Using terahertz (THz) time-domain techniques, we measure the relative coupling and scattering characteristics of surface plasmon-polariton waves for rectangular cross-section grooves on a thick metal foil. This is accomplished by using a single subwavelength aperture surrounded by one or more annular grooves. A unique aspect of the measurement technique is that it allows one to discriminate between the contributions of each groove in the transmitted THz waveform. The measurements are obtained by using structures that contain either a single annular groove, for coupling measurements, or two concentric annular grooves, for scattering measurements. The basic approach should be extendable to other spectral ranges and applicable to different forms of surface electromagnetic waves. Using the aforementioned measurements, we demonstrate the ability to alter the pulse shape of the transmitted waveform using a single aperture surrounded by four annular grooves of varying dimensions. In addition to the value of this capability for pulse shaping applications, the basic technique allows for greater control of the transmission resonance lineshape related to enhanced transmission through a single subwavelength aperture.