Seven-step approach for complete near-surface resistivity survey of farmland with LandMapper

Despite numerous EC-mapping case studies conducted in many countries, only a few studies have demonstrated a complex approach to electrical geophysical site survey. In most studies only one technique of EC-mapping, either EM, GPR or four-electrode method was employed. This is understandable since most commercially available EC/ER measuring equipment operate in limited range of resistivities and depths (1-2 manufacturer-set depths are typical). Purchasing different equipment for each application to measure EC/ER at multiple depths/scales quickly raises the cost of such surveys above the budget of most agricultural, environmental or archeological survey firms and agencies as typical commercially available geophysical devices cost more than 10,000$ per unit. But, LandMapper ERM-02 cost less than 2,500$. Four-electrode probes are custom-made to any specific depth from a few cm to 10 meters and cost less than 100$ each. To further decrease the cost of such a system user can make their own probes from materials available in any hardware store.

Here we present a complete 7-step methodology of ER-mapping and vertical electrical sounding to aid in agro-reclamation mapping. The detail description of this approach can be found in (Golovko and Pozdnyakov, 2009; Kokoreva et al., 2007). All the proposed measurements of soil electrical parameters both in the field and laboratory can be carried out with only one hand-held device, LandMapper ERM-02, and interchangeable probes.

1. Study available soil maps and landscape of the survey area and select locations for a few complete vertical electrical soundings (VES down to 5-10 m).
2. VES of major soils on the territory of survey.
3. Electrical mapping of the territory with 2-5 four-electrode probes sensing specific key depths selected after VES interpretation.
4. Preparation of electrical survey maps in GIS.
5. Selection of key soil pits on the territory of survey based on electrical maps and measurement of electrical parameters on the walls of soil pits. Collection of soil samples from the layers with contrasting electrical parameters.
6. Measurements of electrical parameters and soil chemical/physical properties of samples in laboratory.
7. Transformation and interpretation of field soil survey with the support of laboratory tests and pedotransfer functions.

Seven-step approach is illustrated below in a case mapping project of intensively cultivated potato field near Moscow. Maps of electrical resistivity at four layers were prepared with Surfer and ArcMap software (Fig. 4) in step 4. Next, 10 soil pits were dug out on the survey field in places exhibited the most contrast in electrical resistivity between soil horizons.

Electrical resistivity and other soil properties were measured in soil samples collected from characteristic soil horizons in step 5. Exponential relationships between ER and clay content, filtration coefficient, field capacity and field soil moisture were obtained in step 6 (Fig. 5). Electrical soil properties influencing density of mobile electrical charges are exponentially  related to apparent soil electrical resistivity according to Boltzmann’s Law (Pozdnyakova, 1999; Pozdnyakov et al., 1996; Pozdnyakov and Pozdnyakova, 2002; Pozdnyakov et al., 2006).

Figure 5: Exponential relationship between field ER and clay content of soil samples from different soil horizons.

Finally, using obtained exponential relationships, the field ER maps were transformed into maps of soil physical properties in step 7. Figure 6 shows map of clay content for 480 cm depth. Result of the study was map of redistribution of water and nutrients within the field, which was used by farmer as an aid for site-specific fertilizer applications.

Figure 6: Maps of soil physical properties at 480 cm depth created with non-destructive geophysical ER mapping: clay content map (a); map of redistribution of water and nutrients in landscape (b).

Locations