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Vertical Spatial Sensitivity and Exploration Depth of Low-Induction-Number Electromagnetic-Induction Instruments

^a U.S. Geological Survey, 520 N. Park Ave., Tucson, AZ 85719
^b Hydrology and Water Resources, Univ. of Arizona, Tucson, AZ 85721
^c PetRos EiKon, Inc., 222 Snidercroft Rd., Concord, ON L4K 2K1, Canada

View larger version (58K): [in this window] [in a new window]   Fig. 1. Low-induction-number frequency-domain electromagnetic-induction instruments, two coils with an intercoil separation (s). One coil transmits (Tx) the primary magnetic field and one coil receives (Rx) a combination of the primary and secondary fields. Coils may be oriented either horizontally or vertically with respect to the ground surface as indicated by directional arrows.

View larger version (20K): [in this window] [in a new window]   Fig. 2. Cumulative sensitivity and relative depth to interface between layers (McNeill, 1980). Approximations are for vertical (VCP) and horizontal (HCP) coplanar coil orientations. McNeill's "effective depth of exploration" is indicated by a cumulative sensitivity value of 0.3. Cumulative sensitivity of low-induction-number (LIN) instruments is calculated from infinity to a given relative depth. Relative depth is depth divided by intercoil separation.

View larger version (26K): [in this window] [in a new window]   Fig. 3. Each set of cumulative-sensitivity simulations was based on a two-layer soil. Electrical conductivity of the upper layer in soil model Type 1 and the lower layer in soil model Type 2 were fixed at 0.1 mS m–1 for the electrically resistive case or 100 mS m–1 for the electrically conductive case. The variable electrical-conductivity layer was given values ranging from 0.1 to 200 mS m–1. For a given two-layer soil model in each set of simulations, the thickness of the upper layer increased from 0.001 to 18 m. Tx indicates transmitter coil; Rx indicates receiver coil; and s is the intercoil separation.

View larger version (30K): [in this window] [in a new window]   Fig. 4. Cumulative sensitivity for soil model Types 1 and 2 calculated from forward numerical simulation results compared with low-induction-number (LIN) approximation: (A) horizontal (HCP) and (B) vertical (VCP) coplanar coil orientation. Relative depth is depth divided by intercoil separation. The first number in brackets is the upper layer electrical conductivity, the second number is lower layer electrical conductivity, both in mS m–1. In (B), to show underlying LIN VCP approximation curves, the lines for the VCP [0.1/0.2] and [0.2/0.1] soil models were removed.

View larger version (34K): [in this window] [in a new window]   Fig. 5. Soil model Type 1 cumulative sensitivity using the higher electrical conductivities sometimes detected in environmental investigations. Comparison of results of forward numerical simulation with the low-induction-number (LIN) approximation: (A) horizontal (HCP) and (B) vertical (VCP) coplanar coil orientation. Relative depth is depth divided by intercoil separation. The first number in brackets is the upper layer electrical conductivity, the second number is that of the lower layer, both in mS m–1.

View larger version (33K): [in this window] [in a new window]   Fig. 6. Soil model Type 2 cumulative sensitivity using the higher electrical conductivities sometimes detected in environmental investigations. Comparison of results of forward numerical model simulation results with the low-induction-number (LIN) approximation: (A) horizontal (HCP) and (B) vertical (VCP) coplanar coil orientation. Relative depth is the depth divided by the intercoil separation. The first number in brackets is the upper layer electrical conductivity, the second number is the lower layer electrical conductivity, both in mS m–1.

View larger version (19K): [in this window] [in a new window]   Fig. 7. Relation between apparent electrical conductivity and effective depth of exploration: comparison of simulated and low-induction-number (LIN) approximation results. VCP, vertical coplanar coil orientation; HCP, horizontal coplanar coil orientation. Effective depth of exploration is the depth divided by the intercoil separation.