Because of the disunity of resolution and exploration range in applied geophysics, the effective medium theory (EMT) should be developed to help us to understand the geological microstructure. We extended the EMT to complex permittivity in high frequency, and calculated the imaginary part of effective complex permittivity of composite as the effective conductivity using Finite-Difference Time-Domain (FDTD) numerical method. The result we obtained is the intrinsic property of the equivalent medium, which has explicit geological signification to satisfy the requirement of interpretation.

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With the aggravation of electromagnetic radiation pollution, it is urgent to develop green, lightweight, ultra-thin and high-performance electromagnetic interference shielding materials to eliminate unnecessary electromagnetic interference; however, the construction of wood-based high-performance electromagnetic shielding materials by simple methods remains a challenge. Based on the layer-by-layer assembly strategy, a lightweight Ni/Wood/Ni composite (NWNC) with an interlayer structure was constructed by a simple electroless plating method using natural wood as a substrate for electromagnetic interference shielding. The synthesized NWNC has a smooth surface, and its minimum surface roughness is only 8.34 μm. After 15 min of electroless nickel plating, the contact angle (CA) of NWNC with an ultra-thin nickel layer (65 μm) was 118.3°. When the thickness of the nickel layer is only 0.102 mm, the conductivity can reach 1659.59 S/cm when the three electroless nickel plating time is 15 min. In the L-band, the electromagnetic shielding effectiveness can reach 94.1 dB after three times electroless nickel plating for 20 min. This is due to the conductive loss, magnetic loss and interface polarization loss generated by the electromagnetic network constructed by the nickel layer, which makes the composite material produce an electromagnetic shielding mechanism dominated by absorption. The L-band absorption efficiency can reach 39.01 dB, and due to the porous structure of the original wood, the multiple reflection and absorption inside the wood further lose the electromagnetic wave. This study provides a low-cost and simple method for the design of light, ultra-thin and efficient controllable wood-based electromagnetic shielding materials and has broad application prospects in the fields of construction and aerospace.

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