PNNL

Abstract

Imaging subsurface rock formations or geological objects like oil and gas reservoirs, mineral deposits, cavities or even magmatic plumbing systems under active volcanoes has been for many years a major quest of geophysicists and geologists. Since these objects cannot be observed directly, different indirect methods have been developed. They are all based on variations of certain physical properties of the subsurface materials that can be detected from the ground surface or from boreholes. To determine the density distribution, a new imaging technique using cosmic ray muon detectors deployed in a borehole has been developed and a first prototype of borehole muon detector successfully tested. In addition to providing a static image of the subsurface density in three dimensions (or 3D tomography), borehole muography can also inform on the variations of density with time which became recently of major importance with the injection of large volumes of fluids, mainly water and CO2, in subsurface reservoirs (e.g., aquifer storage and recovery, waste water disposal, enhanced oil recovery, carbon sequestration). This raises several concerns about the risk of leakage and the mechanical integrity of the reservoirs and determining the field scale induced displacement of fluids by geophysical method like muography is thus a priority.