A Receiver Array Evaluation Method Designed For Surface Based Microseismic Monitoring
Authors: Aoki Naoshi , Asakawa Eiich , Abe Susumu 
Published: March 2015

Abstract
Surface based microseismic monitoring during hydraulic fracture stimulations is an emerging technology in the last decade. The method observes acoustic emission (AE) or microseismic events using a surface receiver array and estimates source locations and origin times of the events by analyzing a time series of migration images from observation data. However, the accuracy and monitoring cost of the technology highly depends on the array-geometry. The migration images can be blurred if a too coarse array is used. Monitoring cost can be unnecessarily high if a too dense array is used. Some sort of adequate array evaluation methods are required for designing a cost effective survey plan. We introduce an array evaluation method based on the point-spread function (PSF). The PSF describes the response of an imaging system to a point source. Our proposed method evaluates an array in the following way: First, a point source of which excitation time and source location are known is defined. Second, synthetic seismograms using the array are computed. Third, the synthetic traces are migrated and a time series of source images are obtained. Finally, the images are examined so that the abilities of the array are understood. We demonstrate the method by diplaying attributes calculated from the dataset which represent the precision and accuracy of the source location determination.

Introduction
The use of microseismic monitoring during the hydraulic fracture stimulations is growing worldwide as the development of tight oil/gas shows phenomenal growth. The technology is based on seismic source location methods, which observes microseismic events using a receiver array network and estimates source locations and origin times of the events by analyzing the characterestics of microseismic events. The source location methods can be divideed into two categories: downhole based and surface based (Duncan and Eisner, 2010).
Downhole based method deploys the sensors in the well and configures an array network near the stimulated area. Most of the source location methods applied to the downhole data are based on the Geiser's scheme (Geiser, 1912), which minimizes the arrival-time residuals in an iterative maner. Thanks to a quiet environment in the borehole and short travel paths from the microseismic sources to the receivers, detectability of the method is higher than surface based method. However, because of the limited array configuration, the area that can cover with an array network in a well is also limited around the well. To increase the accuracy, many monitoring wells are necessary to retain an observable area. This may cause the increase of the monitoring cost.
Even though higher noisy environments than downhole based method, surface based microseismic monitoring has been developed as an emerging technology in the last decade. This method plants or buries the sensors to configure a large 2D array network on the surface or near the surface. Migration-type methods are typically used for the surface based monitoring because the signal to noise (S/N) ratio can be improved by stacking of the events (Gajewski et al., 2005). To overcome the higher noise level of surface environments, a temporary array network consisting of 1,000+ geophones, or a permanent sensor array network consisting of several buried sensors per square mile, has been used in these days.