Pitfalls In Seismic Amplitude Vs. Offset Analysis: Case Histories

ABSTRACT During the past ten years a new seismic technology has come of age. This new technology has been called by many names (seismic lithology, amplitude vs. offset, Poisson's ratio analysis, etc.) and has been very effective in reducing exploration risk for gaseous hydrocarbons. However, as with all technologies there are pitfalls which can lead to incorrect interpretation of the amplitude anomalies seen on common depth point gathers. The Terra Linda Group, Inc., has seen many of these pitfalls in the 500 amplitude versus offset projects we have performed during the last five years using the SAMPLE method throughout the world. These pitfalls can be categorized in the following ways:Data CollectionData ProcessingGeological Conditions This paper will address the pitfalls in each of the above categories. INTRODUCTION Early in this decade the new technology of amplitude vs. offset analysis proved to be an effective follow-up to the bright spot technology developed in the previous decade. W.J. Ostrander (1982) pointed out that under proper geologic conditions, gas saturated sandstones showed a marked reflection amplitude increase with increasing offset. G.S. Gassaway and H.J. Richgels (1983, 1984) showed that changing lithologies can also affect the reflection amplitude vs. offset signature. As with all new technologies there is an experience learning curve which is necessary to properly apply this new technology. Some things which at first inspection might appear to affect the amplitude vs. offset signature do not have significant effects; thin bed tuning is an example of this. Other things which are often overlooked, such as geophone and shot arrays, have a large effect on the amplitude vs. offset signature. In what follows, we will try to quantify the pitfalls in amplitude vs. offset analysis based upon our experience in applying the SAMPLE method (G.S. Gassaway and H.J. Richgels, 1983, 1984) to exploration problems throughout the world. I. DATA COLLECTION There are two major problems encountered at the data collection phase which are sometimes fatal to amplitude vs. offset analysis. The first is the automatic gain control (AGC) applied to older (pre 1975) data. The objective of such gain control was to remove the trace-to-trace differences so that the seismic data could be recorded at the same level on the limited dynamic range of the older recording instruments. The AGC attempts to remove the effects to be analysed. It has been our experience that this is always fatal to amplitude vs. offset analysis. It should be noted here (to the credit of field personnel) that the AGCs applied at this stage are almost always noted and recognized; this is not always true in the data processing (see below). Geophone and shot arrays are a different matter. The details of array length and weighting are rarely recorded on observer's logs. These arrays can have a major effect on the observed amplitude vs. offset anomaly observed. See Figure 1.