We present here precise measurement of the resonance frequency, lifetime and shape of confined acoustic modes in the tens of GHz regime in GaAs/AlAs superlattice planar and micropillar cavities at low temperature ($sim 20,textrm{K}$). The subharmonic resonant pump-probe technique, where the repetition rate of the pump laser is tuned to a subharmonic of the cavity resonance to maximize the amplitude of the acoustic resonance, in combination with a Sagnac interferometer technique for high sensitivity ($sim 10 ,textrm{fm}$) to the surface displacement, has been used. The cavity fundamental mode at $sim 20,textrm{GHz}$ and the higher order cavity harmonics up to $sim 180,textrm{GHz}$ have been clearly resolved. Mechanical Q-values up to $2.7 times 10^4$ have been measured in a planar superlattice, and direct spatial mapping of confined acoustic modes in a superlattice cavity micropillar has been demonstrated. The Q-frequency product obtained is $ sim 5 times 10^{14}$ demonstrating the suitability of these superlattice cavities for optomechanical applications.
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