Locating microseismic events using both head and direct waves travelling in vertical transverse isotropic media

Abstract

Microseismic monitoring is widely used to detect hydraulic fractures. Accurate mapping of microseismic events is essential to detect such fractures enhancing productivity. The eikonal solver is an efficient forward-modelling method used to map microseismic events. However, traditional eikonal solvers do not distinguish between head and direct waves, computing only the traveltimes of the waves that arrive first. We developed a new eikonal solver that computes the traveltimes of direct waves by imposing new constraints on the conventional, vertical transverse isotropy solver. We then performed numerical experiments exploiting the traveltimes of direct waves. We used the traveltimes of only the first arrivals, and those of both first and direct arrivals, when performing inverted event mapping. The results showed that the uncertainties of event locations were minimized when both head and direct waves were analysed due to the increased both the number of available data and the travelling path diversity. Also, we found that the use of only direct-arrival traveltimes was valuable when head-wave first arrivals were difficult to detect because the signal-to-noise ratio was low.