Practical modifications to improve the sledgehammer seismic source

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.