One of false positives in seismic amplitude associates with the presence of low saturation gas. The amplitude response is ambiguous prominently in shallow subsurface due to similar amplitude of fizz gas to highly saturated gas reservoir. This pitfall is demonstrated in Gassmann’s fluid substitution modelling which drastic velocity reduction is modeled by 5% of gas. The strong reduction induces anomalous bright spot in seismic similarly manifested by a highly gas reservoir. In pore fluid characterization, seismic amplitude of a brine reservoir is well distinguished from gas by change of response polarity from negative to positive amplitude due to higher brine modulus than gas. To a lesser extent, the negative amplitude of gas sand almost identical between commercial and non-commercial gas trap and difficult to distinguish in seismic. The similar negative amplitude of the gas cases caused by a very low gas modulus resulting to sudden drop of rock bulk modulus despite with the presence of low gas percentage. Thus, seismic velocity slows down when travels through a gas sand medium. Integration of controlled-source electromagnetic (CSEM) method in hydrocarbon prediction reduces exploration risk by only detecting commercial hydrocarbon. It exploits resistivity contrast between overburden and hydrocarbon reservoir which highly controlled by hydrocarbon saturation. CSEM normalized response exceeds the cut off response by a minimum saturation of 70% of gas. Less than 70% of gas, CSEM response is below the cut off response which considered as insignificant anomaly in CSEM measurement. It becomes a key strength as this method reduces amplitude uncertainty in seismic due to low saturation gas and potentially improves the chances of economic discovery in hydrocarbon exploration.
In hydrocarbon exploration, information carried by diving waves and post-critical reflections that are used to reconstruct the long-to-intermediate wavelength of the subsurface is an integral part of successful velocity model building. Diving wave tomography (DWT) is one of the tools for shallow velocity assessment particularly when seismic data has poor signal-to-noise ratio (SNR) with complex geologic settings where no clear reflector is present. Considering the relationship between velocity with time and space, the output from tomography plays a crucial role to align data between time and depth domain and produce a reliable image of the deeper structure where hydrocarbon reservoir is typically located. In geophysics, tomography is primarily used to correct seismic trace alignment to produce a reliable stack section. In advanced imaging it is used as an initial model for waveform inversion in an integrated workflow. In the post-processing stage, it is used to correct the misfit between well logs and seismic data and is crucial for the quantitative analysis of rock physics. In this paper, we focus on tomography and its working principle on near-surface velocity modelling. We restricted our workflow to 2D synthetic data simulating the shallow gas occurrence that is prominent in the offshore Malay Basin to demonstrate how tomography works in velocity reconstruction. Results from synthetic and real data example shows that DWT can recover local large-scale structure and improved stacked data, considering no other seismic data and constraint from well data is included in the iterative process.
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