These acoustic tools send sound pulses out to the formation and measure both the amplitude and the travel time of the returning signals. Because the acoustic image tool relies on sound pulses, they can work in oil based muds, where electrical conductivity is very poor. Their disadvantages when compared to electrical images are a heightened sensitivity to borehole roughness or washouts and a generally poorer quality image overall in part or all of some wellbores. As borehole conditions are often best over the pay zone, they can give useful fracture data where such information is most useful.