A sensor arrangement for monitoring a reservoir, which comprises: a sensor array (1, 404) comprising a plurality of sensor units located over an area to be monitored; an interrogator (6, 30) for obtaining data on the reservoir from the sensor units, which comprises: a. a transmitter for sending optical signals to the sensor array; b. a receiver unit for receiving modulated optical signals from the array in response to the transmitted optical signals; an uplink optical cable for transmitting the optical signals from the transmitter to the sensor array; and a downlink optical cable for transmitting the modulated optical signals from the sensor array to the receiver unit; at least one of the uplink optical cable and the downlink optical cable having an optical amplifier (412, 414) for amplification of the transmitted optical signals and/or amplification of the modulated optical signals; characterised in that the sensor arrangement further comprises: an optical line, and wherein the receiver unit is divided into two parts, and comprises: a first part that includes: a receiver for receiving modulated optical signals from the array in response to the transmitted optical signals; an A/D converter for digitizing the modulated optical signals from the sensor array; and an optical transmitter for transmitting the digitalized modulated optical signals; and a second part that includes: a demodulator for demodulating the received digitalized modulated optical signals; and a recorder for recording the demodulated signals; and wherein the digital modulated optical signals are sent along the optical line that extends at least 100 km from the optical transmitter of the first part to the demodulator of the second part. a sensor array (1, 404) comprising a plurality of sensor units located over an area to be monitored; an interrogator (6, 30) for obtaining data on the reservoir from the sensor units, which comprises: a. a transmitter for sending optical signals to the sensor array; b. a receiver unit for receiving modulated optical signals from the array in response to the transmitted optical signals; a. a transmitter for sending optical signals to the sensor array; b. a receiver unit for receiving modulated optical signals from the array in response to the transmitted optical signals; an uplink optical cable for transmitting the optical signals from the transmitter to the sensor array; and a downlink optical cable for transmitting the modulated optical signals from the sensor array to the receiver unit; at least one of the uplink optical cable and the downlink optical cable having an optical amplifier (412, 414) for amplification of the transmitted optical signals and/or amplification of the modulated optical signals; characterised in that the sensor arrangement further comprises: an optical line, and wherein the receiver unit is divided into two parts, and comprises: a first part that includes: a receiver for receiving modulated optical signals from the array in response to the transmitted optical signals; an A/D converter for digitizing the modulated optical signals from the sensor array; and an optical transmitter for transmitting the digitalized modulated optical signals; and a second part that includes: a demodulator for demodulating the received digitalized modulated optical signals; and a recorder for recording the demodulated signals; and wherein the digital modulated optical signals are sent along the optical line that extends at least 100 km from the optical transmitter of the first part to the demodulator of the second part. an optical line, and wherein the receiver unit is divided into two parts, and comprises: a first part that includes: a receiver for receiving modulated optical signals from the array in response to the transmitted optical signals; an A/D converter for digitizing the modulated optical signals from the sensor array; and an optical transmitter for transmitting the digitalized modulated optical signals; and a second part that includes: a demodulator for demodulating the received digitalized modulated optical signals; and a recorder for recording the demodulated signals; and wherein the digital modulated optical signals are sent along the optical line that extends at least 100 km from the optical transmitter of the first part to the demodulator of the second part. a first part that includes: a receiver for receiving modulated optical signals from the array in response to the transmitted optical signals; an A/D converter for digitizing the modulated optical signals from the sensor array; and an optical transmitter for transmitting the digitalized modulated optical signals; and a receiver for receiving modulated optical signals from the array in response to the transmitted optical signals; an A/D converter for digitizing the modulated optical signals from the sensor array; and an optical transmitter for transmitting the digitalized modulated optical signals; and a second part that includes: a demodulator for demodulating the received digitalized modulated optical signals; and a recorder for recording the demodulated signals; a demodulator for demodulating the received digitalized modulated optical signals; and a recorder for recording the demodulated signals; and wherein the digital modulated optical signals are sent along the optical line that extends at least 100 km from the optical transmitter of the first part to the demodulator of the second part.