The present disclosure relates to methods and apparatuses for evaluating a porous earth formation. The method may include estimating a value of at least one parameter of interest of the earth formation using a signal indicative of acoustic waves generated at a metallic surface (200) in communication with the earth formation when the metallic surface (200) is exposed to a constant magnetic field (250) normal to the metallic surface (200) and a harmonic magnetic field (260) along the metallic surface. The signal may be generated by a sensor responsive to the acoustic waves. The apparatus may include a first magnetic source (230) configured to generate a constant magnetic field (250), a second magnetic source (240) configured to generate a harmonic magnetic field (260), and a sensor (220) configured to generate a signal in response to acoustic waves.