Using Intermediate-Field Terms in Locating Microseismic Events

Microseismic mapping is a passive seismic technique used extensively for assessment of hydraulic fracturing treatments during the last two decades. Basically, microseisms are microearthquakes induced by the changes in stress and pore-fluid pressure associated with the hydraulic fracturing treatment. Current practice to locate events and determine the source mechanism of microseismic events associated with hydraulic fracture treatments only includes far-field terms for the moment tensor inversion. The intermediate-field terms and near-field term are normally ignored, perhaps simply following the tradition in locating distant earthquakes. However, source-receiver distances in hydraulic fracturing are usually 1000 ft (~300m), which is much less than the typical distances in earthquakes; therefore the effect of near and intermediate-field effects are not yet known.

We try to include these near-field effects to improve the precision of locating the events and consequently determining the source mechanism. We find that the intermediate-field term may contribute up to 1/3 of the signal amplitude when the source-receiver distance is about 300 m. The intermediate-field term contributes ~1/20 of the signal amplitude when the source-receiver distance is ~ 2000 m . When the source-receiver distance exceeds ~ 2000 m, the intermediate-field terms can be ignored in our inversion. In our case, we also confirm that the near-field term can be ignored in microseismic analysis. Our results indicate that the intermediate-field terms can improve moment tensor inversion between 2% to 40% at source-receiver ranges less than 300 m. However for the case of large distances, the improvement using this technique is limited to 1%. In the presence of strong environmental noise, intermediate-field terms help to effectively improve the moment tensor inversion: i.e., 15% improvement with noise present vs 3% improvement without noise.