Articles written in Journal of Earth System Science

    • Enhancement of a thumper source far oAset refracted phases using super virtual interferometry (SVI)


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      Super-virtual interferometry (SVI) is a technique in which cross-correlation between consecutive receiver responses is carried out to obtain the virtual head-wave arrivals, which are then convolved with the initially recorded traces to get the super-virtual trace. SVI can be used to enhance the refracted phases by stacking all the arrivals acquired using multiple shots at one position, leading to an improved SNR by a factor of $\surd n$, where $‘n’$ is the number of sources and receivers to generate the head-waves. In this study, we have generated few synthetic common shot gathers (CSGs) using forward modelling over a three-layer velocity–depth model with an embedded spherical anomaly, a complex five-layer velocity–depth model and the Marmousi model. Certain amount of noise is added on these gathers and then SVI technique is applied on the gathers which has resulted in an improved SNR of refracted phases at the far offset. We have further tested this technique on a field dataset acquired from the Kumaon Himalayan region using a 450 kg thumper as an energy source and 111 active channel remote acquisition unites (RAUs) with 5 Hz geophones as sensors. The resulting SVI gathers show the refracted arrivals more clearly. Continuity in the phases is increased after stacking and iterative SVI.

    • A reliable velocity estimation in a complex deep-water environment using downward continued long offset multi-channel seismic (MCS) data


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      The estimation of a reliable velocity–depth model from towed streamer marine seismic data recorded in deep water, especially with a complex seafloor environment, is challenging. The determination of interval velocities from the normal move-out (NMO) of the reflected seismic signals for shallow reflectors (<1 km below the seafloor) is compromised by the combination of a long wave path in the water column and the complex ray paths due to topography, leading to small move-out differences between reflectors. Furthermore, low sediment velocities and deep water produce refraction arrivals only at limited far offsets that contain information about deeper structures. Here, we present an innovative method where towed streamer seismic data are downward continued to the seafloor leading to the collapse of the seafloor reflection and the emergence of refraction events as first arrivals close to zero offset, which are used to determine a high-resolution near surface velocity–depth model using an efficient tomographic method. These velocities are then used to perform pre-stack depth migration. We found that the velocity–depth model derived from tomography of downward continued towed streamer data provides a far superior pre-stack depth migrated image than those produced from velocity–depth models derived from conventional velocity estimation techniques.


      $\bullet$ A comparison in velocity from conventional NMO, sparsely spaced OBS tomography, and tomography of downward continue streamer data is carried out.

      $\bullet$ Accuracy in velocities are proved from the prestack depth migration results.

    • Mapping of hydrocarbon-bearing reservoirs using frequency-dependent amplitude vs. offset (FAVO)


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      The uncertainty associated with seismic interpretation poses a significant challenge to the hydrocarbon industry in accurately identifying the potential hydrocarbon zones. Over the years several new techniques have emerged to address this problem. Amplitude variation with offset analysis is one of the prominent methods to estimate various subsurface properties from pre-stack gathers. Due to the presence of fluid in reservoir pores, seismic velocity dispersion occurs within the seismic bandwidth, resulting in reflection coefficient as a function of frequency. In this study, a prototype inversion scheme is developed for frequency-dependent amplitude vs. offset analysis, which is tested on both the synthetic and field data. The synthetic data are generated using a three-layered model with a spherical anomaly. The application of this method to synthetic data indicates anomalous high amplitude in P-wave dispersion gradient, and helps in identifying effects of spherical inclusion. Further application of this method to field data from North Viking Graben, North Sea clearly delineates the hydrocarbon-bearing reservoir identified on the well log data between ${\sim}$1.8 and 1.9 s two-way travel time.

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