Application of Resistivity Method Wenner-Schlumberger Array to Evaluate Brantas River Embankment as Avoid Landslide

The maintenance of embankment is important to avoid some damages like landslide. Brantas river passes 6 cities, include Mojokerto. Landslide of Brantas embankment was occured at Lengkong and Mlirip, Mojokerto. The location of landslide is near from crowded highway. Therefore evaluation using resistivity method is needed to identification vulnerability of landslide. There are 4 line of data acquisition. First, suspected there are seepage at point 25-35 m and fracture at point 65-75 m from dam. Then, second line, there is a suspect of fracture at point 65-90 m and seepage at point 110-125 m. Third line, at point 20-35 m and 110-140 m are suspected as fracture and at point 55-75 m is suspected as seepage. The last line is indicated that the embankment was good.

I. INTRODUCTION 1 Embankment foundation along the river is useful to protect human life [1]. Maintenance of embankment is important to keep the function of river properly. Body of embankment can suffer some damages that caused by landslide at embankment slope, leakage of embankment foundation, and internal erosion [2]. All of the damages caused by some factors like porosity of the material, seepage, fracture, and overburden at slope of the embankment [3].
Brantas is the second biggest river in Java. One of region that passed by Brantas is Mojokerto [4]. The failed function of embankment cause the landslide was occured at Lengkong and Mlirip, Mojokerto. The location of landslide is near from crowded highway. Therefore evaluation is needed to identification vulnerability to landslide. The evaluation using geophysics survey. One of the method is resistivity. Fundamental of electrical resistivity method is injecting current to the ground and measuring potential difference using potential electrode around current injected [5]. The evaluation using Wenner-Schlumberger array because the research expected obtain the result in lateral and sounding to find sructure of the subsurface.
The research at Brantas, Mojokerto itself has never done before. Researchs about the landslide of embankment have been done by [6] that discussed about scouring as the potency of lanslide at Bengawan Solo embankment used GPR method, [7] that discussed about GPR and resistivity method to evaluated the safety level of embankment, Aitsebaomo et al [2] that written the utilization of electromagnetic method for erosion surveying at embankment, and [8] discussed about investigation and monitoring internal erosion and seepage anomaly at embankment using resistivity method. Hopefully from this research can obtained efective result so that it can investigate the potency of landslide in Brantas embankment at Lengkong and Mlirip Mojokerto. Therefore the research can give information to local society and service to be more cautious to landslide of the embankment.

II. METHOD
The research to evaluate Brantas embankment at Lengkong and Mlirip Mojokerto has been done using resistivity method Wenner-Schlumberger array. The research use four line. The result that obtained from the measurement is resistance value which then calculated to obtain resistivity value as data analysis. Resistivity value is processing by Res2dinv software. The characteristics of subsurface can be known and analysed by compare the resistivity value from inversion result to table reference that describe resistivity value of some material of the earth and data logging of research location.

III. RESULTS AND DISCUSSION
Based on processing data using Res2dinv has been obtained three section. First section is called measured apparent resistivity pseudosection, second section is calles calculated apparent resistivity pseudosection, and the third section is called inverse model resistivity section. First line is described by Fig. 2 below. From inverse model, first line has range of resistivity value 2,22-199 ohm-m. This value (comparing with table reference and data logging) indicates that materials of subsurface consist of water, sand, sand-gravel, and sandclay. The anomaly at first line occured at point 35-45 m (A-B) that suspected as seepage of the water because it has lower resistivity value. The other anomaly occured at point 60-75 m (C-D) that suspected as fracture which consist of air. The inversion result using resistivity then compared by inversion using another method i.e VLF which is done by Realita [9]. Inversion of VLF shown by

IV. CONCLUSION
Based on the research in the line 1,2, and 3 there are suspects of seepages and fracture consists of air. The embankment of these three line can be considered less worthy or less secure so that have high potency of landslide. Meanwhile, fourth line is suspected no fracture or seepage. Therefore these embankment of fourth line still feasible or in a good condition.