The blasts from the seismic survey were detectable by GeoNet as very small local ground tremors along the seismic line last week:

                   PRELIMINARY EARTHQUAKE REPORT

                            GNS SCIENCE
                         GeoNet Data Centre
                      Lower Hutt, New Zealand
                      http://www.geonet.org.nz

       The following earthquake has been recorded by GNS Science:

       Reference number:        3511346/G
       Universal Time:          12 May 2011 at 10:24
       NZ Standard Time:        Thursday, 12 May 2011 at 10:24 pm
       Latitude, Longitude:     41.15°S, 175.38°E
       Location:                10 km south-east of Featherston
       Focal depth:             0 km
       Richter magnitude:       2.3

       Web page: http://www.geonet.org.nz/earthquake/quakes/3511346g.html

Man-made explosion as part of a science experiment in lower North Island                                    
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 Here is a video in which Stuart Henrys explains the seismic survey :