DAS depth calibration with multi-gauge length interrogation
Systems and methods are presented for determining a calibrated vertical seismic profile (VSP) with a distributed acoustic sensing (DAS) system in a borehole. The methods include obtaining a density log and an acoustic slowness log for the borehole; determining a first impedance, a DAS impedance from well log (DIL) curve, from the density log and the acoustic slowness log; obtaining a VSP dataset in the borehole; for each of the plurality of gauge lengths: determining a second impedance, a DAS from vibrations (DIV) curve, from the VSP dataset for the plurality of gauge lengths, and determining a fit between the DIV curves and the DIL curve by using a misfit functional for each of a plurality of depth adjustments. The method further includes determining the calibrated VSP by applying a preferred depth adjustment to the VSP dataset.
1 . A method for determining a calibrated vertical seismic profile (VSP) with a distributed acoustic sensing (DAS) system in a borehole, comprising:
obtaining a density log and an acoustic slowness log for the borehole, wherein each log comprises a value recorded at a plurality of locations common to the density log and the acoustic slowness log;
determining a first impedance, a DAS impedance from well log (DIL) curve, from the density log and the acoustic slowness log;
obtaining a VSP dataset in the borehole, wherein the VSP dataset comprises a plurality of acoustic signals generated by a seismic source and recorded with the DAS system for a plurality of gauge lengths;
for each of the plurality of gauge lengths:
determining a second impedance, a DAS impedance from vibrations (DIV) curve, from the VSP dataset for the plurality of gauge lengths, and
determining a fit between DIV curves and the DIL curve by using a misfit functional for each of a plurality of depth adjustments;
determining the calibrated VSP by applying a preferred depth adjustment to the VSP dataset;
forming, using a seismic processing system, a seismic image based, at least in part, on the calibrated VSP;
planning, using a well planning system, a sidetrack borehole trajectory based, at least in part, on the seismic image; and
drilling, using a drilling system, a sidetrack borehole guided by the sidetrack borehole trajectory.
2 . The method of claim 1 , wherein the misfit functional comprises a plurality of correlation coefficients between the DIV curves and the DIL curve.
3 . The method of claim 2 , wherein the plurality of correlation coefficients comprise a Pearson correlation coefficient.
4 . The method of claim 1 , wherein determining each of the plurality of depth adjustments comprises determining a plurality of depth shifts.
5 . The method of claim 1 further comprising:
selecting the preferred depth adjustment by determining a depth adjustment that maximizes the misfit functional.
6 . The method of claim 1 , wherein the DAS system comprises a fiber-optic cable coupled to a laser light source and to an optical interrogator, wherein the optical interrogator decodes a backscattered laser light from a plurality of locations along the fiber-optic cable.
7 . The method of claim 6 , wherein a gauge length comprises a length of a portion of the fiber-optic cable that determines one of the plurality of acoustic signals.
8 . A system comprising:
a logging system configured to obtain a density log and an acoustic slowness log for a borehole;
a distributed acoustic sensing (DAS) system configured to record seismic data from a vertical seismic profiling (VSP) experiment with a seismic source for a plurality of gauge lengths in the borehole;
a VSP processing system configured to:
determine a first impedance, a DAS impedance from well log (DIL) curve, from the density log and the acoustic slowness log;
for each of the plurality of gauge lengths:
determine a second impedance, a DAS impedance from vibrations (DIV) curve, from a VSP dataset for the plurality of gauge lengths, and
determine a fit between DIV curves and the DIL curve by using a misfit functional for each of a plurality of depth adjustments;
select a preferred depth adjustment based, at least in part, on the misfit functional;
determine a calibrated VSP dataset by applying the preferred depth adjustment to the VSP dataset;
a seismic processing system configured to form a seismic image based, at least in part, on the calibrated VSP dataset obtained by the DAS system;
a well planning system configured to plan a sidetrack borehole trajectory based, at least in part, on the seismic image; and
a drilling system configured to drill a sidetrack borehole guided by the sidetrack borehole trajectory.
9 . The system of claim 8 , wherein the DAS system comprises a fiber-optic cable coupled to a laser source and an interrogator.
10 . The system of claim 9 , wherein the interrogator is configured to decode a backscattered laser light from a plurality of locations along the fiber-optic cable.
11 . The system of claim 9 , wherein the interrogator is configured to use a gauge length to measure a backscattered laser light from portions of the fiber-optic cable to determine a plurality acoustic signals.