Fundamentals of Fluid Flow in Porous Media

Chapter 3

Diffusion Coefficient: Measurement Techniques

NMR Method: Principles of NMR and Processing

Relaxation Process

When the B₁ field is turned off, the net magnetization decreases and system gradually returns to equilibrium. During this process, the protons gradually lose their extra energy and return to equilibrium by emitting radio waves and by transferring energy to surrounding molecules. The processes by which nuclei transfer energy to their surroundings to return to equilibrium state are called relaxation. The relaxation processes are exponential in time and are described by two time constants, T₁ as the longitudinal magnetic relaxation time constant and T₂ as the transverse magnetic relaxation time constant. These two constant values are seldom equal. Transverse relaxation is always faster than longitudinal relaxation; consequently, T₂ is always less than or equal to T₁. In general, for protons in solids, T₂ is much smaller than T₁.