A fluid measurement system, comprising: a fluid flow circuit that comprises at least two liquid sources (102, 106), a gas source, and a fluid mixer that mixes at least two liquids (104, 108) from the at least two liquid sources (102, 106) to form a liquid phase of a mixed-phase fluid that also includes a gas phase from the gas source; a pre-polarizing magnet positioned to receive the mixed-phase fluid flow from the fluid mixer; an Earth's field nuclear magnetic resonance EFNMR detector that comprises a radio-frequency RF coil, a first electromagnet, and a second electromagnet, the EFNMR detector positioned to receive the mixed-phase fluid from the pre-polarizing magnet; and a control system communicably coupled to the pre-polarizing magnet and the EFNMR detector and configured to perform operations comprising: controlling the pre-polarizing magnet to polarize at least the gas phase of the mixed-phase fluid flow to an initial polarization; controlling the EFNMR detector to measure a plurality of free induction decay - FID - values of the polarized gas phase; determining a velocity of the gas phase based on the plurality of FID values of the polarized gas phase; controlling the first electromagnet to produce a pulsed magnetic field gradient to suppress one or more signals acquired by the EFNMR detector; controlling the EFNMR detector simultaneously with the production of the pulsed magnetic field gradient to measure a plurality of FID values of the liquid phase of the mixed-phase fluid; controlling the second electromagnet to generate a homogeneous polarizing field to polarize the liquid phase of the mixed-phase fluid; determining a velocity of the liquid phase based on the plurality of FID values of the polarized liquid phase of the mixed-phase fluid; and determining a content of the liquid phase based on the plurality of FID values of the liquid phase of the mixed-phase fluid. a fluid flow circuit that comprises at least two liquid sources (102, 106), a gas source, and a fluid mixer that mixes at least two liquids (104, 108) from the at least two liquid sources (102, 106) to form a liquid phase of a mixed-phase fluid that also includes a gas phase from the gas source; a pre-polarizing magnet positioned to receive the mixed-phase fluid flow from the fluid mixer; an Earth's field nuclear magnetic resonance EFNMR detector that comprises a radio-frequency RF coil, a first electromagnet, and a second electromagnet, the EFNMR detector positioned to receive the mixed-phase fluid from the pre-polarizing magnet; and a control system communicably coupled to the pre-polarizing magnet and the EFNMR detector and configured to perform operations comprising: controlling the pre-polarizing magnet to polarize at least the gas phase of the mixed-phase fluid flow to an initial polarization; controlling the EFNMR detector to measure a plurality of free induction decay - FID - values of the polarized gas phase; determining a velocity of the gas phase based on the plurality of FID values of the polarized gas phase; controlling the first electromagnet to produce a pulsed magnetic field gradient to suppress one or more signals acquired by the EFNMR detector; controlling the EFNMR detector simultaneously with the production of the pulsed magnetic field gradient to measure a plurality of FID values of the liquid phase of the mixed-phase fluid; controlling the second electromagnet to generate a homogeneous polarizing field to polarize the liquid phase of the mixed-phase fluid; determining a velocity of the liquid phase based on the plurality of FID values of the polarized liquid phase of the mixed-phase fluid; and determining a content of the liquid phase based on the plurality of FID values of the liquid phase of the mixed-phase fluid. controlling the pre-polarizing magnet to polarize at least the gas phase of the mixed-phase fluid flow to an initial polarization; controlling the EFNMR detector to measure a plurality of free induction decay - FID - values of the polarized gas phase; determining a velocity of the gas phase based on the plurality of FID values of the polarized gas phase; controlling the first electromagnet to produce a pulsed magnetic field gradient to suppress one or more signals acquired by the EFNMR detector; controlling the EFNMR detector simultaneously with the production of the pulsed magnetic field gradient to measure a plurality of FID values of the liquid phase of the mixed-phase fluid; controlling the second electromagnet to generate a homogeneous polarizing field to polarize the liquid phase of the mixed-phase fluid; determining a velocity of the liquid phase based on the plurality of FID values of the polarized liquid phase of the mixed-phase fluid; and determining a content of the liquid phase based on the plurality of FID values of the liquid phase of the mixed-phase fluid.