a method of seismic investigation using the swept impact seismic technique involves measuring the response of a site to be investigated to a series of controlled impact sequences applied at different points of the site , each sequence using impacts generating pulses of energy at 100 - 2500 hz ., the impacts of the sequence having a repetition frequency swept between frequencies in the range of 10 to 500 hz ., the end frequencies having a ratio of at least 1 . 5 to 1 , and the number of impacts in the sequence being at least 100 . the invention also extends to apparatus for implementing such a method , comprising an electrically - actuated hammer having an impact frequency variable through a range of at least 1 . 5 : 1 within the range 10 to 500 hz ., and generating impacts characterized by substantial energy within a frequency range of 100 - 5000 hz ., a drive circuit for energizing the hammer to deliver a sequence of at least 100 impacts having a repetition frequency swept through a ratio of at least 1 . 5 to 1 , and means for repositioning the hammer between sequences .