An apparatus for monitoring a submarine reservoir, which comprises: a sensor array comprising a plurality of sensor units adapted to be located over an area of the seabed in the region of the reservoir to be monitored; and an interrogator (11, 80) for obtaining data on the reservoir from the sensor units, which comprises: a transmitter for sending optical signals to the sensor array, the transmitter including an optical switch for receiving optical radiation from an optical source and transmitting optical signals generated thereby along an uplink optical fibre, and at least one splitter (36, 38, 40) for splitting the uplink optical fibre into a plurality of optical fibres that extend to the sensor units over the area to be monitored; and a receiver for receiving modulated optical signals from the array in response to the transmitted optical signals, the receiver including an optical-to-electrical converter for converting optical signals from the array to electrical signals, a phase demodulator, a multiplexer for multiplexing the electrical signals from the phase demodulator, and a signal processing and recording unit (68, 70) for recording the multiplexed signals; wherein the optical source and optical switch of the transmitter, and the signal processing and recording unit (68, 70) of the receiver are incorporated in an interrogator hub (11, 20) adapted to be located on a platform or on shore, and wherein one or more of the splitters (36, 38, 40), the optical-to-electrical converter, the phase demodulator and the multiplexer of the receiver are incorporated in at least one concentrator adapted to be located underwater; and the apparatus further including a riser (9, 10) for transmitting signals from the or each concentrator to the interrogator (11, 20). a sensor array comprising a plurality of sensor units adapted to be located over an area of the seabed in the region of the reservoir to be monitored; and an interrogator (11, 80) for obtaining data on the reservoir from the sensor units, which comprises: a transmitter for sending optical signals to the sensor array, the transmitter including an optical switch for receiving optical radiation from an optical source and transmitting optical signals generated thereby along an uplink optical fibre, and at least one splitter (36, 38, 40) for splitting the uplink optical fibre into a plurality of optical fibres that extend to the sensor units over the area to be monitored; and a receiver for receiving modulated optical signals from the array in response to the transmitted optical signals, the receiver including an optical-to-electrical converter for converting optical signals from the array to electrical signals, a phase demodulator, a multiplexer for multiplexing the electrical signals from the phase demodulator, and a signal processing and recording unit (68, 70) for recording the multiplexed signals; a transmitter for sending optical signals to the sensor array, the transmitter including an optical switch for receiving optical radiation from an optical source and transmitting optical signals generated thereby along an uplink optical fibre, and at least one splitter (36, 38, 40) for splitting the uplink optical fibre into a plurality of optical fibres that extend to the sensor units over the area to be monitored; and a receiver for receiving modulated optical signals from the array in response to the transmitted optical signals, the receiver including an optical-to-electrical converter for converting optical signals from the array to electrical signals, a phase demodulator, a multiplexer for multiplexing the electrical signals from the phase demodulator, and a signal processing and recording unit (68, 70) for recording the multiplexed signals; wherein the optical source and optical switch of the transmitter, and the signal processing and recording unit (68, 70) of the receiver are incorporated in an interrogator hub (11, 20) adapted to be located on a platform or on shore, and wherein one or more of the splitters (36, 38, 40), the optical-to-electrical converter, the phase demodulator and the multiplexer of the receiver are incorporated in at least one concentrator adapted to be located underwater; and the apparatus further including a riser (9, 10) for transmitting signals from the or each concentrator to the interrogator (11, 20).