Converted modes in subsalt seismic exploration

Abstract

Salt has unusual properties which complicate seismic exploration. A Model study examines some problems encountered in subsalt amplitude analysis and demonstrates that traditional techniques for detecting hydrocarbons and overpressured sediments using amplitude and velocity analysis are severely limited subsalt. Strong mode conversions generated at salt-sediment interfaces intrinsically limit amplitude versus offset techniques by diverting a significant amount of energy away from P-wave reflections. Some of the information lost from P-wave reflections is recouped by examining mode converted reflection amplitudes. Mode conversions are sensitive to different rock properties than P-waves, and those generated at the base of salt bodies are easily identified using acoustic and elastic modeling comparisons. Models demonstrate the use of base of salt converted mode amplitudes to gauge the strength of P-wave reflections for hydrocarbon discrimination. Further, mode converted amplitudes are used in conjunction with P-wave amplitudes from base of salt to detect overpressuring directly beneath salt.