In a nutshell, AVO is a way of high-grading brightspots on your seismic sections and of finding subtle anomalies that can be lost in run-of-the-mill CDP stacking and migration. It stands for Amplitude Versus Offset and is something of a misnomer since we typically look at seismic reflection amplitude as a function of incidence angle rather than source-receiver offset. However, the acronym AVO has been in use for a long time and it’s probably futile for anyone to try to change it now.
AVO can help to answer the question of whether that amplitude anomaly you see on the workstation screen is due to 50 feet of gas or 50 feet of tight carbonate cement, or coal, or salt, or the top of overpressure, or ….
It may be riskier out there than you think.
Whether you use bright spots, flat spots, dim spots, frequency anomalies, attenuation anomalies or AVO you can’t escape risk. Risk is usually seen as the probability of drilling a dry hole but there is also the risk of passing over a profitable discovery. It can be risky to limit your use of technology. You don’t escape it by simply making “conservative” drilling decisions. Over the long run, prudent use of technology can reduce the risk inherent in estimating reserves.
Dealing with risk – How AVO can help.
- Include AVO attribute analysis in geographic areas that have a good track record like much of the Gulf of Mexico and the North Sea.
- Model the expected AVO signature from several wells in the area
- Use those model results to guide your efforts in processing or re-processing
- Prepare special purpose AVO displays from your seismic data to screen for hydrocarbon anomalies based on your model.
- Interpret your data with an understanding of what to look for and what to avoid.
An AVO Checklist
Consider the following points with respect to your project area:
- Do the data exhibit bright spots or other attribute anomalies? If so, then take a look at the data in the prestack domain to help validate the interpretation.
- Have others used AVO in this area in the past? Most of the Gulf Coast, onshore and offshore, has a published track record of AVO examples.
- Have the data been migrated with an amplitude preserving pre-stack algorithm? If not consider having it done before commencing work beyond the synthetic modeling stage.
- Am I drilling an anomaly near the top of overpressure? Recent experience suggests that top of overpressure is the site of complex and problematic AVO signatures.
- Is the target in an interval where the sands are faster than shales? slower than shales? The relative interval velocity of sands in nearby wells can provide a quick estimate of the type of anomaly to look for.
- Can I reduce my risk by employing an AVO specialist?