Passive seismic approaches have gained traction in hydrocarbon exploration following reports of low-frequency spectral anomalies that may be linked to fluid-bearing subsurface systems. Yet their underlying physics, spatial coherence, and temporal persistence remain contentious. Here we examine these issues using an eight-month continuous dataset recorded by a dense array of 106 three-component borehole stations deployed in the Dehdasht area of the Zagros sedimentary basin (SW Iran).

Dehdasht is situated within the Zagros fold-and-thrust belt, where detachment folding and thrusting are accompanied by pronounced along-strike variability in structural style. This variability is strongly controlled by evaporite-dominated décollement levels, most notably within the Gachsaran Formation. Thick evaporitic successions and complex three-dimensional geometry substantially degrade conventional reflection-seismic imaging, motivating evaluation of passive seismic attributes as complementary observables.

The array operated continuously during 2016–2017, with a dense network spanning ~400 km² at ~2 km station spacing. Each site comprised a triaxial borehole seismometer installed at ~10 m depth and recorded by a Spidernano data logger. After instrument-response removal, STA/LTA-based transient and event detection and excision, and selection of quiescent nighttime interval, we estimated power spectral densities (PSDs) using Welch’s method, computed band-integrated energies, and formed vertical-to-horizontal (Z/H) spectral ratios over 1–5 Hz. Spectral ratios were smoothed using Konno–Ohmachi logarithmic smoothing, and station-level statistics were summarized by the median of daily estimates.

Absolute power maps show a strong spatial maximum co-located with the city of Dehdasht, with amplitudes ~2 orders of magnitude above surrounding levels, demonstrating pervasive anthropogenic contamination that precludes direct hydrocarbon interpretation. The tight correspondence between vertical and horizontal power patterns indicates that cultural sources energize the full three-component wavefield, consistent with broadband urban noise.

By contrast, Z/H ratio maps exhibit laterally variable anomalies that recur across three consecutive ~80-day intervals, indicating quasi-stationary behavior with moderate temporal modulation. Frequency-resolved diagnostics show that elevated Z/H values are primarily associated with diminished horizontal-component energy in the 2–3 Hz band rather than enhanced vertical energy, favoring mechanisms that suppress or attenuate horizontal motion (e.g., site/near-surface effects) over direct reservoir-generated emission.

The resulting anomaly distribution shows no compelling alignment with mapped structural traps or regional Zagros tectonic trends, implying that near-surface heterogeneity, site response, and/or cultural noise dominate the observed signatures. Although the repeatability of the Z/H anomalies confirms the presence of a stable wavefield component, their non-unique origin limits their utility as standalone hydrocarbon indicators. Overall, the study provides a practical framework for quantifying the repeatability of passive-seismic spectral anomalies and highlights the necessity of rigorously separating anthropogenic and site-related contributions from any deeper subsurface signal in structurally complex basins.