1. The decrease in amplitude of a wavefront because of geometrical spreading. The energy in body waves spreads out as the spherical wavefront expands, causing the energy density to vary inversely as the square of the distance (spherical divergence). With surface waves the energy density varies inversely as the distance (cylindrical divergence). Tube waves do not suffer energy loss because of divergence. Energy density also decreases because of raypath curvature, absorption, and other reasons. 2. The divergence of a vector field is expressed in Figure C-14 for rectangular, cylindrical, and spherical coordinates.