Efficient CO₂monitoring by parsimonious refraction interferometry

We determine how parsimonious refraction interferometry (PRI) can be used to efficiently monitor a CO₂sequestration project in near real time. The simple refraction survey only requires a pair of reciprocal shots, one shot at each end of a survey line populated by receivers. Changes in the refraction traveltimes recorded at different injection times can be used to monitor the zone of CO₂leakage in the caprock. This estimation of the leakage zone also can be validated by using PRI that creates velocity tomograms of the plume zone. That is, two reciprocal shot gathers of refraction traveltimes can be interferometrically transformed into hundreds of traveltime shot gathers, creating a virtual source at each receiver location. This results in thousands of virtual refraction traveltimes that can be inverted to give a velocity tomogram below the injection site. The two reciprocal shots can be excited every few minutes to give a new tomogram for a near real-time movie of the gas flowing through the rock. Alternatively, receivers can be placed in the injection well and be used to monitor changes in refraction times due to CO₂leakage. This procedure also can be used for efficient 4D monitoring of fluid injection in shallow geothermal fields, shallow drilling hazards, hydrofracturing fractures in unconventional oil and gas fields, and environmental remediation of contaminated sites.