A method for measuring the movement of boundaries between different portions of a heterogeneous rock body is disclosed. The method comprises placing a plurality of blast movement monitors 109 in a rock body prior to blasting and noting the position of each blast movement monitor. The rock body is then blasted to break it up into a plurality of pieces. Thereafter the position of the blast movement monitors 109 is located and based on this the boundaries of rock portions can be adjusted to account for the blast. This leads to a more accurate reporting of different ore bodies to the appropriate processor in a heterogeneous rock body. An apparatus for carrying out the method is also disclosed. The apparatus comprises broadly a said monitor 109 and a receiver. The monitor 1 comprises a transmitter 109 received within a casing 111 that in turn is received within a housing 126. Further the casing 111 can move within the housing 126 and self right so that it always transmits its signal in an upright direction.