Ghyben–Herzberg relationship Beneath oceanic islands, the thickness (d) of a lens of fresh groundwater (density ϱw) overlying sea water (density ϱm) can be determined if the height above sea level (h) of the top layer of the lens is known and the conditions are static. The relationship is d = ah = ρw/(ρm − ρw) where a is typically about 38. The relationship was discovered independently by W. Badon (sometimes spelled Baydon) Ghyben, who described it in 1888–9, and B. Herzberg, who described it in 1901.

Ghyben—Herzberg relationship Beneath oceanic islands, the thickness (d) of a lens of fresh groundwater (density ρ_w) overlying sea water (density ρ_m) can be determined if the height above sea level (h) of the top layer of the lens is known and the conditions are static. The relationship is d = ah = ρ_w/(ρ_m −ρ_w) where a is typically about 38.