEM3D solver. The LSM method characterizes the 3D geometry of the Moho interface with improved vertical resolution compared to the conventional migration approach. For field data application, we apply our method to recordings acquired by a dense nodal array across the Qaidam Basin, located on the northeastern margin of the Tibetan Plateau. The resulting image reveals distinctive structural layering in the crust, including 1) a 12 km thick sedimentary sequence, in excellent agreement with the basement depth constrained by the high‑resolution seismic reflection data acquired for hydrocarbon exploration; 2) a long-wavelength Moho variation from ∼50 km to ∼60 km beneath the basin, as well as a small-scale ∼6 km Moho offset beneath the Yingxiong Range. Overall, this study highlights the great potential of the migration imaging in resolving fine-scale crustal structures using dense array data.