Deep magmatic plumbing system beneath Harrat Al-Madinah revealed by borehole seismic ambient noise interferometry

Ambient seismic noise interferometry is increasingly used for crustal imaging, but its application to intraplate volcanic systems remains limited because seismic networks are often sparse, ambient-noise sources are variable, and strong scattering in volcanic terrains makes stable reflectivity retrieval challenging. We analyze 70 days of continuous borehole seismic records from the VORiSA network in Harrat Al-Madinah, western Saudi Arabia, to investigate the crustal structure beneath a large basaltic volcanic field. Eight three-component seismometers deployed at approximately 120 m depth provide stable recordings with reduced near-surface noise. We apply conventional autocorrelation and phase autocorrelation with phase-weighted stacking (PAC–PWS) to the vertical component. Conventional autocorrelation provides an amplitude-preserving reference based on linear correlation and stacking, whereas PAC–PWS reduces the influence of amplitude fluctuations and emphasizes phase-coherent arrivals. Their combined use allows evaluation of reflector consistency across complementary processing schemes. The results identify six principal reflectors above approximately 50 km depth. These include boundaries corresponding to the Conrad discontinuity at approximately 22 km and the Moho at approximately 38 km, consistent with previous seismic studies, as well as additional reflectors near 10 km, within the mid-crust at 10–25 km, and near 46–47 km. Comparison with magnetotelluric imaging suggests that several mid-crustal reflectors may be associated with conductive zones interpreted as possible magma reservoirs. These results indicate that borehole ambient-noise autocorrelation, supported by complementary phase-based processing, can provide useful constraints on crustal structure beneath intraplate volcanic regions.