Abstract
On 4 September 2010, a MW 7.1 earthquake struck the Canterbury region of the South Island of New Zealand. Widespread liquefaction caused major damage to many structures, including the flood-control stopbanks along the lower reaches of the Waimakariri and Kaiapoi Rivers. Additional damage occurred during the subsequent MW 6.2–6.3 earthquakes of 22 February and 13 June 2011. Repeated LiDAR surveys indicated that up to 1 m of subsidence occurred in places. Visual inspections identified areas of significant damage, which have been repaired. However, internal damage to the stopbanks cannot be recognized by visual inspection. Thus electromagnetic (EM) and ground penetrating radar (GPR) surveys were undertaken.A pilot study was completed upstream of the confluence of the two rivers, along the northeast segment of the Waimakariri stopbanks and the southwest section of the Kaiapoi stopbanks. A complementary horizontal loop EM (HLEM) survey was carried out in advance of the GPR surveys. The HLEM measurements were done with the instrument oriented both parallel and perpendicular (transverse) to the stopbanks. Anomalous HLEM responses were noted at one location; subsequent GPR surveys indicated a change in the style of stopbank construction and repair, and possibly some internal cracking. HLEM readings were also taken at high and low tide levels along the tidally-influenced lower Kaiapoi River, and significant differences were observed. Finally, during the surveys, a surface crack was observed at one location, and a GPR line across that site suggested that the crack extended to depth.The results were complemented by velocity analyses using subsurface diffractions, and velocity variations were noted along the lengths of the stopbanks. The velocity changes appear to be broadly correlated with changes in the HLEM conductivity, which is not unexpected given the effects of water content and clay content on both the electrical conductivity and the GPR velocity.
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