Abstract: Subsurface hydrocarbon reservoir-sized structures are detected or monitored by ambient noise tomography. Interface wave data are recorded for interface waves excited by seismic ambient noise. The data are recorded simultaneously at pairs of locations with the locations of each pair being spaced by less than or equal to a wavelength at the frequencies of interest. The recorded data are processed (3-7) by tomography to obtain group-velocity and/or phase-velocity tomograms, which are inverted to obtain seismic parameters values, such as seismic velocity. The seismic parameters may then be used to form a geological model (8) of a subsurface region of interest.