Safe Operating Procedures for Land Seismic Survey

Throughout the world there are vast areas covered by basalt flow that are under-­‐explored for hydrocarbons due to poor sub-­‐basalt imaging. High reflectivity at the top of the basalt and high energy attenuation within the basalt cause the sub-­‐basalt reflections to be very weak and masked by different orders of multiples generated from above the basalt. In order to attempt to image below the basalt in onland Cambay basin, seismic data with large maximum offset were acquired by ONGC. Sub-­‐basalt reflections do become visible on long-­‐offset data in a cone formed between the refraction from the basalt top (Trap Top) and the refraction from shallower layers. Processing of long-­‐offset data does have challenges associated with it such as mode conversion and anisotropy (both intrinsic as well as long-offset effect). In the present study a model-­‐based noise attenuation technique has been applied to suppress different kinds of noise and VTI anisotropy has been modeled and accounted for in Kirchhoff Pre-­‐Stack Depth Migration imaging with encouraging results.

Isotropic pre-stack depth migration (PSDM) in the presence of anisotropy in the subsurface, suffers from positioning errors; vertical as well as lateral. Layering in shaly formations produces anisotropy which can be modeled as transverse Isotropy (TI). In the present paper we present a case study from shallow offshore India. The subsurface in the area is characterized by massive shale (800-1000 m) underlain by thick limestone – shale alternations. The isotropic PSDM showed a 50 m mismatch at this level with hockey-stick effect visible at far offsets on PSDM gathers. We have estimated the heterogeneity and anisotropy parameters i.e. velocity gradient (G), and Thomson parameters (e and d) from seismic and well data. These parameters, estimated in a number of wells spread over 1000 sq. km., were found to be practically constant. These parameters were incorporated in depth velocity modeling and PSDM along 2D profiles. The study of PSDM depth image gathers has shown that in this area non-hyperbolic effect due to heterogeneity dominates beyond offsets of 1140 m (offsetdepth ratio of 1.4). The similar effect due to anisotropy is visible only after offsets of 1440 m (offset depth ratio of 1.8). However after Anisotropic PSDM (APSDM) the gathers are flattened up-to 1940 m (offsets depth ratio 2.5). APSDM images match well with well-tops within seismic resolution with accompanying improvements in overall image quality at target horizons.

This paper provides a method for extracting Vp & Vs of sea bottom sediments from P-wave seismic data and, simultaneously, for doing a QC of amplitudes for pre-stack inversion of seismic data. The method is based on extracting Vp & Vs of sea bottom sediments, from seismic data. As a by-product, this information, in conjunction with 3-D seismic image can provide a pre-drill warning on shallow drilling hazards.

This I consider as my most useful work in Geophysics. I developed a theory to describe wave propagation in a two-phase medium and obtained an analytical expression for the velocity of P-waves in terms of elastic parameters of the two phases. The theory takes into account all multiple scatterings among the inclusions. For the case of spherical inclusions, solved exactly the integral equation for the total wave field up to order (ka) **3 – where “k” is the wave number and “a” is the radius of inclusions. - Purnell, another author, commented on this work: “I compared seven published mathematical models for wave propagation in two-phase media. One given by Mehta most closely agrees with the P-wave velocities I observed. The agreement is sufficiently close to merit the use of Mehta’s model in the design of physical model materials” –W G... Purnell, Geophysics, Vol.51, p.2193, 1986.

Report on technical lecture titled " Seismic processing to Image processing" organized by SPG, Chennai chapter on 09.03.2011 and delivered by Dr.G.V.R.Kumar, CG(S), ONGC, Chennai