Method of and apparatus for exploring a region below a surface of the earth

A method of passively exploring a region below a surface of the Earth, comprising;

• using a single sensor located in turn at a plurality of locations to passively obtain seismic data by recording naturally occurring ambient seismic waves in a frequency range whose lower limit is greater than or equal to 0 Hz and whose upper limit is less than or equal to substantially 1 Hz;
• selecting for processing portions of the data obtained at each of the plurality of locations as portions of the data at each location having least amounts of energy;
• processing only the selected data obtained from the ambient seismic waves to remove and/or suppress temporary seismic events;
• and
• processing only the selected data obtained from the ambient seismic waves at each location to obtain measures of the energy in a plurality of different frequency bands within the frequency range to obtain information for different depths in the region at each location.