1. A method for adjusting sensor signal data detected in a structural health monitoring (SHM) system to compensate for the effects of an environmental variable acting on the data, the method comprising: detecting one or more sets of a sensor signal data from an array of transducers mounted on a structure being monitored by the SHM system; constructing a baseline data space comprising the one or more sets of the sensor signal data detected; detecting one or more current sets of the sensor signal data from the transducers at a later time at which the environmental variable is acting to affect the data; amending the baseline data space with the current sensor signal data sets while they are being detected; and, determining if the current sensor signal exceeds a selected threshold value on the basis of the amended baseline data space; and repeating the detecting one or more current sets of the sensor signal data, the amending, and the determining for each of a plurality of signal propagation paths in the structure respectively extending between associated pairs of the transducers.