There is an amplifier circuit on the PSN site. Quite high sensitivities can be archived using pulse signals and coherent detection. Commercial IR intruder beam sensors are available. Ultrasonic detectors working in the 40 KHz range will also detect animals and can be used to measure velocity. Small animals, some birds, bats and insects may be able to sense these signals.
Electrostatic detectors can be of the type which react to static charges or to RF capacity changes.
10 GHz Radio Intruder Detectors modules are available, as are pyroelectric heat detector modules. Radio signals may penetrate sandy soil quite well. None of these techniques need be very expensive, but it is a great help if you have the services of someone really competent in electronics. You can also use the small surveillance CCD cameras with Infra Red illumination to monitor animal movements. The modules are fairly reasonably priced.