Controlled Amplitude Processing and Offset Amplitude Variation
Gary Yu · Offshore Technology Conference · 1985
ABSTRACT The partition of plane seismic waves at plane interfaces introduces changes in seismic amplitude which vary with angle of incidence. These amplitude variations are a function of the elastic parameters of rocks on either side of the interface. Controlled amplitude processing is designed to obtain the true amplitude information which is geologic in origin. The offset amplitude information may be used successfully to predict the fluid type in reservoir sands. Various tests were carried out on one seismic profile from the Gulf Coast, The processing comparison emphasized the effects and pitfalls caused by trace equalization, coherent noise, offset, and surface related problems~ Two wells drilled at amplitude' anomaly locations confirmed the predictor of hydrocarbons from offset amplitude analysis. Furthermore, controlled amplitude processing provided clues in evaluating reservoir quality, which were not observed on the conventional relative amplitude data. INTRODUCTION Churline and Sergeyev (1963} and Ballakh et al (1970) used direct hydrocarbon indicators long before they became widely recognized in the Western world. But it was a great idea whose time had arrived. Since the early 1970's the introduction of bright spot technology~ to the Western world brought a profound revolution in seismic acquisition, processing, and interpretation throughout the petroleum exploration industry. The search for high amplitude anomalies in stocked seismic sections has successfully led to many discoveries of hydrocarbon accumulations, particularly gas fields in the Gulf of Mexico. It was not long before bright spots had been categorized as a series of visual signatures such as high amplitude anomaly, phase change, flat spot, time sag, shadow zone, chimney effect, etc., that geophysicists employed them as direct indicators, the high amplitude anomaly is the most obvious, powerful, and often used. Unfortunately, it is also difficult to differentiate a high amplitude reflection caused by a gas layer from that caused by a non-gas layer like a high velocity hard streak. In recent years, considerable research has been dedicated to determine lithology and pore fluids from the analysis of offset dependence of reflection amplitude data. Ostrander (1982) and Gassaway, et al (1983, 1984) effectively used the relationship of reflection amplitude versus offset in a common mid-point (CMP) gather to distinguish between gas generated and non-gas generated amplitude anomalies. A key factor for success in evaluating either direct hydrocarbon indicators or offset. amplitude variations is amplitude balancing. Conventional seismic data processed for structural interpretation are automatic-gain-control (AGC) balanced, whereas for direct hydrocarbon interpretation, seismic. data are relative amplitude balanced to preserve amplitude information. Sometimes, relative amplitude processing (RAP) is referred to as true amplitude processing (TAP) or reserved amplitude processing (PAP). The main purpose of these schemes is to restore amplitude losses due to spherical divergence, transmission, absorption, etc., but not to apply any AGC scaling in the processing sequence.