Signal configurations used in the study. (a): Transmitter-receiver pairs of the 92 measurement points which are used in the 92/1 and 10/10 data sets. (b): the 363 transmitter-receiver pairs forming the 363/1 and 10/40 sets. (c): example of a sparse 10-point-pair sub-configuration. The different colours in (a) and (b) indicate the sub-systems into which each measurement point belongs to. The target has been magnified for illustration purposes.