We describe the acquisition, processing, and inversion of a multitransient electromagnetic (MTEM) single-line survey, conducted in December 2004 over an underground gas storage reservoir in southwestern France. The objective was to find a resistor corresponding to known gas about 500m below the survey line. In data acquisition, we deployed a 100-m inline bipole current source and twenty 100-m inline potential receivers in various configurations along the 5-km survey line; we measured the input current step and received voltages simultaneously. Then we deconvolved the received voltages for the measured input current to determine the earth impulse responses. We show how both amplitude and traveltime information contained in the recovered earth impulse responses reveal the lateral location and approximate depth of the resistive reservoir. Integrating the impulse responses yields step responses, from which the asymptotic DC values were estimated and used in rapid 2D dipole-dipole DC resistivity inversion to find the top of the reservoir. A series of collated 1D full-waveform inversions performed on individual common midpoint gathers of the step responses position the top and bottom of a resistor corresponding to known gas in the reservoir and also obtain the transverse resistance. The results imply that the MTEM method can be used as a tool for hydrocarbon exploration and production.