Borehole Radar (BHR) uses ultra-wideband electromagnetic (EM) waves to
image discontinuities in formations. It has been a major bottleneck to
extend BHR applications to obtain a clear and high-resolution radar profile
in a complex and noisy environment, which increases ambiguity in the geology
interpretation. To avoid this increased ambiguity in the geology
interpretation, we proposed a scheme based on the empirical mode
decomposition (EMD) and complex signal analysis theory to process the BHR
data with low signal to noise ratio (SNR). The scheme includes four steps.
First, the original radar profile is pre-processed to avoid mode confusion
and noise interference to the radar echo. Next, the EMD method is used to
process a single-channel radar dataset and to analyze the frequency
components of the radar signal. Various intrinsic modes of the
pre-processing radar profile are also obtained by using EMD. Finally, we
reconstruct the intrinsic mode profile, which contains information about the
formation, calculate the complex signals of the reconstructed radar profile
using the Hilbert transform, extract the three instantaneous attributes
(instantaneous amplitude, instantaneous phase, and instantaneous frequency),
and draw the separate instantaneous attributes profiles. This processing
scheme provides both the conventional time-distance profile also in addition
to the three instantaneous attributes. The additional attributes reduce
ambiguity when evaluating the original radar profile and avoid the deviation
relying solely on a conventional time-distance profile. An actual radar
profile, which was obtained by a BHR system in a limestone fracture zone, is
used to verify the effectiveness of instantaneous attributes for improving
interpretation accuracy. The results demonstrate that the EMD method is
superior in processing the BHR signal under a low SNR and has the capability
to separate the high-low components of the radar echo effectively.
