A sensor assembly for the detection of substances by means of nuclear quadrupolar resonance (NQR) and in the presence of environmental interference. The sensor assembly has a transmitter T.sub.x which comprises excitation means which generate excitation signals and a switch connected at the outlet of the Transmitter. A first phase-shifted signal divider/adder is connected to the outlet of the switch. First and second sensor elements are connected to the respective outlets of the first phase-shifted signal divider/adder. First and second coupling circuits, preferably of common mode, are connected to the outlet of the first and second sensor elements, respectively. First and second amplifiers are connected to the outlets of the first and second coupling circuits, respectively. First and second bandpass filters are connected to the outlets of the first and second amplifiers, respectively. A second phase-shifted signal divider/adder is connected to the outlets of the first and second bandpass filters, respectively. A receiver R.sub.X is connected to the inlet of the second phase-shifted signal divider/adder. The method associated to the sensor assembly is also disclosed.