Techniques are described in which the intensity distribution of an ultrasound beam is measured with high resolution by means of the radiation reflected from a target whose effective backscattering diameter is smaller than one wavelength. Correlation receivers are used to improve the signal-to-noise ratio of the echo. As the target is scanned across the beam, the phase of the reference signal applied to the correlation receiver must be adjusted to compensate for changes in the time of flight of the ultrasound echo. The broadband beam intensity mapping technique employs a noise signal in which phase compensation is adjusted manually. The other technique, which is used with narrow band signals, applies a phase quadrature technique to achieve phase compensation.