Estimating sedimentary thickness of Lanka Basin using digitised scanned seismic sections

In exploration geophysics, seismic surveys and their interpretations provide the most reliable information of the subsurface structures. Extensive work on the determination of the subsurface structure has been carried out in both available satellitegravity and magnetic data suggest that the Lanka Basin may have the necessary sediment volume for the occurrence of hydrocarbons. However, without seismic data, this suggestion cannot be tested further. As an alternative to acquiring additional seismic data in the region, this research focused on digitising 50,000 km of vintage seismic lines from the National Geophysical Data Centre (NGDC) data repository and converting them into Seg-Y format. Key seismic horizons on the digitised sections were interpreted to provide details of the seabed and acoustic basement in the time domain. Regional sedimentary thickness maps were compiled in the time domain interpolating the interpreted horizons using ‘ ’ method. The velocity data acquired in the region were used to convert the maps from time to depth domain. The results indicate that average thickness of sediments in the region varies around 5000 m to 6000 m, where the maximum thickness is around 8000 m in the North Eastern part of the Lanka basin. The thickness maps can be used as a reference dataset to plan commercial seismic surveys in the future for hydrocarbon exploration in the Lanka Basin.


INTRODUCTION
Geophysical techniques such as seismic, gravity, magnetic and electrical surveys are employed in the present scenario mainly in prospecting for natural resources buried underneath. They also provide information on the existence of structural and lithological variations in the subsurface. Exploring for these resources could countries.
all the data belonging to the upstream industry of the country (Tantrigoda, 2010). Extensive exploration work has been carried out in both Cauvery (Chandra et al., 1991;Baillie et al et al., 2013;Kularathna et al., 2015a;Ratnayake et al., 2014) basins. Although the interest in exploration for hydrocarbon in the ultra-deep Lanka Basin has been recently established, it has been hindered by the lack of data in the region (Rathnayaka et al., 2017;Gamage et al., 2018;. The Lanka Basin is thought to have opened up when Antarctica separated from the eastern India and Sri Lanka 142 Ma ago (Kularathna et al., 2015b). During the rifting phase, Cretaceous sediments may have been deposited in the basin. Thereafter, the sediment supply to the passive margin might have when the Indian and the Eurasian plates collided to form the Himalayan mountain range and started to form major rivers like the Ganges and Brahmaputra (Nairn & Stehli, 1982;Curray, 1994). Furthermore, the freely available satellite gravity and magnetic data suggest that the Lanka Basin could have possessed the necessary sedimentary thickness and structures required for hydrocarbon occurrences. However, without seismic data, the above claim and the existence of other major elements to have a working petroleum system cannot be proven. As an alternative to acquiring more seismic data, which involves a large cost and extensive national planning, a study using vintage seismic data acquired in the past may provide useful information about the region as well. The preliminary data for the interested region was obtained from the National Geophysical Data Centre (NGDC) of the USA. If a sedimentary thickness model can be created for the Lanka basin using existing seismic data available in the NGDC website, we can get some basic idea about sedimentary distribution in the Lanka basin. Thereafter, based on those details we can plan the buried in the Lanka Basin.

METHODOLOGY
The data for the interested region ( Figure 1) was obtained and the second section contained time-migrated seismic ( Figure 2) was developed to carry out the digitisation of 50,000 km of scanned seismic images (Silva et al., 2018).
After creating this comprehensive database of the seismic sections, the data was interpreted to generate a sedimentary thickness map in the passive margin area of to be the basement (acoustic basement) were interpreted using the IHS Kingdom suit software and a sedimentary thickness grid in Two Way Time (TWT) was generated. The velocity details to convert the TWT into depth domain were obtained from previous seismic studies conducted in the Eastern side of the Bay of Bengal (Hamilton et al., 1974;Rao et al., 1997). Therefore, the average velocity (V A ) of the sedimentary section was obtained by converting the interval velocities using the following equation. using the following equation. ... (2) For the purpose of enhancing the details of the digitised seismic database, topography and gravity data were accessed from and used to create time and depth domain grids. To create these grids, kriging algorithm was applied and the grid cell size of 1000 m by 1000 m was selected. Topography and gravity data were used to get a better understanding of the sea basement.

RESULTS AND DISCUSSION
surveys and about 400 scanned paper copies were digitised and converted to SEG-Y format. Two examples of seismic sections including the interpreted seabed and basement horizons are given in Figures 3(a) and 3(b). The sedimentary thickness maps generated in both time and depth domain for the Lanka Basin area are given in Figures 4(a) and 4(b). The acoustic velocity in the water column was taken as 1500 ms -1 and the calculated average velocity in the sedimentary section was around 3300 ms -1 . The average sediment thickness in the region varies from 5000 m to 6000 m whereas the maximum sediment thickness was around 8000 m in the North Eastern part of the Lanka basin.
that the seismic data have non-unique standards and the presence of considerable amount of noise, since Hewlett, 1993), which had to be solved using various to interpret the basement horizon because of the extensive noise that had occurred due to poor quality of the images and the fact that the older equipment failing to generate and record lower frequency data, which provide information on the deeper targets. The depth conversion would have been more accurate if a velocity model was built using more points and by considering the increment of velocity with depth.
Along the sedimentary thickness grid in the depth domain, crosslines were drawn to view the sediment thickness variation in the Lanka Basin. Using the Figure 7 shows the details of sedimentary thickness, values were direct observations and they have higher accuracy compared to the sedimentary thickness values, which were calculated using digitised seismic images. Gravity values have been scaled (*100) to correlate with thickness and bathymetry values. Here, the attempt was to increase the validity of sedimentary thickness values using gravity and bathymetry values in the region.  Figure 7, the sedimentary thickness value increased along the F-F1 line, from the coastal margin to the outer edge of the thickness grid. Bathymetry values started from 0 and decreased (depth increasing) up to 4000 m and remained constant along the line. Gravity values showed the negative gravity anomaly until that the higher dense land masses were replaced by the lesser dense water column causing the negative gravity anomaly shown. The focus here was that after increment of gravity values (decrease negatively) in the constant value, another parameter should have caused that sudden increment in gravity values. In Figure 7, there was an increment in the sedimentary thickness in also shows a gravity increment. After considering these facts, a conclusion can be made to depict the increment of Using these directly observed gravity and bathymetry values, the accuracy of the sedimentary thickness model created can be increased and could be used to create more accurate hypotheses.

CONCLUSION
to 6000 m is present in the Lanka Basin and the maximum sedimentary thickness is around 8000 m in the North Eastern part of the Lanka Basin. A sudden