An apparatus and procedure for passively performing a geophysical survey. The RAP Equipment complex comprises an A/D Conversion Device, an external sensor, an external 12 volt battery, and a Notebook Computer in which resides a Fourier analysis program. The A/D Conversion Device is connected to the serial port of the Notebook Computer. The A/D Conversion Device comprises an Analog/Digital Converter modified to replace its quartz resonator with a Frequency Generator for providing a predetermined sampling rate, and a Signal Amplifier. An external sensor is attached by a cable to the input of the Signal Amplifier. A 12 volt power supply powers the Signal Amplifier, the modified A/D converter, and the Notebook Computer. In use, the external sensor is placed on the ground at the first spot it is desired to take a reading. The ground is then lightly tapped in the near vicinity of the sensor either with one's finger or a hammer. This tap causes the natural vibrations within the subsurface layers to be amplified. The sensor picks up these vibrations from the resonating layers, passes them on to the Signal Amplifier which in turn amplifies these signals and passes them on to the A/D Conversion Device. The A/D conversion Device converts these acoustical signals from analog form to digital form and passes them on to the serial port of the Notebook Computer which in turn creates a file of these digital signals.