By Alfonso R. Analfino • Giuseppe Decaro • Francisco J. Gutiérrez

The coastal and seabed environments surrounding Sicily are layered with geological complexity and millennia of human history. In a recent high-resolution sub-bottom profiling survey conducted in Porto Empedocle harbor, high-resolution acoustic data were acquired that reflect this dual legacy. The work was part of a project encompassing underwater archaeological surveys for archaeological risk assessment in preparation for the creation of a route for a submarine pipeline near Porto Empedocle.

Using a GeoAcoustics GeoPulse Compact sub-bottom profiler paired with a Trimble Applanix POS MV WaveMaster inertial navigation system, the team mapped the stratigraphy of the harbor seabed with high resolution. The data set revealed soft silt deposits draped over Pliocene-aged gray clay from the Monte Narbonne Formation. A strong acoustic return at approximately 22-m depth—anomalous in character and potentially anthropogenic—was of particular interest.

This article outlines the tools and methodology employed, the workflows developed for data acquisition and processing, and a representative example from the data set. The Porto Empedocle survey demonstrates how modern remote sensing technologies, when applied with geological and historical awareness, can support both environmental monitoring and the safeguarding of submerged cultural heritage.

Tools and Methodology

Survey operations were conducted aboard Neptune 1, a 6.5-m workboat configured for high-resolution geophysical investigations. The primary system used was the GeoPulse Compact sub-bottom profiler, chosen for its ability to resolve fine sedimentary layering in shallow marine environments. Positioning and motion compensation were provided by a POS MV WaveMaster system, integrating RTK-GNSS, gyrocompass, and MRU sensors for centimeter-level accuracy. Data acquisition was managed using QPS Qinsy version 9.6.2 and Geo Marine Survey Systems GeoSuite Acquisition, with post-processing performed in GeoSuite AllWorks. Additional tools—including AutoCAD Map 3D, Blue Marble Geographics Global Mapper, and Golden Software Surfer—supported mapping and spatial data handling. Power on board was supplied by a portable Honda EU22i inverter generator.

Survey methodology followed best practices for harbor sub-bottom profiling. Survey lines were planned to ensure full coverage of the proposed pipeline corridor. Acquisition parameters were adjusted in real time to match seabed conditions. Prior to acquisition, all systems were calibrated and time-synchronized to guarantee alignment between acoustic, motion, and positioning data.

Setup, Deployment and Configuration

Mobilization took place over two days in June 2025. On June 10, the vessel was launched, and all instrumentation was installed, tested, and calibrated. Data acquisition began the following day and was completed within the same operational window.

Neptune 1 was selected for its compact size and maneuverability, enabling precise navigation within the harbor and along the planned corridor. All survey systems—including the POS MV, GeoPulse Compact, and antennas—were installed on a rigid pole, deployed over the side of the boat to minimize mechanical offsets and reduce motion artifacts.

The POS MV WaveMaster was configured with dual GPS antennas (2-m separation) to ensure heading accuracy, and its IMU was aligned relative to the vessel’s center of gravity and transducer. Offsets were configured in POSView and Qinsy software to maintain consistency across coordinate systems and enable real-time compensation of roll, pitch, heave, and yaw.

The GeoPulse Compact was pole-mounted and submerged to a consistent draft of 0.5 m. Pre-survey trials were conducted to optimize source power, gain and firing rate based on local sediment properties. Survey lines were spaced at 5-m intervals and oriented parallel to the shoreline, as requested by archaeological oversight, ensuring high lateral resolution and full coverage.

 

 

Pre- and Post-Survey Accuracy Checks

Accuracy validation was integral to both mobilization and demobilization. The POS MV, equipped with a GPS azimuth measurement system (GAMS), provided stable and accurate position and attitude data throughout. GAMS was particularly critical in the nearshore environment, where wave-induced heave can degrade sub-bottom data quality. Real-time heave compensation significantly enhanced signal clarity and interpretability.

Before acquisition, figure-eight maneuvers were performed to calibrate heading and verify antenna alignment. During the survey, continuous monitoring ensured roll, pitch, yaw and heave remained within operational thresholds. Post-survey analysis of navigation and motion logs confirmed consistent accuracy, with no drift or latency. The process delivered subdecimeter horizontal and vertical accuracy, meeting the resolution requirements for archaeological assessment.

Operating in a low-noise environment is essential for high-resolution profiling. The EU22i portable generator provided clean, true-sine wave power with minimal electromagnetic interference. During quiet periods (0 percent transmit volume), the GeoSuite Acquisition software’s power spectral density display enabled noise baseline assessment and confirmed system installation quality. This ensured the profiler operated at peak sensitivity, maximizing data quality and interpretability.

Online Data Optimization

During acquisition, optimal system configuration was achieved through iterative testing informed by prior knowledge of the local environment and real-time data quality monitoring. The GeoPulse Compact sub-bottom profiler was configured in chirp mode, operating across a 1- to 18-kHz frequency range. This setup offered excellent vertical resolution, enabling clear identification of sedimentary structures while maintaining good penetration—essential for interpreting buried features in shallow marine environments.

Real-time visualization in GeoSuite Acquisition allowed the team to assess signal strength, noise levels and seabed response. Parameters such as waveform type, volume output, and ping interval were fine-tuned accordingly. The software’s ability to display power spectral density during “listening” periods proved especially useful for identifying environmental or onboard noise sources. This online optimization process ensured a high signal-to-noise ratio and data fidelity throughout the survey.

Data Processing Workflow

Post-survey, sub-bottom profiler data were processed using GeoSuite AllWorks, an integrated software platform designed for high-resolution, single-channel seismic analysis. Initial quality control included seabed tracking, envelope scaling, and filtering to remove low-frequency noise (e.g., 50-Hz interference) and frequencies beyond the system’s 18-kHz range. An amplitude envelope and automatic gain control were also applied to enhance reflector visibility.

GeoSuite’s real-time layback correction feature ensured precise positioning of the seismic source and receiver relative to the GPS antenna, compensating for dynamic offsets. The dual-channel acquisition capabilities, though not exploited in this survey, are designed to improve horizontal resolution or reduce noise when using multiple sources. Additional spatial and projection data were managed through the software’s built-in coordinate reference system library and compatibility with external cartographic layers (e.g., bathymetry, satellite overlays). These tools allowed for effective visualization, navigation verification, and eventual integration of processed lines with geological interpretations.

The Geology

The harbor of Porto Empedocle is located along the southern coast of Sicily, within a geologically complex area influenced by its position at the southern edge of the Hyblaean Plateau and near the Gela-Caltanissetta Foredeep Zone. This transitional setting between the Apenninic-Maghrebian orogenic belt to the north and the African Continental Shelf to the south has given rise to a diverse and tectonically active stratigraphy. The region is marked by compressional and extensional structures, with a prominent northwest-southeast fault separating units with distinct depositional histories.

The stratigraphy observed in the area reflects alternating marine, transitional and continental depositional environments. At depth, the sedimentary sequence includes Pre-Miocene basement units, such as Mesozoic limestone forming the substrate of the Hyblaean carbonate platform. These are overlain by Lower to Middle Miocene globigerina marks and limestone (Ragusa and Tellaro Formations), deposited in a medium- to deepwater marine setting during the early phases of the Miocene transgression.

Late Miocene sequences, particularly those associated with the Messinian Salinity Crisis, are represented by the Gessoso-Solfifera Formation—composed of laminated and saccharoidal gypsum with interbedded clay and calcarenite. These layers are often identifiable in seismic profiles as strong, continuous reflectors.

The Pliocene is marked by pelagic gray and bluish clays (Trubi and Monte Narbonne Formations), which are rich in microfossils and serve as excellent stratigraphic markers. These units form much of the local coastal relief, including features such as the nearby Scala dei Turchi cliff. A gradual transition to more turbidite facies is also observed in this interval.

Shallower layers, from the Pleistocene to the Holocene, consist of bioclastic calcarenite (e.g., Marsala and Terranova Formations), as well as marine terraces, lagoonal silts, coastal sands, and recent alluvial deposits—reflecting glacial sea level cycles and deltaic-littoral processes.

Sub-bottom profiler data confirmed the presence of a well-layered surficial cover of soft silts and sands, typically 1- to 10-m thick, resting atop a more rigid substrate of Pliocene clay or calcarenite. Internal reflectors often appear horizontal or gently undulating, in some cases suggesting erosional paleo-surfaces or submerged paleo-channels formed during lower sea levels in the Pleistocene.

An Anthropogenic Anomaly?

Among the notable features identified in the seismic profiles was a high-amplitude reflector at approximately 22 m below the seabed. Its geometry—marked by clear boundaries, localized convex shape and stratigraphic discontinuity—raises the possibility of anthropogenic origin.

In archaeological terms, the Agrigento Coast was part of ancient Akragas and has yielded numerous submerged artifacts, including Greek and Roman shipwrecks, amphorae, and port infrastructure. Given sea level rise since the Holocene, the anomaly’s depth suggests it may lie within a paleo-environment that has been submerged for more than 8,000 years.

Three plausible interpretations include: a buried shipwreck, possibly Greek or Roman, fully encased in sediment; some structural remains of a submerged port—such as a quay, breakwater, or foundation platform; or a pre-Holocene coastal settlement now buried beneath marine transgressive layers.

While these hypotheses are speculative, the anomaly exhibits acoustic signatures consistent with man-made objects, such as high reflectivity, acoustic shadowing and deviation from natural stratification. Full confirmation would require targeted archaeological investigation, including broader sub-bottom profiler coverage and, potentially, direct sampling or excavation.

Survey Challenges

Sub-bottom profiling in coastal and harbor settings presents numerous operational challenges, particularly when seeking both high resolution and sufficient penetration. The success of the Porto Empedocle survey relied on carefully managing these trade-offs.

A key challenge was optimizing frequency selection, with the chirp mode (1 to 18 kHz) offering the best compromise between vertical resolution and sediment penetration. Adjusting pulse length, gain settings (especially time-varying gain), and trigger intervals required field testing and active monitoring to maintain consistent quality across varying substrates.

Environmental conditions also posed limitations. Surface turbulence and propeller-induced vibrations can reduce signal clarity, particularly in soft or gas-rich sediments. Careful control of survey speed, transducer immersion depth and data windowing were essential in overcoming these effects.

Instrument synchronization and offset calibration were critical to maintaining spatial accuracy. The POS MV’s integration with GAMS helped mitigate heading and heave uncertainties, while software configuration ensured alignment between the transducer, IMU, and GNSS antennas. Acoustic interference from nearby systems or vessel electronics was minimized through frequency management and by using low-noise power from an inverter generator.

Altogether, the survey required constant adjustment and quality control, but with careful tuning and experienced operation, high-quality stratigraphic data were successfully obtained.

Conclusion

The high-resolution sub-bottom profiling survey conducted in the harbor of Porto Empedocle successfully met its dual objectives: mapping sedimentary stratigraphy in a geologically complex coastal environment and supporting archaeological risk assessment ahead of infrastructure development. The use of the GeoPulse Compact system, combined with precision positioning via the POS MV and real-time monitoring through GeoSuite software, enabled the collection of clean, high-fidelity seismic data despite the operational challenges of a shallow, dynamic marine setting.

The resulting imagery provided clear identification of stratified Holocene and Pleistocene deposits and confirmed the presence of underlying Pliocene clay from the Monte Narbonne Formation. These geological units not only align with regional stratigraphy but also revealed features such as paleo-surfaces and possible buried channels indicative of past sea level changes.

The detection of a strong, geometrically distinct reflector at approximately 22 m below the seabed raises the possibility of an anthropogenic structure, such as a buried wreck or submerged port element. Further investigation is needed to confirm its nature, and this finding underscores the value of sub-bottom profiling in coastal archaeological assessments.

The survey demonstrates how modern marine geophysical methods—when carefully configured, optimized in the field, and integrated with geological and archaeological context—can produce actionable insights for environmental monitoring, heritage protection, and engineering planning in sensitive nearshore areas.

References

For a full list of references, contact Alfonso Ricardo Analfino at alfonso@geonautics-srl.com, Giuseppe Decaro at g.decaro@geonautics-srl.com or Francisco J. Gutiérrez at francisco.gutierrez@geoacoustics.com.

Alfonso Ricardo Analfino co-founded Geonautics srl in 2008 and has served as technical director and sole director since 2011.

Giuseppe Decaro has been part of the Geonautics srl team for several years, engaged in data acquisition and analysis for environmental studies, coastal monitoring, and onshore and nearshore projects.

Francisco J. Gutiérrez is a product specialist at GeoAcoustics Ltd., where he was previously a product manager.