The main gas condensate and oil bearing fields are concentrated within the polar area of Western Siberia. The target depth varies from 900 m for Cenomanian gas deposits up to 4000 m for Upper-Middle Jurassic oil, gas and gas condensate sediments. The target structure is very complex that is why 2 and 3D seismic survey is required to explore the fields. However, seismic survey is complicated by environment.
In respect to seismology the main factor that impedes seismic study of sediments occurred in the north of Western Siberia is presence of permafrost of complicated structure. Taliks in permafrost developed in the area under survey depending on their size and properties may both introduce long-period distorting constituents into seismic wavefield and make CMP coherent summation impossible. Besides, dynamic characteristics of reflected waves are also distorted, because the existent true-amplitude processing techniques «succeed» only in a conventional model obtained with reflection CDP method.
As an example a series of seismic sections and maps from the north of Western Siberian oil fields is demonstrated.
Presence of thawed ground results in abrupt change in both wave pattern and frequency content of waves recorded. It causes wide expansion of diffraction events, reflection and conversion of waves at the heterogeneities' boundary and complexity and instability of noise field associated with it. Another cause of presence of permafrost heterogeneities is strong distortion of reflected waves: drastic distortion of seismic events are observed at the heterogeneities' boundary, the frequency content of images changes, and this change causes problems for statics autocorrection.
Analysis of near-surface impact correction approaches
Known methods of anomalies impact correction associated with permafrost are applied either at interpretation stage or at the stage of processing and interpretation. Application of anomalies impact correction methods at interpretation stage is considered to be inefficient due to signal shape distortion when summing along a travel time curve that differs from a hyperbola. The approaches more frequently used at processing stage are based on the following:
Visual analysis of coherence in CDP, CSP and CRP sections and their iterative manual correction. Application of source and receiver statics correction;
Reflection tracking from relic permafrost top, establishment of correlation ratio between reflection geometry from permafrost top and below regulated reflectors and calculation based on the data derived from compensating statics correction;
Modeling of heterogeneous velocity thickness including permafrost by replacing heterogeneous velocity layer by homogeneous one with the help of statics calculation and introduction;
Building of velocity-depth model of near-surface with further migration of seismic gathers to the level lower than near-surface;
Application of reflection first breaks data for subsurface model calculation and corresponding corrections.
Each of abovementioned methods has its advantages and disadvantages. That is why their application results in ambiguous structural imaging and various dynamic «patterns». It happens due to lack of information on near-surface structure for correct solution of heterogeneities impact on seismic survey results.