Method for monitoring flood front movement during water flooding of subsurface formations

A method for determining the profile of a fluid flood front traveling through earth formations located between a plurality of injection wells and a production well, comprising the steps of:

• traversing a plurality of monitor boreholes located between said injection wells and said producing well with a source of high energy neutrons and a radiation detector;
• irradiating the formations surrounding each of said plurality of monitor boreholes with high energy neutrons emitted from said source;
• detecting radiations emanating from said formations irradiated by said neutrons;
• generating first electrical signals functionally related to the relative distribution of silicon to calcium contents of said formations, said first electrical signals indicative of salinity of the water within said formations;
• generating second electrical signals functionally related to the relative distribution of carbon to oxygen contents of said formations, said second electrical signals indicative of oil saturation of said formations;
• and
• recording said first and second electrical signals for determining the arrival of said flood front at each of said plurality of monitor boreholes.