Pre-stack Time Migration (PreSTM)

For areas of moderately complex geological conditions or velocity fields, Pre-stack migration can lead to superior amplitude and bandwidth preservation, resulting into better event coherency and improved lateral resolution.
At Samit, we use an approach which is not constrained by straight ray approximation and properly takes care of ray bending at dipping surfaces. An algorithm which uses RMS velocities analytically ensures there is no bias in the velocity estimates or in the positioning of steep dips. The main steps include-

Pre-stack Depth Migration (PreSDM)

Strong lateral velocity variations associated with complex overburden structures require earth imaging in depth. Example of complex overburden includes diapiric structures formed by salt tectonics, imbricate structures formed by overthrust tectonic and irregular water-bottom topography. All three are characterized as structure-dependent lateral velocity variations. Their also exist structure independent lateral velocity variations, often associated with facies changes; for instance changes in lithology from shale to sandstone to carbonate include lateral changes in acoustic impedance.

An algorithm which uses interval velocities and accounts for ray bending at interfaces is utilized at Samit. Depending upon the requirement, our approach include-

• Kirchhoff depth migration
• Beam migration
• Wave equation migration (WEM)

Because our PreSDM algorithm preserves amplitude, its gathers are ideal for AVO studies.

Inversion

Simply stated, seismic inversion is the process of determining what physical characteristics of rocks and fluids could have produced the seismic record you are viewing. Transforming a processed seismic trace (with remnants of noise in it) into Acoustic Impedance log is the inverse of transforming sonic and density logs into a synthetic seismogram, hence the name inversion. Seismic Inversion is very important tool for reservoir characterization.

Seismic inversion is done both on Post Stack data as well as on Pre stack data. Depending upon the requirement, we utilize modules of Promax, Strata and Rock Solid Attributes (RSA) for this purpose.

Amplitude Versus Offset (AVO)

In Geophysics, amplitude versus offset (AVO) or amplitude variation with offset is variation in seismic reflection amplitude with change in distance between shot point and receiver. It is also referred as AVA (amplitude variation with angle).

The most important application of AVO is the detection of hydrocarbon reservoirs. Rising AVO is typically pronounced in oil-bearing sediments, and even more so in gas-bearing sediments. Most companies use AVO routinely as a tool to reduce risk of exploration targets and to better define the extent and composition of existing hydrocarbon reservoirs.

At Samit, we generate Angle gathers, Angle Stacks, Offset Stacks, different Attribute Stacks (Intercept, Gradient, Fluid Factor, P-wave Stack, S-wave Stack etc) for AVO Analysis. The Cross plots can also be generated and analyzed to find out the class of reservoir. The different offset Stacks are also analyzed for observing whether DHI character is preserved or not in the Seismic volume.

AVO Anomaly observed in a gas field in Cambay Basin, India