© 2006 European Association of Geoscientists & Engineers 135 Multi-electrode measurements at Thai Binh dikes (Vietnam) Andreas Weller 1* , Tran Canh 2 , Katrin Breede 1 and Nguyen Trong Vu 3 1 Technical University Clausthal, Institute for Geophysics, Arnold-Sommerfeld-Strasse 1, D-38678, Clausthal-Zellerfeld, Germany 2 Institute of Geological Sciences, Vietnamese Academy of Science and Technology, Hanoi, Vietnam 3 Institute of Geophysics, Vietnamese Academy of Science and Technology, Hanoi, Vietnam Received January 2004, revision accepted May 2005 ABSTRACT The province of Thai Binh is located in the delta of the Red River in Vietnam where an extensive system of river and sea dikes protects the population and infrastructure from flooding in the rainy season. The integrity of the river dikes is affected by a diversity of problems. Importantly, various termite species dig their nests in the dikes causing water leakage. Leakage can also occur near sluices and passages. Geoelectrical multi-electrode profiling has been applied to locate defects in the dikes. Surveys carried out on several dikes demonstrated that a combined half-Wenner configuration yields high-resolution images in which termite nests are indicated by resistive anomalies. Petrophysical investigations of soil samples and density logs yield additional information on the dike material. Elsewhere, imaging surveys confirmed the subsurface extent of a buried sluice structure in the dike body. clays in the dike foundation may also affect the stability of the dike body and leakage can occur near sluices and passages. Another problem in Vietnamese dikes is related to the activity of termite infestation and in this case the origin of the defect is usually not directly visible at the ground surface. Fortunately, no severe accidents, such as dike breaks, have occurred in the province of Thai Binh during the last flood and storm seasons. However, the dike segments, Hong Ha I, Hong Ha II, right Tra Li, and left Tra Li (Fig. 1b) are affected by thousands of termite and mouse nests. Seepage effects are observed at hundreds of locations where many kilometres of stone jetty have been destroyed by flooding. Modern methods of dike inspection are urgently needed to assess the stability and the state of dikes. This paper reports the results of geophysical measurements at two dike segments in the province of Thai Binh. The investigations at Hiep Hoa were aimed at the locating of termite nests. A second survey was car- ried out to reveal the extent of an old sluice which is buried in the dike near the village of Cu Lam. TERMITE NESTS IN RIVER DIKES In recent years, the Thai Binh dike system has been improved considerably, but 70% of the total length, including 143 km of river dikes, may be classified as weak segments, some of which are severely affected by termite infestation. Investigations in northern Vietnam have shown that, of the 16 species of termite that have been found in dikes, nine species should be regarded as dan- gerous to the dike body (Vu Van Tuyen 2003). The level of danger caused by a termite nest depends on the depth, the volume of holes INTRODUCTION A network of river dikes having a total length of more than 5000 km covers the northern part of Vietnam along the Red River and its tributaries. The dike system in the province of Thai Binh, which is located 120 km south-east of Hanoi in the Red River delta (Fig. 1a), alone has a total length of 584 km, includ- ing 211 km of river dikes and 152 km of sea dikes. Other dikes run along smaller water courses. During the monsoon period from June to September, total precipitation in the mountainous region in the north-west of Vietnam reaches 3000 mm. These heavy rainfalls cause a rise in the river water-level of up to 13 m above the normal level in some regions. The province of Thai Binh, which has more than 2 million inhabitants, is one of the main rice-producing regions in Vietnam. In the rainy season, most of the flat land is located more than 2 m below the water-level in the main rivers. Therefore, the social and economic development of the province depends on the safety of river dikes. Thousands of people endeavour every year to repair and strengthen the dikes in order to protect their villages, towns and crops against flooding. The water flow in the monsoon period results in strong pressure on the dikes and their foundations. Despite all efforts, damage to the integrity of the dikes is caused by seepage, land- slides or fractures. The instability of dikes results from defects in the dike foundation or in the dike body, and severe problems are often related to dike sections that cross old river-beds, where sandy lenses with higher permeability cause water seepage. Soft * Andreas.Weller@TU-Clausthal.de Near Surface Geophysics, 2006, 135-143