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A novel scheme using multi-wavelength coherent detection for SNR enhancement in a Brillouin optical time domain reflectometer (BOTDR) is presented. The probe pulse of the BOTDR contains multiple wavelengths with small equal-wavelength spacing, and the Brillouin scattering light waves of the multiple wavelengths beat with the corresponding local reference light waves with the same equal spacing to generate overlapped multiple Brillouin scattering spectra. The overlapped scattering spectra can be detected by a conventional photodetector. A three-wavelength heterodyne detection BOTDR is demonstrated and it provides 4.2 dB SNR enhancement compared to a conventional single wavelength heterodyne detection system.