Abstract: A data telemetry system for use in a plugged exploratory borehole to transmit measurements of subsurface conditions after long delays by use of seismic waves. The system includes a sensor for detecting a downhole condition, such as temperature, and converting it to electrical signals, a timer, and a series of explosive charges. The timer detonates a first of the charges at a preselected time delay and a second charge at a further time delay corresponding to the electrical output of the sensor. The seismic waves generated by the explosions are detected by a seismometer on the earth's surface near the exploratory borehole.