From Individual Gathers to Source-Composite Inversion in Active MASW: A paired field comparison of 8kg hammer and gun acquisition protocols

Prince Larbi Akor , Nelson Foli, Ruth Araba Tawiah Annan, Emmanuel Teye Mensah, Edmund Okoe Amartey, Voitey Aku Essinam, Naomi Tawiah Quashie, Judith Tansi Addy, Jeffery Nelson, Emmanuel Kwame Nuamah

In multichannel analysis of surface waves (MASW), the suitability of an active source is evaluated by its ability to generate coherent and continuous surface-wave dispersion over a usable frequency and wavelength range, as these characteristics influence the reliability of mode identification and the attainable depth of investigation. We present a hierarchical paired comparison of hammer and gun acquisition protocols using 13 matched source positions and identical 12-receiver geometry (5m spacing, 55m aperture and 30m nearest offset). All 26 SEG-2 gathers were processed individually using the same phase-shift imaging and automated ridge-extraction criteria, followed by independent manual fundamental-mode picking to verify the automatic interpretation before acceptance and inversion. The verified individual gathers were then combined through normalized common-offset trace stacking and normalized dispersion-image stacking for each source. Every gather yielded an accepted fundamental-mode path. At the individual-gather level, mean maximum usable wavelength was 30.14m for the hammer and 29.45m for the gun (gun minus hammer = −0.69m; 95% paired bootstrap interval −3.81 to 1.50m). The hammer contained a greater proportional 4.5–10Hz spectral fraction (0.967% versus 0.672%), but this exploratory contrast did not remain significant after Holm correction [14]. Source-composite curves spanned 21.0–46.0Hz for the hammer and 20.5–43.5Hz for the gun, with maximum wavelengths of 32.14 and 33.41m, respectively. The stacked gun-minus-hammer wavelength difference was 1.27m (paired spatial-bootstrap 95% interval −1.39 to 2.50m), equivalent to upper screening depths of 16.07 and 16.71m under a 0.5 wavelength convention. Fixed-layer conditional inversions produced overlapping shear-wave velocity intervals in all four model layers; gun-minus-hammer differences ranged from −17.6 to +14.1m/s and all 95% intervals included zero. Agreement between independently extracted trace-stack and image-stack curves was 7.16m/s for the hammer and 4.15m/s for the gun. These results show that source ranking is endpoint- and threshold-dependent: the hammer supplied more relative low-frequency energy, whereas the gun stack gave a slightly longer wavelength, but neither a reliable depth advantage nor a distinct velocity structure was resolved. The proposed individual-to-stack consistency framework, strengthened by manual verification of automated modal picks, provides a reproducible way to prevent source-energy observations from becoming unsupported claims of deeper MASW investigation