--

The Israelis surely have the expertise (and can afford the investment of hardware and human effort) to drive perpendicular countersaps against these tunnels, running bores parallel to the Egypt-Gaza border at depths commensurate with existing and likely tunnels.  Listening gear can be emplaced all along the border in narrow bores without endangering Israeli sappers, allowing the Israelis to monitor traffic therein.

 

Once these tunnels are located, "aimed" drilling can be used to place explosive charges that will collapse the detected tunnels - as and when the Israelis desire - without much disturbance of the surface geography.  Timed properly, these can even catch and kill Palestinian tunneling crews in action or to destroy high-priority traffic while in transit.\

--

Geophones have major limitations.

 

First, as long as it is dry, even compacted sand makes a good insulator (both acoustically and thermally).

 

Second, the Egypt side of the border is urbanized in many areas with high levels of traffic noise that drown out the sound of digging.

 

Lastly, these tunnels are being dug by hand.  This is the tunneling equivalent of a submarine running on batteries, extremely difficult to detect by passive means.