## 1 Introduction  
Electrical resistivity tomography (ERT) is a method for determining the electrical resistivity distribution in a volume from discrete measurements of current and voltage made within a volume or on its surface [1]. ERT conception was first proposed by [PERSON] [2] in 1987, and it has been widely used in China since 1990s as a programmable automatic switching controller. Right now, distributed ERT is the most popular. The ERT method is a commonly used geophysical prospecting manner which is widely applied in hydrology, ocean exploration, engineering, and environment geological exploration and investigation [3, 4]. Single-channel ERT has been used for many years [5]. Multi-channel ERT was developed in the past twenty years or so but is not so widely employed. It is extremely suitable for super high density ERT. The ERT in this article refers to a single ERT system. Multi-channel ERT can access all channel data at the same time and can obtain all possible combination tests, therefore it is time saving and has high precision. Simultaneously, it needs more complicated hardware and more fussy inversion calculations. It is normally used in 3D tests with many electrode requests, which may take several hours for one round of testing. Whereas for the ERT applications that do not have too many electrodes, single-channel ERT has widely been employed [5], e.g., long-time ERT monitoring systems with relatively minimal data requirements [4, 6]. Substantially, the ERT system consists of the usual electric logging and electrode switching device. Therefore, it is called a super multi-meter. Distributed ERT is the most efficient ERT monitoring method at present. The electrode switching device (ESD)is an important component of ERT, and it is also a key differentiator between ERT and conventional electrical resistivity. A monitoring instrument has been developed along with the electronic technique. The development of ESD, to a great extent, affects the future of ERT. In a conventional serial testing mode, each testing sample is sent to the main control end of testing circuit for data acquisition based on a predetermined sequence [7]. Distributed ERT significantly decreases the cost of cable-laying because it does not require a cable for each electrode [8]. The parallel ERT instrument [9] began deployment in 2008 [8]. It synchronizes multiple tests in a parallel method, which could save time dramatically [8; 9; 10]. However, effectiveness of the parallel testing needs further investigation [11]. In the past twenty years, research on ERT has mainly focused on operation efficiency based on testing speed and data analysis [12; 13]. However, very little attention has been paid to save the hardware overhead on the ERT system, which means easy maintenance and miniaturization. It is very important to note that the 1D test device is widely used in a variety of fields, such as in soil science [14], ocean erosion [15; 16] and contaminated surveys [17], etc. For 1D tests, research has paid more attention to the test results and little concern has been given to ESD [15; 18; 19; 20; 21; 22]. ERT takes the same ESD with a 2D conventional system and requires four switches for one electrode. In this study, we designed a hierarchical ESD (HESD) for a distributed ERT, which saved three fourths of the hardware overhead for the 1D test. In the HESD, a hierarchical electrode switch structure was used to set it apart from the conventional ESD. An intermediate part, Line 1-Line 4, was added between the test parts (AMNB) and tested electrodes. The elaborate hierarchical structures were designed and rooted before and after the intermediate of the two layers, respectively. HESD only requires one switch for one electrode when conducting a 1D test or an odd layer monitor. Two extra switches for one electrode are required when monitoring all layers. Only three switches were used in total. Comparatively, HESD could save hardware expenditure for switches and corresponding control circuits and lower the power dissipation.