A new method and apparatus is described which allows remote sensing and in-situ volumetric measurement of liquid petroleum reserves. A small fraction of the nuclear magnetic moments of protons in a liquid oil reserve align in the earth's magnetic field. This nuclear paramagnetism slightly alters the strength of the earth's static magnetic field above the oil reservoir. The size of the paramagnetic contribution to the earth's magnetic field is proportional to the volume of the petroleum reserve. An artificial A.C. magnetic field is applied to the bulk oil reserve at the Larmor frequency. This A.C. magnetic field causes the condition appropriate for nuclear magnetic resonance which under certain circumstances, causes the paramagnetic contribution to the earth's magnetic field to vanish. Different phenomena causing this paramagnetic contribution to vanish include saturation effects and the conditions described by rapid passage.Under these circumstances, frequency sweeping the A.C. magnetic field through magnetic resonance while simultaneously measuring the variation earth's magnetic field yields a direct measure of the total volume of the oil reserve. Numerous means are utilized to produce the appropriate A.C. magnetic field including simply passing an A.C. current through the oil formation between two boreholes. In this case, a sensitive induction magnetometer senses the change in the earth's magnetic field as the A.C. current is frequency swept through resonance. Methods are disclosed which also allow the determination of some characteristic dimensions of the oil deposit. Appropriate measurement of the relaxation of the earth's magnetic field after passage through resonance yields the oil/water ratio under certain circumstances.