changes occurring downhole during a fracturing process can create or reflect pressure signals . capturing and evaluating such pressure waves during fracturing enables personnel to monitor , in real time or later , what happens downhole . when a fracture extends , a burst of acoustic noise is embodied in a pressure wave or signal , as is noise coming from other sources . by transforming time - based pressure signals to a frequency base , one can monitor this acoustic noise . in a particular implementation , a waterfall plot of frequency spectra at successive time slices of the original signal is used to determine frequency ridges , such as a ridge of decreasing frequencies indicates fracture extension and a ridge of increasing frequencies indicates either closure or proppant backing up in the fracture . filtering , such as wavelet filtering , can be used . a fracturing process can be controlled in response to determining whether the fracture is extending .