A review of the results of over 3,000 measurements of soil ethane emanation rate made over the past five years has revealed a number of conclusions which have an important bearing upon the validity of geochemical methods of prospecting for oil and gas fields. A number of factors have been found to be highly disturbing, namely: earth topography, ground water percolation and seepage, barometric pressure variations, etc. These effects result in fluctuations of the rate of escape of hydrocarbons accompanied by horizontal shifts of leakage which give rise to the creation of artificial leakage highs altogether meaningless from the point of view of oil and gas accumulation at depth. Certain qualitative rules of interpretation have been established which permit weeding out the meaningless anomalies provided sufficient information is at hand on the topography and watertable movement. A further observation has been made indicating that the artificial anomalies are often of greater magnitude than the significant measurements. Hence the problems, so common in other geophysical methods, of making important reductions on the observed figures are imperiously facing the exploration geochemist if he is to develop a valuable prospecting tool. A serious attempt is presently made to solve this problem.