Assessing Alternative Scenarios for the Cause of Underpressures in the Ordovician Sediments along the Eastern Flank of the Michigan Basin

Figure 4

(a) Helium isotope ratios in deep geologic repository (DGR) DGR-2 (green circles) and DGR-3 (red circles) cores normalized to air [xRa = (³He/⁴He) sample/(³He/⁴He) air; where (³He/⁴He) air is 1.38 × 10^–6], calculated isotope ratios (black squares) based on decay of U and Th (⁴He) and fission of ⁶Li (³He) from calculations of neutron flux (Ballentine and Burnard, 2002), and groundwaters (blue circles). (b) ⁴He concentration (cm³ STP g^–1 rock; STP—standard temperature and pressure) in DGR-2, measured by step-wise extractions, corrected for losses using exponential extrapolation model (red squares) and three-dimensional numerical diffusion model (blue circles) (mbgs—meters below ground surface). Figure and caption from Clark et al. [9].