An apparatus for measuring pore pressure in a formation, comprising: a first signal generator configured to generate a pulsed sinusoidal signal, the pulsed sinusoidal signal having a frequency; a first transceiver disposed in a borehole in the formation and configured to receive the pulsed sinusoidal signal, and to transmit a pulsed sinusoidal acoustic signal in accordance with the received pulsed sinusoidal signal; a second transceiver disposed in the borehole and configured to receive the pulsed sinusoidal acoustic signal and to generate a first electrical signal therefrom; one or more microprocessors configured to receive the first electrical signal and to time reverse the first electrical signal to generate a time-reversed electrical signal; a second signal generator configured to receive the time-reversed electrical signal, and to direct the time-reversed electrical signal to the second transceiver, the second transceiver being further configured to transmit a second acoustic signal in accordance with the time-reversed electrical signal, whereby the second acoustic signal forms a focal volume centered on the first transceiver, the second acoustic signal including second and third harmonics of said frequency, the first transceiver being further configured to produce a second electrical signal based on reception of the second acoustic signal, the second electrical signal having an amplitude; and memory storage configured to store the amplitude of the second electrical signal and/or information derived from the amplitude of the second electrical signal, wherein a hysteretic nonlinearity parameter α relating to hysteresis strength is determined based on the amplitude of the third harmonic of the second electrical signal, a nonlinear elastic parameter β relating to acoustoelasticity is determined based on the amplitude of the second harmonic of the second electrical signal and the pore pressure in the formation is determined based on the hysteretic nonlinearity parameter α and the nonlinear elastic parameter β. a first signal generator configured to generate a pulsed sinusoidal signal, the pulsed sinusoidal signal having a frequency; a first transceiver disposed in a borehole in the formation and configured to receive the pulsed sinusoidal signal, and to transmit a pulsed sinusoidal acoustic signal in accordance with the received pulsed sinusoidal signal; a second transceiver disposed in the borehole and configured to receive the pulsed sinusoidal acoustic signal and to generate a first electrical signal therefrom; one or more microprocessors configured to receive the first electrical signal and to time reverse the first electrical signal to generate a time-reversed electrical signal; a second signal generator configured to receive the time-reversed electrical signal, and to direct the time-reversed electrical signal to the second transceiver, the second transceiver being further configured to transmit a second acoustic signal in accordance with the time-reversed electrical signal, whereby the second acoustic signal forms a focal volume centered on the first transceiver, the second acoustic signal including second and third harmonics of said frequency, the first transceiver being further configured to produce a second electrical signal based on reception of the second acoustic signal, the second electrical signal having an amplitude; and memory storage configured to store the amplitude of the second electrical signal and/or information derived from the amplitude of the second electrical signal, wherein a hysteretic nonlinearity parameter α relating to hysteresis strength is determined based on the amplitude of the third harmonic of the second electrical signal, a nonlinear elastic parameter β relating to acoustoelasticity is determined based on the amplitude of the second harmonic of the second electrical signal and the pore pressure in the formation is determined based on the hysteretic nonlinearity parameter α and the nonlinear elastic parameter β.