material is introduced into the borehole inside an annulus 702 such as a metal casing at some location such as the bottom of the borehole the entrained material moves to the outside of the annulus and changes direction i e moves back toward the surface when positioning cement outside the annulus during well completion it is useful to know the location of both the leading and trailing edges 704 706 of the entrained cement it should be noted that the distance between the leading and trailing edges is variable because the cross sectional area outside the annulus can vary significantly because of fractures in order to monitor the progress of both edges acoustic disturbances 108 d 1 108 e 1 are introduced both inside and outside the annulus the edges 704 706 will reflect at least a portion of the acoustic disturbances such that location can be determined from round trip time and modeling of reflected acoustic disturbances 108 d 2 108 e 2 as already described above the disturbances may be generated by operating a valve 708 connecting the fluid inside the annulus with the fluid outside the annulus or alternatively by separate acoustic sources 702 a 702 b with the valve in a closed position it will be expected that there should be a pressure differential across the annulus hence by opening the valve it is possible to generate acoustic disturbances on either side of the annulus which will be equal in amplitude and opposite in polarity the reflected acoustic disturbances are detected by separate sensors 110 a 110 b fig8 and 9 illustrate an embodiment for monitoring an.