Practical modifications to improve the sledgehammer source

This is the publisher's version, also available electronically from "http://onlinelibrary.wiley.com".We have examined frequency and amplitude changes in high-resolution seismic-reflection data associated with practical modifications to the sledgehammer method. Our seismic data, acquired at three sites with different near-surface geology, demonstrate the effects of seating the plate prior to recording, of centered versus noncentered impacts, of subsurface plate emplacement, of various plate-surface covers, and of aluminum versus steel impact plates. Impacts on well-seated plates produced as much as 4 dB higher seismic amplitude than data recorded using unseated plates, and increased the ratio of high-to-low frequencies. Sledgehammer impacts on the edge of the plate decreased seismic amplitude by 6 to 12 dB for frequencies above 100 Hz compared to centered impacts. Placement of the impact plate 1 meter below the ground surface produced a 12 dB amplitude increase for frequencies above 130 Hz at one test site. Plates made of either steel alloy or aluminum produced equivalent seismic signals. The site-dependent nature of some of our results suggests that other investigators may benefit from conducting similar experiments prior to acquiring shallow seismic-reflection data when using the sledgehammer source