Detection of subsurface defects (e.g., delamination, cracking) using microwave/radar sensors (e.g., ground penetrating radar or GPR) is an important and promising technique for the effective and efficient maintenance of civil infrastructure. In this technique, reflected and scattered electromagnetic signals are typically collected and used for interpreting the size and property of subsurface damages. The objective of this paper is to investigate the feasibility of using finite measurements in the reflected signal for size estimation, through the geometric analysis of waveform. Simulated transient electromagnetic response was generated by finite difference time domain (FDTD) methods in two dimensional domain. A modulated Gaussian impulse at a center frequency of 3.5 GHz was used as the source. Rectangular delaminations with a width ranging from 3.048 cm to 16.256 cm and a thickness of 0.762 cm were considered. The depth of subsurface delamination was also studied. The curvature of reflected waveforms, obtained by three measurements, was used to correlate with the width of subsurface delamination. A relative width parameter was defined and used in the proposed equations for estimating the delamination width with less than 10% error. It is found that the relative width parameter is linearly proportional to the difference in waveform curvature. The proposed approach is potentially applicable to other subsurface defects with different shapes.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.