<p>Shallow marine seismic surveys commonly rely on portable seismic sources such as small air-gun and high-frequency sparker sources, each characterized by distinct frequency content and imaging capabilities. While air-guns provide superior penetration depth and structural continuity, sparkers offer high vertical resolution for shallow subsurface imaging. Integrating these complementary datasets to obtain broadband seismic responses remains challenging, primarily due to differences in source wavelets and phase characteristics. This study proposes a phase-consistent broadband seismic synthesis method for a shallow marine coastal area, based on simultaneously acquired small air-gun and sparker data along the same survey line. To ensure physical consistency prior to synthesis, deterministic pre-processing steps were applied, including source wavelet estimation, de-bubbling, de-signature, and band-limited spectral whitening. Phase alignment was then achieved using a convolution-based approach, where the air-gun source wavelet served as a low-frequency-stable reference. Multiple candidate reference wavelets were examined, and the applicable frequency band was selected for synthesis. The resulting broadband seismic section demonstrates improved reflector continuity and enhanced shallow resolution without phase distortion, compared to the individual datasets. Spectral analysis confirms balanced frequency band expansion. The results highlight that phase consistency is a critical prerequisite for physically meaningful broadband synthesis and demonstrate the effectiveness of the proposed method for shallow marine seismic imaging.</p>