passive radar theory refrence needed

["Peter Divos" <pdivos@xxxxxxxxx>]

Hi,

I heard there are some types of radar which operate passively, this
means, they dont send any signal and dont wait for the echo, but they
get some random electromagnetic waves from the air which are reflected
on objects as well and it is possible get some image in exchange of
some computation.
I also heard of a swiss company who are using the same principle but in
geology, seismology, oil exploration. They dont produce any signal
(like explosion under the surface) but instead use very sensitive
detectors and catch the random seismic noise from the earth which
reflects as well and get the image based on this.

So. I am interested in the mathematical and algorithmical part of the
topic. Does anyone know any good references? Like keywords or book
titles.

Many thanks
Peter