Mapping basalt lava flows at Dive Ghat (Pune, India) using multichannel analysis of surface wave (MASW) technique

Lava-flow structure and morphology provide insights into eruption styles, emplacement mechanisms, and post-emplacement alterations like weathering. However, the challenge lies in quantifying subsurface structures of buried lava flows beneath soil and vegetation cover. We present a case study of lave flow mapping employing seismic methods from the Dive Ghat region of Pune, situated in the Deccan Volcanic Province. Seismic data was collected using a 48-channel engineering seismograph with 1m receiver spacing and 2m shot spacing via a sledgehammer source. The seismic profile is taken parallel (roughly 4 m away) to the approximately 10 m vertical exposed section, which is used for cross-validation of the flow geometry, and it shows the presence of roughly six inches of red bole layer overlaying on a highly weathered basalt. The observed data is analyzed through refraction and multichannel analysis of surface wave (MASW) techniques. The refraction method yielded a high-resolution P-wave velocity model in the near subsurface with P-wave velocity (Vp) ranging from 0.3–4.5 km/s. However, the refraction method fails to image highly weathered basalt, which is expected as no critically refracted wave will be generated from the top of a low-velocity layer. The MASW method delivered a horizontally smeared S-wave velocity model where S-wave velocity (Vs) varies from 0.15–2.2 km/s. The subsurface S-wave model has a low-velocity layer of around 0.7 km/s Vs velocity embedded between 8.5–14m, which correlates to a red bole layer and underlying fractured basalt formed through weathering. Furthermore, a Vp-Vs cross plot is calculated to characterize different seismic units in velocity sections. The results obtained in this study may serve as an analogue to map near-surface flow geometry in unexposed sections in the Deccan Volcanic Province region. The MASW method successfully imaged the buried red bole layer and underlying weathered zone, which is crucial for agriculture due to its organic content. The imaging also holds implications for slope stability studies, emphasizing the importance of seismic characterization for hazard zonation, especially concerning the red bole and fractured basalt.