Virtual Source Method for Imaging and Monitoring Below Complex Near-Surface

We propose the Virtual Source method (VSM) that is designed to provide imaging and monitoring in case of very complex overburdens.<br>VSM acknowledges upfront that our data inversion techniques are unable to unravel the details of the complex overburdens to the extent necessary to remove the distortions caused by them. Instead VSM suggests placing downhole geophones below that most complex overburden while still exciting signals with a surface sources. Receivers may sit in horizontal or slanted wells, which may be producers/injectors or dedicated sidetracks and first applications include drilling such wells below complicated near-surface, basalt or salt layers. By performing data-driven redatuming with measured Green\'s functions, these data can be recast into complete downhole dataset with buried Virtual Sources located at each downhole geophone. This step can be effectively thought of as a time reversal and it's remarkable feature is that velocity model between sources and receivers is not required to perform it. Therefore not only we can image through the overburden of any complexity, but also we can effectively monitor even if those overburdens change between the surveys. We illustrate VSM by applying to synthetic and monitoring 4D field case.<br>