A Common Framework to Calculate Uncertainties of Microseismic Events from Downhole and Surface Recordings

The results of microseismic surveys are almost exclusively presented as “dots” in three-dimensional space. Although it is well known that each of these microseismic event locations has an individual level of accuracy, the location accuracy is rarely quantified. In cases where the location uncertainty is represented by bars around the “dot”, they often represent only the precision, i.e. repeatability, of the event location. For the purpose of interpretation it is much more useful to specify the accuracy, i.e. the difference between the calculated and the true location. By using a methodology based on probability density functions (PDFs) it is possible to calculate meaningful confidence intervals around the most probable solution, i.e. the “dot”, which are much more representative of the overall location accuracy. In addition, this methodology allows similar calculations for the two main types of localization algorithms used throughout the industry, i.e. migration-type algorithms and algorithms requiring explicit arrival time picks. This allows creating a common framework that can be applied equally well to recordings from surface and downhole arrays. With this approach it is possible to include the individual location uncertainties in the overall interpretation of the microseismic map.