Identification and application of PmP multiples from onshore-offshore seismic surveys in Eastern Guangdong

Onshore-offshore deep seismic exploration is a significant method for studying the crustal structure and geological tectonics in the land-sea transition zone. Special conditions, such as large offsets and differences in geologic structures, allow for the observation of seismic multiples. Multiples are reflections from subsurface interfaces that include additional information about subsurface structures, which can improve imaging resolution. In particular, it is able to make up for the limitation of ray coverage induced by unilateral source excitation in onshore-offshore deep seismic exploration. In this work, a systematic analysis of seismic multiples associated with the reflection phase of the Moho interface (PmP) is carried out using the seismic data from the L1-NW03 survey line of an onshore-offshore seismic experiment conducted in eastern Guangdong in 2021. First, based on the features that multiples have comparable frequency characteristics to PmP (4–6 Hz) and that their travel times grow as offset increases, it is suggested that the multiples are secondary PmP phases and that the reflection occurs on the sedimentary basement interface. Then, using theoretical raypath simulation, it is verified that the multiples' reflection interfaces are the seafloor interface and the sedimentary basement interface (multiples is named PmP2PsP). Finally, a RAYINVR forward P-wave velocity model is derived from all reflection and refraction seismic phases. The VMONTECARLO method is used to assess the constraint accuracy of the sedimentary basement interface based on the forward model. According to the results, using the PmP2PsP seismic phase reduced the interface depth errors from ±0.21–1.16 km to ±0.11–0.58 km. The imaging resolution of the shallow crust and sedimentary layer was also considerably improved. In addition, a time-depth conversion formula for the sedimentary basement interface was obtained by fitting multi-channel reflection seismic data and the data of time difference between PmP2PsP and PmP.