How deep does an EM field reach? Skin depth sets the answer, and it depends on frequency and ground conductivity.
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skin depth δ (field falls to 1/e ≈ 37%)
$\delta = \sqrt{\dfrac{2}{\omega\mu\sigma}} \approx 503\sqrt{\dfrac{\rho}{f}}$ (metres, with $\rho$ in Ω·m and $f$ in Hz). Lower frequency and more resistive ground both push the field deeper.