1. A subsurface feature detection method comprising: receiving a sensor signal indicative of a signature of a signal imparted to the earth by a movable member responsive to a vibratory source and concurrently responsive to reflected acoustic waves in a single substantially lineal direction; wherein the vibratory source operates at a plurality of frequencies at different moments in time; 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.