1. A subsurface feature detection method comprising: receiving a sensor signal comprising information representing motion of a movable member due to vibrations having a plurality of frequencies at different moments in time, the vibrations comprising a combination of acoustic waves transmitted to the earth from the movable member and reflected vibrations received from the earth; for two or more frequencies among the plurality of frequencies, determining magnitudes of the sensor signal; identifying at least one point of zero slope for the sensor signal using the magnitudes; and providing information regarding a subsurface feature of the earth using the at least one point of zero slope.