Magnetic influence sensing. The magnetic field of a ship can be controlled by periodically degaussing the ship. However, this is a time consuming process which involves wrapping the entire hull in large cables and applying a magnetic field for several hours. Most ships therefore have some detectable magnetic field and are vulnerable to magnetic influence mines. Only ships made out of wood or fiberglass are considered immune.
Mines can also detect targets by the wake they create in the water.
As the ship moves through the water, a pressure wave is formed which is visible as a wake on the surface. The same thing occurs underwater but is not visible. The pressure wave can be detected by a mine, however. The faster the ship is moving, the greater the effect. Pressure influence sensing.