Where is true time zero ?

The accurate setting of the zero time position relative to the ground surface is a parameter which concerns all GPR operators and it is an essential factor for conducting accurate shallow depth measurements in GPR. But where is the true ground surface on our radargrants? This paper examines the setting of the true zero time position. It presents the results of a survey defining where GPR users and equipment manufacturers currently set their zero time position. In field operations it is necessary to place the zero time at a clearly definable and stable location in the early wavelet. This is commonly at either the negative or positive maximum peaks of the first wavelet, or the zero amplitude point between these two peaks. This position needs to be corrected for in subsequent data processing in order to make accurate and reproducible depth or thickness measurements in materials. In reality the precisely calibrated true ground surface position in time is not where is it commonly thought to be located on the early time wavelet, due to the inherent characteristics of the signal generating electronics, the transmitting and receiving antennae and variations in the electrical properties of the investigated medium in close proximity to the antenna. An understanding of the near-field characteristics of a bowtie antenna, coupled with extensive comparisons via empirical observations from numerous coreholes, enables us to predict where the true zero time actually occurs in a pulse GPR system.