1. A method for evaluating a proposed design for a geophysical survey of a subsurface region, comprising the steps of (a) obtaining existing geologic and economic information about the subsurface region and about potential target formations therein; (b) simulating a plurality of calibration surveys of the subsurface region, using the existing geologic information and the proposed survey design, each simulated survey assuming target properties consistent with the existing information but otherwise random; (c) using the simulated calibration survey results and the existing economic information to train a selected classifier algorithm to convert the results of a survey into an indicator of presence of economic size targets; (d) simulating a plurality of decision surveys of the subsurface region, using the existing geologic information and the proposed survey design, each simulated survey assuming target properties consistent with the existing information but otherwise random; (e) applying the trained classifier to the results of each of the plurality of simulated decision surveys, generating an indicator of likelihood that such survey contains at least one economic target; (f) selecting a threshold value for the target indicator based on a count of correct indications of at least one economic target and a count of incorrect indications of at least one economic target from said plurality of simulated decision surveys; and (g) calculating an expected value for a geophysical survey based on the proposed survey design, using probabilities derived from said count of correct indications and said count of incorrect indications corresponding to the selected threshold indicator value.