Method of calibrating depths of a seismic receiver array
A seismic receiver array has a plurality of seismic receiver channels, each coupled to a local surrounding in an earth formation. A formation-material-dependent response of each seismic receiver channel is determined, and associated with an assumed depth for the corresponding seismic receiver channel. The formation-material-dependent responses as function of the assumed depth are compared to an independent depth log of at least one petrophysical parameter of the earth formation as a function of depth along the borehole. Based on the comparison, a set of lags between said assumed depth and depth in the independent depth log is determined, that provides the best correlation between the formation-material-dependent response and the independent depth log of the at least one petrophysical parameter of the earth formation. The assumed depth of each seismic receiver channel can thus be aligned with corresponding depths in the independent depth log.
1. A method of calibrating depth of a plurality of seismic receiver channels in a seismic receiver array disposed in a borehole in an earth formation, said method comprising:
selecting a seismic receiver array disposed in a borehole in an earth formation, wherein said seismic receiver array comprises a plurality of seismic receiver channels, whereby each seismic receiver channel is locally coupled to the earth formation that is present adjacent to the seismic receiver channel location;
attributing an assumed depth to each seismic receiver channel in the plurality of seismic receiver channels;
determining a formation-material-dependent response of each seismic receiver channel induced by seismic waves propagating through the earth formation adjacent to each respective seismic receiver channel, wherein the formation-material-dependent response of each seismic receiver channel is determined as a value of signal amplitude;
providing an independent depth log of at least one petrophysical parameter of the earth formation as a function of depth along the borehole;
for a number of the seismic receiver channels determining a set of lags between said assumed depth and depth in the independent depth log that provides the best correlation between the formation-material-dependent response of the seismic receiver channels and the independent depth log of the at least one petrophysical parameter of the earth formation; and
outputting a set of corrected depths or correction values for the plurality of seismic receiver channels to align the assumed depth of each seismic receiver channel of the plurality of seismic receiver channels with corresponding depths in the independent depth log.
2. The method of claim 1 , wherein the independent depth log comprises values for the at least one petrophysical parameter of the earth formation for a number of depths per unit length that is larger than the number of seismic receiver channels per said unit length, said method comprising upscaling the independent depth to obtain an upscaled depth log that has a single value of the at least one petrophysical parameter of the earth formation per seismic receiver channel.
3. The method of claim 1 , wherein at least one petrophysical parameter of the earth formation of the independent depth log is dependent on an elastic property of the earth formation.
4. The method of claim 1 , wherein the independent depth log is one of: a sonic log, a density log, and an acoustic impedance log.
5. The method of claim 1 , wherein the value of signal amplitude corresponds to an inverse root-mean-square average amplitude of a plurality of seismic events.
6. The method of claim 1 , wherein the value of signal amplitude is determined using a channel-consistent scalar technique.
7. The method of claim 1 , wherein the seismic receiver channels in the seismic receiver array form a string of seismic receiver channels.
8. The method of claim 1 , wherein the seismic receiver array is formed by a Distributed Acoustic Sensing (DAS) system which subdivides an optical fiber in a plurality of DAS receiver channels whereby the seismic receiver channels are said DAS receiver channels.
9. The method of claim 8 , wherein said optical fiber is packaged in a cable and operated as a DAS optical fiber.
10. The method of claim 9 , wherein a plurality of optical fibers is packaged in said cable, all operated as DAS optical fibers.
11. The method of claim 9 , where said optical fiber is helically wound around a core in said cable.
12. The method of claim 9 , wherein said formation-material-dependent response of each seismic receiver channel is determined using actual measurements of signals of back-scattered light from the optical fiber.
13. The method of claim 12 , wherein said assumed depth corresponds to an optical distance as determined by measuring time-of-flight of the back-scattered light and taking into account a fiber trajectory through the earth formation.