Self-calibrated method of determining borehole fluid acoustic properties

A method of determining an acoustic parameter of a downhole fluid using an acoustic assembly comprising a plurality of acoustic reflectors each having at least one acoustically reflective surface at least partially immersed in the downhole fluid, the method comprising:

• estimating the acoustic parameter using a value for at least one wave property measured for:
• a first near reflection received at a first acoustic receiver (R1) of a first pulse from a first near reflector located a first distance from the first acoustic receiver, the first near reflection having a first amplitude, the first near reflector having a first impedance value;
• a first far reflection received at the first acoustic receiver (R1) of the first pulse from a first far reflector located a second distance from the first acoustic receiver different than the first distance, the first far reflection having a second amplitude, the first far reflector having a second impedance value;
• a second near reflection received at a second acoustic receiver (R2) of a second pulse from a second near reflector located a third distance from the second acoustic receiver, the second near reflection having a third amplitude, wherein the first near reflector and the second near reflector are physically separate surfaces, the second near reflector has a third impedance value, and the third impedance value is substantially different than the first impedance value;
• a second far reflection received at the second acoustic receiver (R2) of the second pulse from a second far reflector located a fourth distance from the second acoustic receiver different than the third distance, the second far reflection having a fourth amplitude;
• wherein the first pulse, the first near reflection of the first pulse, the first far reflection of the first pulse, the second pulse, the second near reflection of the second pulse, and the second far reflection of the second pulse are each transmitted through the downhole fluid, and wherein estimating the acoustic parameter comprises estimating an acoustic impedance for the downhole fluid using a cross-product ratio, the cross-product ratio ratio being a ratio of a first product to a second product, wherein:
• the first product is a product of the first amplitude of the first near reflection and the fourth amplitude of the second far reflection;
• and
• the second product is a product of the second amplitude of the first far reflection and the third amplitude of the second near reflection.