Method of using semblance of corrected amplitudes due to source mechanisms for microseismic event detection and location
A method for determining hypocenters of microseismic events includes entering as input to a computer seismic signals recorded by a plurality of seismic sensors disposed proximate a volume of subsurface to be evaluated. For each point in space in the volume, and for a plurality of preselected origin times, a seismic energy arrival time at each seismic sensor is determined. Event amplitudes for each arrival time are determined. A synthetic event amplitude is calculated for each arrival time. A semblance between the determined event amplitudes and the synthetic event amplitudes is determined. Existence of an actual microseismic is determined event when the semblance exceeds a selected threshold.
1. A method for determining hypocenters of microseismic events, comprising:
entering as input to a computer seismic signals detected by a plurality of seismic sensors disposed proximate a volume of subsurface to be evaluated;
in the computer, for each point in space in the volume, for a selected seismic velocity model and for a plurality of preselected origin times, calculating an expected arrival time of seismic energy at each seismic sensor position;
in the computer, determining event amplitudes for each arrival time from the detected seismic signals;
in the computer, calculating a synthetic event amplitude for each arrival time;
in the computer, calculating a semblance using the determined event amplitudes corrected using the synthetic event amplitudes;
in the computer, determining existence of at least one actual microseismic event when the semblance exceeds a selected threshold; and
displaying or recording a position and origin time of the at least one actual microseismic event.
2. The method of claim 1 wherein the semblance is calculated as a ratio between a squared sum of amplitudes from signals from all the seismic sensors and a sum of squared amplitudes divided by a number of the seismic sensors.
3. The method of claim 1 further comprising determining a plurality of hypocenters of microseismic events during pumping of an hydraulic fracturing fluid into a wellbore and generating a map of microseismic events caused by the hydraulic fracturing fluid over time.
4. The method of claim 1 wherein only a polarity of the determined event amplitudes is corrected by the synthetic event amplitudes.
5. The method of claim 1 wherein only a polarity of the determined event amplitudes is corrected by both a polarity and a size of the synthetic event amplitudes.
6. The method of claim 1 wherein signals from seismic sensors are selected by using values of the synthetic event amplitudes.
7. The method of claim 1 wherein the synthetic event amplitudes are determined by determining a scalar product of a vectorized derivative of Green's function with a seismic moment tensor determined from the determined event amplitudes.
8. The method of claim 7 wherein the seismic moment tensor is determined by inversion processing the event amplitudes.
9. The method of claim 1 wherein the seismic signals are generated by pumping a fluid into formations disposed in the volume to cause fracturing thereof.
10. The method of claim 9 wherein the seismic sensors are deployed in subgroups along a direction of radiation of acoustic noise generated by devices disposed at the Earth's surface.
11. The method of claim 1 wherein the seismic sensors are deployed proximate Earth's surface.