Virtual Real Source-Jyoti Behura
      Estimation of the seismic source signature is an important problem in reflection
seismology. Existing methods of source signature estimation (statistical meth-
ods and well-log-based methods) suffer from several drawbacks. For example,
assumptions of whiteness of the earth response, stationarity of the data, and the
phase characteristics of the wavelet have no real theoretical justification and the
extracted wavelets may not be reliable. Here, I introduce a method of extracting
the source signature based on the theory of seismic interferometry, also known
as the virtual source method. Interferometry can be used to extract the scaled
impulse response between two receivers. 
       This is the Green’s function scaled by the power spectrum of the source wavelet. If a source location coincides with one of the receiver locations (not necessarily a zero-offset receiver), the recording
at the other receiver would be the Green’s function convolved with the source
signature. The scaled impulse response, thus differs from the real recording by
having an extra source term convolved with it. Deconvolving the real recording
with the scaled impulse response gives the source signature, and so this method
is named as “Virtual Real Source”. 
     Through modeling examples, I show that the
Virtual Real Source method produces accurate source signatures even for com-
plicated subsurface and source signatures. The quality of the extracted wavelet
can be improved by using particular time windows and stacking wavelets ex-
tracted from different time windows. Source variability within a seismic survey
does not pose any problems because interferometry averages the source signa-
tures and the individual source signatures can be extracted reliably using this
method. Source signature of each shot can be extracted reliably if they all have
similar amplitude spectra even though their phase spectra might be completely
different. This method of source signature estimation not only gives accurate
traveltimes and amplitudes of reflection events, but also has the potential to
solve other issues, such as finding source radiation patterns, measuring intrinsic
attenuation, and estimating statics.