PREFACE Ground~motion studies conducted as Project 6.5 of Operation Ivy yielded useful information of limited scope because complete data were not recovered. It was considered desirable to extend and supplementthe data fromOperationwguwith several close-in, ground-motion measuring stations (Project 1.7). df Operation Castle The data from Project 6.5 of Operation Ivy have been delayed in processing. Consequently, it is convenient and pertinent to combine the reports on the original and supplementary studies as Chaps. 1 and 2 of a single report.