## 1 Introduction  
With the rapid development of China's economy and society, the problem of soil pollution is becoming increasingly serious. In total, 82.8% of the polluted soil is contaminated with heavy metals such as chromium (Cr), cadmium (Cd), lead (Pb), copper (Cu), zinc (Zn), and nickel (Ni), and the exceedance rate of Cr is 1.1% [1]. Cr has nine valence states in soil but is mainly present in two stable valence states, trivalent and hexavalent, with hexavalent Cr being 100 times more toxic than trivalent Cr. Hexavalent chromium is highly toxic, non-degradable, soluble, mobile, concealed, and can have an adverse impact on ecological sustainability and human health [2]. When humans consume plants and animals containing hexavalent Cr, it can cause skin irritation [3], allergic dermatitis [4], DNA damage [5], and, in serious cases, cancer [6]. Soil quality is a strategic issue related to the sustainable development of the national economy and the overall progress of society. During the 13 th Five-Year Plan period, China has stepped up its efforts to combat pollution [7]. The Chinese authorities have issued the documents \"Soil Pollution Prevention and Control Action Plan\" and \"13 th Five-Year Plan\" to enhance environmental protection. For this reason, it is essential to research and develop advanced and effective source prevention, contaminated area detection, and remediation technologies for chromium heavy metal soil pollution. The commonly used laboratory methods for monitoring heavy metals in soils include ectopic monitoring methods and in situ monitoring methods [8]. The former is expensive, complex, time-consuming, and destructive to the soil structure; the latter has poor accuracy and resolution and does not allow for quantitative analysis of contaminated areas. Common physical methods for detecting heavy metal contamination sites include ground-penetrating radar [9], electromagnetic [10], resistivity [11], and excitation [12]. Ground-Penetrating Radar (GPR) method is not applicable to the detection of environments containing clay, saline groundwater, and sandy clay strata. If there are structures such as steel reinforcement under the ground, it will also cause multiple strong reflections and result in a confusing and indistinguishable image. The electromagnetic method of measurement requires the emission of a large amplitude of current; electromagnetic noise will also distort the measurement data, weakening the effect of deep detection. The Direct Current (DC) resistivity method has good sensitivity to the contaminated area. However, due to the porous structure of the soil itself, the moisture content and porosity, as well as the different concentrations of contamination, can interfere with the resistivity. Identifying the extent of the contaminated area and the degree of contamination by a single resistivity characteristic quantity is difficult. The dual frequency IP method is a frequency domain-induced polarization method proposed by academician [PERSON] [13]. Dual frequency IP instrument is lightweight, fast, highly accurate, and resistant to interference. The observation parameters of the dual frequency IP method are apparent amplitude frequency and apparent resistivity, which can accurately reflect the extent and properties of the target body through multi-parameter observation. Based on the dual frequency IP method principle, [PERSON] et al. proposed a focused dual frequency IP method over-detection technique at the subsurface full-space three-dimensional level [14; 15; 16]. They conducted theoretical and experimental research on detecting coal mines with water-bearing geology. With the development of the dual frequency IP method theory, some domestic scholars extended it to the site of heavy metal contamination site detection. [PERSON] investigated the effects of different electrode devices and Cole-Cole model coefficients on the induced polarization anomaly of heavy metal contamination sites [17].