Magnetic anomaly detection (MAD) technology has been widely used in geological exploration, unexploded ordnance exploration, wreck salvage, and other fields because of its attractive advantages such as all-weather implementation, high concealment, and signal propagation in water and soil. The most classic method in MAD is orthonormal basis functions (OBF) detection which can effectively detect the magnetic target in a comparatively low signal-to-noise ratio (SNR) environment. In this paper, attracted by the excellent properties of magnetic gradient tensor (MGT) invariant, the I ₁ -OBF and I ₂ -OBF methods are developed. Considering strong dependence of the I ₂ -OBF method on the angle between magnetic moment vector and displacement vector, and moderate detection performance of the I ₁ -OBF method, an effective invariants-OBF method through adaptively integrating search energy of the I ₁ -OBF and I ₂ -OBF methods is proposed. Simulations and field experiments witnessed a great improvement in the SNR of the proposed method from the perspective of signal energy. It is proved that the proposed method can stably produce promising results even under the shaking-platform condition or the specific geometric relationship between the magnetic moment vector and displacement vector.

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