EXAMPLE 8
A protected particle was formed of a base particle having an average diameter of 20 μm. The base particle is a hollow sphere having a shell thickness of 1 μm. The interior pressure in the hollow cavity of the base particle is 14.7 psi. The density of the base particle is 0.38 g/cc and the crush strength of the base particle is 1,500 psi. The outer surface of the base particle was coated with a PLGA by suspension deposition. The coating thickness was 18 μm. The protected particle had a crush strength of about 8000 psi. The protected particles were subjected to well conditions of 30,000 ppm brine solution at a pH of 7.5, a temperature of 90° C., and under 6,000 psi hydrostatic pressure. After about 12 hours and over the period of 2 hours thereafter, the base particles were fractured or crushed. The acoustic sound or emission created by the fractured or crushed base particles had a traceable harmonic resonant frequency peak at 3,000 Hz. As is evident from Examples 7 and 8, the protected particle can be tailored by using different sized base particles to create a certain frequency or range of frequencies when the base particle fractures or crushes.
