Determination of Setback Distances for On-Land Seismic Reflection Survey in Thailand
Explosives are commonly used to generate strong impulsive waves for on-land petroleum exploration. For each exploration area in Thailand, a concessionaire shall submit an environmental impact assessment report that commits minimum distances, or setback distances, between an explosive and various types of structures to a regulatory authority. However, the basis for determining the setback distances is not described and or based on surface mine blasting researches. To establish a guideline for exploration contractors in Thailand, ground vibrations were measured from ten exploration sites across the country which can be grouped into three geology conditions, namely sandy ground, clayey ground and sandy ground with rock boulders. Explosives in this study were buried and packed in boreholes at depths of 9 ~ 23 m, having weights of 1 ~ 4 kg and length-to-diameter ratios of 6 ~ 25. The measured data were used for determining the attenuation of particle velocities over distance and the frequency content of vibrations. The geometric damping was found to be the main cause of attenuation while the material damping was negligible. The attenuation in sandy ground with rock boulders was found to be stronger than sandy ground and clayey ground. Since the dominant frequencies of vibration in each ground type varied over wide range, the frequencies at the 2.5th percentile were used for the conservative sake in the vibration assessment according to DIN 4150. Based the proposed formula, the setback distances between residential buildings and a 2-kg explosive are 53, 149 and 221 m for sandy with rock boulders, sandy and clayey grounds, respectively.