When velocity logging oil wells, a sonde is raised from the bottom of the well borehole at a slow, but variable and unpredictable rate. This sonde transmits pulsed acoustic energy at a variable pulse repetition rate into the earth surrounding the borehole and, in addition, detects the refracted acoustic pulses after they have traveled a few feet through the earth surrounding the borehole. In order to ascertain certain characteristics of the earth, it is desirable to record photographically those pulses occurring, not at specified time intervals, but at specified depth intervals. This is accomplished by converting the sonde depth to electrical gate pulses occurring at the desired depth intervals. These gate pulses are used to open a gate which permits only the first acoustic pulse following a gate pulse to pass through the gate and be photographed on a triggered oscilloscope. The equipment is fully automatic in that the camera film is automatically advanced between recorded pulses and precautions are automatically taken to prevent double exposures. Similar instrumentation should be applicable to related problems involving the photographing of system responses as a function of some measurable parameter which varies slowly but unpredictably with time.