An Amplitude Scaling Methodology for 2D/3D Migrated VSP Data

VSP data can provide a significantly high resolution image. However, VSP data is characterized by a highly uneven distribution of both CDP fold and reflection incident angles laterally away from the borehole. One end effect of this uneven distribution is a migration image characterized by amplitude decay as a function of offset from borehole in migrated VSP data. This paper presents an amplitude scaling methodology to compensate the offset-dependent amplitude decay. We first locate a geologically stable and flat event above a target zone, and calculate its RMS amplitude distribution as a function of offset. From calculated RMS amplitudes we then characterize the amplitude-decay pattern by least-squares fitting. While each dataset may be analysed by various polynomial or Gaussian functions for best fit, this paper we suggest the amplitude-decay pattern can be characterized by a four-term Gaussian function. Finally, by reversing the best fit function, an amplitude scaling factor is applied to each trace of the migrated VSP data. We demonstrate the methodology greatly enhances the dimming amplitudes at long offsets in migrated VSP data. The amplitude distribution of migrated events after scaling is laterally more homogenous, potentially allowing the data to be used for lithological inversion and interpretation.