[en] The invention describes a method which provides information concerning the location and recoverability of hydrocarbons in subsurface earth formations. This is done by logging the neutron characteristic and combining this with gamma ray spectral data. Measurment of a time-dependent neutron characteristic of the formation is derived by pulsing the neutrons and analysing at least a part of the resulting gamma spectra. Then the timing of neturon pulses is related to the derived time-dependent neutron characteristic of the formation. (OT)