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A controlled-source electromagnetic method with a synthetic aperture is proposed to improve the ability to detect a deep target body in the subsurface. First, we deduced the formulas of three-dimensional diffusion equations in the frequency domain, and gave the expression of synthetic aperture of the controlled-source electromagnetic array. An amplitude weighting coefficient, a phase shift coefficient, and a synthetic aperture range are selected based on model tests, and then, the optimum value of the error range is determined. A typical geo-electrical model is established, and the electric response, the normalized amplitude of the background field, and the apparent resistivity with and without a synthetic aperture are compared. The results show that a synthetic aperture can focus electromagnetic energy and enhance the electromagnetic response of the target body. This is beneficial when extracting weak anomaly information for a deep geo-electrical body, especially for a resistive target object. Finally, we apply the synthetic aperture to field data for gold ore in northern China, and the amplitude of a deep resistive anomaly is strengthened. This demonstrates that our method can be effectively applied to real field data processing using a synthetic aperture. • Synthetic aperture technique applied to wave electromagnetic field is used to low frequency diffuse electromagnetic field. • The optimized amplitude coefficient and a phase shift coefficient can greatly improve the resolution of deep target body. • Weak anomaly of resistive target body can be strengthened greatly, which is beneficial for exploration of deep oil and gas. [ABSTRACT FROM AUTHOR]