An Integrated Method of Subsurface Illumination Analysis for Shallow Gas Anomaly Data
Authors: Abdul Latiff A. Abdul , Ghosh D. P. , Tuan Harith Z. 
Published: May 2016

Abstract
In near surface anomaly region such as shallow gas accumulation and salt diapir, the acquired seismic data often suffer from poor quality image (shadow zones) due to low illumination coverage at subsurface reflector. Degraded signal information within these zones often been associated with multiple scattering of ray propagation or absorption of wave energy during seismic data acquisition stage. The distorted image underneath gas zones was caused by three factors:properties of near surface anomaly;location of target reflector (both lateral and vertical position); andsource-receiver configuration at surface level.
Previously, the quality of subsurface data was determined by extrapolating source and receiver wave field, thus creating a pair of focal beams which can be used for evaluation of acquisition design. The highly successful focal beam method demonstrate the relationship between subsurface illumination and acquisition configuration by analyzing resolution and amplitude versus ray parameter (AVP) functions of target locations for a given geometry set-up. Another way to analyze subsurface illumination is through ray tracing method, where rays were propagated from source to target depth and reflected back towards the surface level. During reflection process, illumination will be measured by counting the number of rays hit the target reflector while taking into account incidence ray angle and travel-time measurement. Although both analyses will estimate the amplitude coverage of target reflector, confidence level of illumination quality is still lacking. With this view in mind, we are proposing an integrated method of illumination analysis known as Illummination Factor which based on focal beam and ray tracing methods for better subsurface illumination guidance. The Illumination Factor is formulated based on seismic attributes information from two forward modelling methods; amplitude distribution in spatial domain (focal beam analysis) and number of rays hit at target subsurface (ray interpolation). This new illumination indicator will be used for evaluating a near surface anomaly velocity model, in terms of its confidence level. Evaluation of Illumination Factor in this seismically complex region will set a basis for improving seismic acquisition design, enhance seismic data through better illumination while provide an insight for reservoir characterization.

Introduction
In recent years, the task of finding new hydrocarbon reserve in shallow anomaly region such as shallow gas accumulation has becoming increasingly difficult due to poor seismic data quality. A couple of seismic section examples taken from a gas cloud affected field in Malay Basin (Figure 1) clearly indicate that our ability to image the true subsurface data remain limited even though with leading seismic data processing sequences and advance imaging algorithms. The weak seismic reflection signal underneath gas anomaly was caused by irregular ray path travel from surface to target reflector and vice versa. Another factor that contributes to poor illumination data comes from absorption effect as the wave propagating through shallow anomaly will experience internal losses as the part of wave energy was transformed into heat.