A seismic event detection system, comprising: a first fiber optic cable (150, 155, 202) disposed in a borehole (152, 153) extending from an earth surface into a formation, the first fiber optic cable configured to react along its length to incident seismic signals originating outside the borehole and propagating through the formation; an optical source configured to launch optical pulses into the first fiber optic cable (150, 155,202) while the seismic signals are incident on the first fiber optic cable; a second fiber optic cable (151, 157) disposed in the borehole (152, 153) and coupled to the optical source, the second fiber optic cable (151, 157) configured to react along its length to incident seismic signals, wherein the first fiber optic cable (150, 155, 202) is disposed at a first location within a transverse cross-section of the borehole and the second fiber optic cable (151, 157) is disposed at a second location within the transverse cross-section of the borehole that is spaced apart from the first location and a data acquisition system coupled to the first fiber optic cable (150, 155, 202) wherein the data acquisition system includes: a receiver configured to detect coherent Rayleigh noise generated in response to the optical pulses and characterised by the data acquisition system further including: a processing circuit that is coupled to the receiver and configured to process the CRN, wherein processing the CRN includes determining at least the time of arrival of the seismic signal, and wherein the data acquisition system is configured to detect a phase difference between seismic signals incident on the first and second fiber optic cables at each of a plurality of locations along a length of the borehole and to determine characteristics of the formation based on the phase difference. a first fiber optic cable (150, 155, 202) disposed in a borehole (152, 153) extending from an earth surface into a formation, the first fiber optic cable configured to react along its length to incident seismic signals originating outside the borehole and propagating through the formation; an optical source configured to launch optical pulses into the first fiber optic cable (150, 155,202) while the seismic signals are incident on the first fiber optic cable; a second fiber optic cable (151, 157) disposed in the borehole (152, 153) and coupled to the optical source, the second fiber optic cable (151, 157) configured to react along its length to incident seismic signals, wherein the first fiber optic cable (150, 155, 202) is disposed at a first location within a transverse cross-section of the borehole and the second fiber optic cable (151, 157) is disposed at a second location within the transverse cross-section of the borehole that is spaced apart from the first location and a data acquisition system coupled to the first fiber optic cable (150, 155, 202) wherein the data acquisition system includes: a receiver configured to detect coherent Rayleigh noise generated in response to the optical pulses and characterised by the data acquisition system further including: a processing circuit that is coupled to the receiver and configured to process the CRN, wherein processing the CRN includes determining at least the time of arrival of the seismic signal, and wherein the data acquisition system is configured to detect a phase difference between seismic signals incident on the first and second fiber optic cables at each of a plurality of locations along a length of the borehole and to determine characteristics of the formation based on the phase difference. a receiver configured to detect coherent Rayleigh noise generated in response to the optical pulses and characterised by the data acquisition system further including: a processing circuit that is coupled to the receiver and configured to process the CRN, wherein processing the CRN includes determining at least the time of arrival of the seismic signal, and wherein the data acquisition system is configured to detect a phase difference between seismic signals incident on the first and second fiber optic cables at each of a plurality of locations along a length of the borehole and to determine characteristics of the formation based on the phase difference.