Figure 2.

Schematic of CRUST image reconstruction. S, the VPS position and the origin of the global coordinate system S‐xyz; D_D , the imaging plane defined by the USTA; D_i , the i‐th element position of the USTA at (x_i , y_i , z_i ); T, a point scatterer position in D_D at (x, y, z); L ₁, the distance between S and T; L ₂, the distance between T and D_i ; τ, the time of flight along L ₁ and L ₂; c, the speed of sound; Γ, a spheroid originating at o with its symmetric axis along the y′‐axis of the coordinate system o‐x′y′z′; a_i (τ), the equatorial radius of Γ; c_i (τ), the distance between the center of Γ and the pole on the symmetric axis. Ω _i (τ), a partial ellipse formed by the intersection of D_D and Γ.