Each electrotelluric survey is carefully designed to meet specific project objectives. For example, the number and distribution of survey stations may be different for each exploration problem. Most surveys begin with calibration of the instrument for "true depth", using known lithologic or electric log data and topographic data. The surveyor scans through a range of frequencies calibrated for depth, listening for changes in the signal, which indicate the crossing of a resistivity boundary, or conductivity interface. Responses are classified as very low, low, medium or high. These are related to the relative resistivity contrasts. The data are logged on a Pro/Log graph, which is scaled to any available electric logs. Pro/Log graphs are similar, but not identical, to conventional downhole logs.