Reginald Fessenden is famous as the Canadian who claimed the first invention of radio. The first is hard to determine. Many people were working on wireless from different angles. Fessenden later made one of the first voice broadcasts in 1906, using a high-freq alternator instead of a spark-gap transmitter.
Around 1912 he moved into a more commercially practical field, using sound for locating objects underwater and underground. Working for Submarine Signaling Co, he developed a high-power speaker and microphone and associated circuitry, the first use of electronically controlled sound to look for fish and minerals and oil.
The speaker is just like a normal dynamic loudspeaker, except that the diaphragm is heavy metal instead of paper, and the field coil is far more powerful.
This setup is described in his 1917 patent 1240328.
Polistra is controlling a GenRad audio oscillator to feed the speaker transducer, and watching a GenRad oscillograph to record the responses from the microphone transducer. A real exploration rig would have several microphones in different locations, then analyze the results together.
Here’s a simplified animation of the sending and receiving.
The shaft on right is filled with water, and the speaker transducer is submerged. Water is incompressible, so the pushes and pulls are transferred without loss to the walls of the shaft. The waves then radiate through the ground, reflecting off harder objects like a large rock or a heavier layer.
Polistra's Mill 