Usage of normalized total gradient (NTG) method for the analysis of seismic data and comparison with complex envelope curves

SUMMARY Usage of Normalized Total Gradient (NTG) Method for the Analysis of Seismic Data and Comparison with Complex Envelope Curves The time depending variations of amplitudes of seismic reflection recorded from thin-layered media are investigated with analytic approaches. The seismic reflections recorded from thin-layered media have an interferenced character. The analysis of these reflections has been done by calculating their envelopes (instantaneous amplitude). The envelope display is important in the determination of the place of the reflector on a seismic trace. Also, envelopes point out the variation of amplitude and the shape of the reflection wavelet according to physical features of media in the course of time. Especially, resolution of envelopes is more important for media including oil and gas which cause strong reflections. Conventionally, calculated seismic envelope traces are effected by interference, coherent and incoherent noises and it doesn't indicate qualified resolution. In this study, Seismic reflections data were examined with Normalized Total Gradient (NTG) method to increase resolution of seismic envelopes and a find solution to interference problems. In this method seismic wave field was defined with the Fourier sine series and the derivations in respect to dependent variables of the obtained analytical functions were obtained analytically and Gn normalized total gradient values, which belongs to initial seismic trace, were calculated numerically. The drawing,according to time, of these Gn values has denoted more resolution envelopes curves of seismic trace and also energy distribution. The obtained results from applications on the synthetic seismic data calculated for thin-layered media models (sandwich, wedge, pinch-out and fault type structures) frequently containing interference problems as well as on the field data have indicated that calculated Gn curves are similar to complex trace envelope as image but having better resolution and smoothed surfaces. Thus, in addition to conventional seismic attributes derived from an analytic approach, Gn curves are advised to be used in the evaluation of stacked seismic reflection data as gradient attribute which indicate interfaces and lithological variations. Key Words: Analytical Seismic Trace, Complex Envelope, Thin-Layered, Reflection Interference, Normalized Total Gradient, Harmonically Analysis VII