Fig. 1.  Pressure–depth plots relating to the mechanics of sand intrusion. (a) Diagram illustrating the build up and decline of pressure during propagation and cessation: note that the fluid pressure gradient during intrusion, which determines the elevation difference between source and sill, is not hydrostatic. (b) Diagram illustrating how the variables of source-bed pressure decline (P) and the instantaneous dynamic gradient of the injecting slurry (F) yield a vector which determines the sill position.