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Levorson first used trend analysis in 1927 as a method to remove the regional structural component in the mid-continent. Trend Surface Analysis is a form of multiple regression, where a surface is mapped using polynomials to create the best fit with observed data. Trend analysis has proved to be an important technique for enhancing map data. Manual trend analysis in the past was time consuming and not always accurate. Advanced computer software packages today eliminate the limitations of manual methods and speed up the trend analysis process. The most used trend analysis techniques are polynomial curve fitting and Fourier analysis.
Geologists are very familiar with trend and residual mapping, which is a technique often used to enhance subtle anomalies in areas of strong regional dip. Now even novice users can apply this mapping technique simply with the use of current industry software packages.
With this process, structural closures can be more prominent once the regional surface is removed. In this technique, typically a 1st, 2nd, or 3rd order polynomial is used to "fit" the regional structure. When mapped, this surface is referred to as a trend map. A residual map is then constructed by subtracting the trend surface from the original structural surface. The residual map will have positive and negative contour values centered about zero.
Trend Map: The First Order trend surface is a plane tilted in space.
Residual Map (1st Order): The first residual calculates the values between the smooth regional trend and the original contour map. Values will be +/- zero around the "trend" surface.
To create a trend map, take the formation top of interest and create a trend map on the structure of that formation of interest. Then, select an algorithm to grid with and create a trend surface setting the "order of fit" to a first order polynomial.
The following trend map shows the overall regional dip.
To create a residual map, we will need to choose the "Create Trend Residual Surface" from the Trend/Bias tab, if we are using Petra. Again, use the 1st order polynomial and the same gridding algorithm as before. We can also choose a blue-red-highlight color scheme, with the reds and yellows as positives. With this process, the subtle closures are enhanced with the trend residual surface. The lower structural areas are shown in blue, and the higher structural features are shown in reds and yellows.
Now we can compare the map above to the original structure map using the Red-Blue-Highlight color scheme.
Now we can create a second residual trend map for further comparison and analysis.
The evaluation of trends and residuals is an important analysis to identify anomalies. Anomalies may represent such features as structural highs, reef structures, zones of mineralization, avenues of fluid migration, and/or faulted regions. The trend analysis of a very simple surface may show areas of large anomalies. As the surface increases in complexity, the anomalous features may become smaller, but more numerous. The purpose of a trend analysis is to detect anomalies that may then be interpreted as to their importance in the regional geologic context.
Thus we have painlessly demonstrated a fairly complex geologic modeling technique that might be very beneficial to you if you are working in areas of strong regional dip, to help identify subtle anomalies. If CEC Energy Consultants can be of any assistance in helping your staff get up to speed to accomplish these tasks or to perform them for you on an outsourcing basis, please do not hesitate to contact us.
Back to March 2004 Newsletter
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