Offset and walkaway VSPs are used to "look to the side" of the borehole to see if a major change, such as a fault or reef, occurs near the borehole and hence that the findings from the borehole may not apply to the nearby region. The greater resolution of a VSP may help delineate small faults, stratigraphic changes, and thin reservoir sands. An objective formation may have been faulted out or a well may be on a nonproductive side of a fault so that sidetracking may encounter production. Detection of reservoirs just missed is especially useful where hydrocarbon indicators are observed. Surveys using geophones in deep holes are also used in searching for and defining nearby features such as salt domes.