Titli was the cataclysmic storm to afflict Indian coast during 2018. Tropical cyclones may inflate nutrients on the surface layer of the ocean by entrainment and mixing, as a result, the upper oligotrophic and euphotic layers may be recharged with rich nutrients resulting in triggering phytoplankton blooms and significant increase in primary production. The biological response using the chlorophyll concentration from Oceansat-2 OCM and the Net Primary Productivity from MODIS-A to this cyclone and its relation with the cyclone intensity has been studied. Role of physical oceanography parameters has been studied using multiple satellite-derived parameters like SST, TCHP, SLA, Ekman pumping velocity, wind stress and wind vectors and eddies during the passage of cyclone Titli. The results showed an increase in the surface chlorophyll concentration from 0.08 (before) to 0.32 mg m$^{-3}$ (after), whereas NPP enhanced from 457 to 1313 mgC m$^{-2}$ day$^{-1}$ with an SST drop of ${\sim}$2.58$^{\circ}$C (30.25$^{\circ}$–27.67$^{\circ}$C). The study confirms that the physical parameters are the key components that influence the biological response in addition to the cyclone intensity.

Analyses of the marine geophysical data collected in four oceanographic cruises during 1974–2008 inside the Gulf of Kachchh, northwest India was done, and geology and crustal structure were interpreted for the first time. Seabed topography deduced from satellite-derived bathymetry and bathymetric investigations has revealed the tide-dominated Gulf (> 6 m high) has ${\sim}$ 50 m maximum depth along the axial part. The satellite-derived free-air gravity anomalies and their horizontal gradients together with earlier published results helped in noting major crust features like faults. Analyses of single and multi-channel seismic reflection data had enabled identifying reflection strata pattern and their continuity, wave-field and acoustic characters. 2-D model studies of ocean-bottom seismic refraction data (following ray-tracing technique), satellite-derived free-air gravity anomalies, magneto-telluric data along a profile in middle of the Gulf and magnetic anomalies near Gulf mouth have led to determine various physical properties: velocity, density, resistivity and the magnetic of the subsurface layers. They have been utilized in interpreting three-layered crust structure of the Gulf. Further, presence of ${\sim}$ 3.0 km thick low,  ${\sim}$ 3.7 km/s velocity and 2.2 gm/cc density layer overlain by about 2.1 km thick high, 4.6 km/s velocity, and 2.55 gm/cc density layer noted. Considering the physical properties in correlation with mainland geology and crustal structure from onshore Suthri-1 and inner shelf GK-01-A1 well logs and coast parallel Mandvi–Mundra deep seismic study results, low and high-velocity layers have been noted as low velocity Mesozoic sedimentary rock formations overlain by high velocity Deccan volcanic rock formations. The volcanic rock thickness increases to the east are noted. The Tertiary sediments above them are ${\sim}$ 1.2 km thick. The major crustal structures and geology identified contribute to understand regional geology, structure and tectonic framework of the Kachchh region and in particular, of the study area. The fractured Deccan volcanic rocks and their intervening intra-trappean sediments are possibly considered as potential hydrocarbon-bearing formations (?) in view of the underlying thick Mesozoic sediments.

$\bf{Highlights}$

$\bullet$ Seismic reflection, OBS, and magneto-telluric data revealed three layered crustal structure of the Gulf

$\bullet$ We have identified fractured Deccan volcanic rocks and their intervening intra-trappeans sediments inside the Gulf

$\bullet$ Presence of low velocity Mesozoic sedimentary rock formations overlain by high velocity Deccan volcanic rock formations