DENSITY TOMOGRAPHY OF SUBSURFACE RESERVOIRS

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. Electrical resistivity, seismic wave’s velocities and density are certainly the most used properties. If we look at density, indirect estimates of density distributions are performed currently by seismic reflection methods - since the velocity of seismic waves depend also on density - but they are expensive and discontinuous in time. Direct estimates of density are performed using gravimetric data looking at variations of the gravity field induced by the density variations at depth but this is not sufficiently accurate. After discussing the merits of these two methods, a new imaging technique using cosmic-ray muon detectors deployed in a borehole, will be presented.

In addition to providing a static image of the subsurface density in three dimensions (or 3D tomography), these methods can also inform on the variations of density with time which became recently of a major importance. The injection of large volumes of fluids, mainly water and CO₂, in subsurface reservoirs is indeed increasingly performed in various applications (e.g., aquifer storage and recovery, waste water disposal, enhanced oil recovery, carbon sequestration). This raises several concerns about the mechanical integrity of the reservoirs themselves and their surroundings. Determining the field-scale-induced displacement of fluids and the temporal and spatial deformations of the ground surface is thus a priority. Finally, to improve imaging of 3D subsurface structures, a combination of seismic data, gravity data, and muons can be used and this promises to be a powerful way to improve spatial resolution and reduce uncertainty. Different applications will be presented and discussed.