NanoVISA: Probabilistic inference for micro and induced seismic event location and uncertainty quantification
Vertically Integrated Seismic Analysis (VISA) is a physically accurate, statistical process for the automatic identification of seismic events. It accurately determines event location, time, magnitude and uncertainty. NanoVISA is a proposed VISA system particularly tuned for micro and induced seismic events. It is inspired by the NET-VISA system currently installed to monitor for nuclear events at the International Data Centre (IDC) of the Comprehensive Test Ban Treaty Organization (CTBTO) being developed by Bayesian Logic Inc.
Compared to standard IDC data analysis, NET-VISA has been shown to:
• Reduce event location errors by half
• Find more small events
• Reduce the false event rate by 40%
• Accurately estimate uncertainty
• Significantly reduce the human labor involved in producing an accurate seismic bulletin
Drawing on this successful technology, NanoVISA will:
>>Improve fracture monitoring during oil and gas well stimulation allowing engineers to design well treatments which optimize production
>>Allow for finer understanding of fracture patterns, leading to better calibration of production models
>>Produce event maps that are less noisy than current images, reducing the risk of fracture networks and well footprints overlapping
>>Return defensible estimates of uncertainty regarding the causes of alleged induced seismic events
>>Significantly reduce the cost of producing high quality data by reducing the need for human intervention during data analysis
Expected Performance
NanoVISA has not yet been built. However, its performance is expected to be similar to that of NET-VISA, which has shown dramatic improvement over current IDC processing. Figure 1 shows events produced by standard processing from a week of data compared to events from the same period that were listed in the Late Events Bulletin (LEB) in which each event was reviewed and corrected by a human analyst. The bottom figure shows events produced by NET-VISA. The NET-VISA bulletin is more complete and clearly contains fewer spurious events than the IDC product.
In June of 2012, a test was performed in which IDC analysts evaluated 26 hours’ worth of NET-VISA events using the same tools they use to evaluate standard events. Out of 135 events that were included in LEB for that day, the standard program found 101 and NET-VISA found 117, many of significantly smaller magnitude.
Brief Technical Description
VISA systems are composed of two parts: a probabilistic Knowledge Representation (KR) and an Inference Algorithm (IA). The KR is a rigorous mathematical framework which is used to compute the probability of any configuration of factors which affect the measurement of a seismic event across a given seismic array. These factors include location and magnitude of the event; seismic travel times as predicted by simple earth models; detection probabilities; and errors in azimuth, slowness, and phase labeling of arrivals. A pictorial representation of the KR for NET-VISA is given in Figure 2. The relationship between factors which do not have a known physical dependence that can be numerically modeled is learned from historical data and represented by conditional probability dependence.
Given a set of measured data (which in the case of NET-VISA are arrivals that have been automatically identified at stations on the CTBTO seismic array), the IA proposes events and then uses the KR to compute the probability of these events and the arrivals associated with them, searching for the best set of events and associations. NET-VISA is the only program of its kind which not only maximizes the probability of events and associations, but actually proposes them. Other Bayesian programs for locating seismic events, such as PEDAL, are capable only of evaluating events that have been pre-proposed, usually in a regular pattern, and so are constrained to finding events on a coarse grid. Others, like BAYESLOC, are designed to relocate events that have already been listed in a previous seismic bulletin.
Development Schedule
A research proposal has been submitted to the Research Partnership to Secure Energy for America (RPSEA) in response to a call for an Unconventional Resources RFP in the first quarter of 2013. (The full proposal will cover development of an integrated Reservoir Model combining NanoVISA with statistical production analysis also developed by Chatelet Resources.) If this contract is secured, commercial development of NanoVISA will start in fiscal 2014. Due to the timeliness of the project and the importance of microseismic fracture monitoring to industry, we are also open to partnership opportunities bringing independent funding if it would enable us to develop NanoVISA earlier.
Link to NET-VISA Interactive Model Visualization