APPARATUS AND METHOD FOR COMBINED ACOUSTIC AND SEISMOELECTRIC LOGGING MEASUREMENTS

A method for estimating porosity of an earth formation from measurements of acoustic energy traversing the earth formation and from measurements of seismoelectric voltages generated in the formation in response to the acoustic energy. The method includes the steps of measuring the acoustic energy traversing the earth formation and measuring said seismoelectric voltages generated in response to the acoustic energy traversing the formation. A seismoelectric signal is synthesized from the measurements of the acoustic energy using an initial value of the porosity. A difference is determined between the synthesized seismoelectric voltages and the measured seismoelectric voltages. The initial value of porosity is adjusted, and the steps of synthesizing the seismoelectric voltages from the acoustic signal, determining the difference, and adjusting the value of porosity are repeated until the difference drops below a predetermined threshold or the difference reaches a minimum value. The adjusted value of porosity which results in the difference being at the minimum is taken as the formation porosity. A particular embodiment includes estimating conductivity of fluid in the pore spaces of the formation by calculating the synthetic seismoelectric voltages using an initial value of conductivity; determining a difference between the synthetic seismoelectric voltages and the measured seismoelectric voltages; and adjusting the initial value of conductivity, and repeating the steps of calculating the synthetic seismoelectric voltages, determining the difference and adjusting the value of conductivity until the difference reaches a minimum.