Deep beneath a quiet valley in the Western Italian Alps, a slab of once-buried lower continental crust lies exposed like the cross-section of a geological sandwich. For more than a century, the Ivrea–Verbano zone has drawn geoscientists from around the world because it offers one of the most complete exposed transitions from the lower continental crust into the uppermost mantle anywhere on Earth. Now, an international team has taken a decisive step toward drilling directly into this remarkable rock record, using high-resolution seismic surveys to peer beneath the surface before the drill bits ever start turning.
The work, published in the journal Scientific Drilling, describes a series of active seismic experiments carried out in the Ossola Valley in support of the International Continental Scientific Drilling Program project known as DIVE, short for Drilling the Ivrea–Verbano zonE. Led by Andrew Greenwood of the University of Lausanne and Montanuniversität Leoben, together with colleagues from Switzerland, Italy, and Austria, the campaign was designed to characterize the subsurface at two proposed borehole sites, one near the town of Ornavasso and another at Megolo di Mezzo. The planned boreholes, each roughly one kilometer long, will penetrate the upper reaches of the lower continental crust, a zone that normally lies hidden many tens of kilometers beneath our feet.
The scientific stakes are considerable. Drilling is the only way to recover continuous fresh rock cores from the deepest, non-exposed portions of the Ivrea crustal section, and the DIVE project aims to answer long-standing questions about the composition, structure, and physical and chemical properties of the lower continental crust. The project also intends to explore potential microbiological niches in these deep rocks, probing for life in one of the planet’s most extreme rock environments. Phase I of DIVE targets two complementary sections of the lower crust, while a future phase II in the neighboring Sesia Valley aims even higher, with plans to drill through the crust–mantle boundary itself, the Moho.
Before any of that could happen, the team needed to know exactly what lies beneath the valley floor. Surface geology is mapped in exquisite detail in the Ivrea–Verbano zone, and the steep topography allows some downward extrapolation of structures, but the researchers needed to bridge a stubborn gap in scale between regional geophysical images, which resolve features on the scale of kilometers, and the centimeter-to-meter scale information that borehole instruments will eventually provide. Active seismic reflection surveying, in which a controlled seismic source sends waves into the ground and geophones record the echoes that bounce back from subsurface layer boundaries, was the tool of choice.
Conducting the surveys, under the project name MicrO-SEIZE, was no simple matter. The Ossola Valley is a deep glacial valley flanked by steep mountain slopes inclined at roughly 25 to 30 degrees and cloaked in dense vegetation. The valley floor hosts pastures, the Toce River, major roads, railways, and water canals, all of which constrain where instruments could be placed and all of which generate anthropogenic noise that contaminates seismic recordings. Over 12 days in June 2019, the team deployed a 12,247-kilogram EnviroVibe vibrator truck, sweeping frequencies from 12.5 to 150 hertz over 12 seconds at each source point. Geophones with a resonant frequency of 15 hertz were planted along roads, disused highways, and open fields, connected to cabled acquisition systems totaling hundreds of channels.
To cope with the complex, steeply dipping geology, the team employed a crossed-array method, combining a central receiver line oriented across the strike of the target structures with shorter orthogonal crosslines. This arrangement provided sparse three-dimensional coverage and allowed the researchers to capture reflections dipping obliquely to the main profile. Where sources and receivers were closely co-located, the seismic fold, a measure of how many independent measurements contribute to each image point, reached values as high as 70, and up to 195 at the center of the Ornavasso primary profile. A supplementary ultra-high-resolution survey, dubbed nanO-SEIZE, used a 35-kilogram portable vibrator and geophones spaced just 1.5 meters apart to characterize the immediate vicinity of the Ornavasso drill site right up to an adjoining cliff face.
The processing chain was rigorous and iterative. After vibroseis correlation, trace editing, filtering, deconvolution, and velocity analysis, the team built site-specific velocity models for each profile. Refraction analysis of first-arrival times revealed a simple two-layer near-surface structure: a thin veneer of dry sediments, only about 5 to 10 meters thick, overlying water-saturated sediments with an average velocity of 1,520 meters per second. Constant-velocity stack tests refined the stacking velocities, and a velocity gradient of 0.00167 per meter proved capable of coherently imaging horizontal reflectors down to roughly 500 meters. For the Megolo profile, which runs nearly parallel to the valley walls and to the dipping rock fabric, the team went further and constructed a full three-dimensional velocity model, using a representative metagabbro velocity of 7,000 meters per second derived from field samples and published laboratory measurements.
The results delivered a striking picture of the valley itself. Along the transect running from Premosello–Chiovenda toward Megolo, the seismic sections revealed a wide, U-shaped glacial valley carved into crystalline bedrock, with the trough centered beneath the middle of the Ossola Valley. The base of the Quaternary glacial and glacioflacial sediments sits at approximately 550 meters depth, equivalent to about 300 meters below sea level. Within the sedimentary fill, the reflectors are far from simple: irregular, onlapping layering points to thick proglacial lake sediments preserved from older glacial cycles, along with evidence of paleochannels left behind as the Toce River meandered across the valley floor. At the two drill sites themselves, the sedimentary overburden was found to be up to 50 meters thick, and the surveys prompted a practical adjustment: the Ornavasso borehole location has been moved roughly 20 meters closer to an exposed bedrock cliff to minimize the amount of soft sediment the drill must pass through and to better target the axis of the tightly folded Massone Antiform.
Equally important was what the surveys revealed about the drilling targets themselves. At Megolo, where an inclined borehole is planned to cross the flank of the broad Proman Anticline, the seismic data confirmed the east-southeast dip of the metagabbroic rocks at roughly 20 degrees and guided the planned borehole inclination of 15 to 20 degrees from vertical, ensuring the hole will cross the rock layering as orthogonally as possible and reach the deepest structural levels achievable. At Ornavasso, the target is the hinge of the narrow Massone Antiform, and a near-vertical borehole is planned. Critically, the surveys found no evidence of major geological drilling hazards: no shallow-dipping reflectors indicative of large open-fracture systems, fault zones, or cataclastic zones were observed in the crystalline basement at either site, although the geometry of the surveys means that steeply inclined faults cannot be entirely excluded.
The surveys also exposed a fundamental challenge of imaging crystalline terrains. The Ivrea lower crustal rocks have very low acoustic impedance contrasts between conformable layers, with reflection coefficients below 0.05, so the basement rocks return almost no coherent specular reflections. Only the gently dipping fabric of the Proman Anticline, where the target rocks lie close to the surface, could be directly imaged. Elsewhere, the team relied on secondary indicators such as diffraction tails from steeply dipping irregular surfaces and sharp terminations. With the borehole geometries now determined and agreed upon, drilling can proceed, and the seismic database will serve as an essential reference for interpreting the forthcoming borehole logging results, complemented by two years of passive seismic monitoring from the DIVEnet network of 12 stations surrounding the sites. What began as a practical site-characterization exercise has thus produced a detailed portrait of a glacial valley and its hidden bedrock, setting the stage for humanity’s closest look yet at the deep roots of the continents.
Subject of Research: Active seismic characterization of drilling targets in the Ivrea–Verbano zone for the ICDP DIVE continental drilling project
Article Title: Active seismic surveys for drilling target characterization in Ossola Valley: International Continental Scientific Drilling Program (ICDP) project Drilling the Ivrea–Verbano zonE (DIVE) phase I
Article References: Greenwood, A., Hetényi, G., Baron, L., Zanetti, A., Müntener, O., & the MOS field team (2024). Active seismic surveys for drilling target characterization in Ossola Valley: International Continental Scientific Drilling Program (ICDP) project Drilling the Ivrea–Verbano zonE (DIVE) phase I. Scientific Drilling, 33(2), 219-236. https://doi.org/10.5194/sd-33-219-2024
Image Credits: AI Generated
Keywords: Ivrea–Verbano zone, scientific drilling, ICDP, seismic reflection, lower continental crust, Ossola Valley, DIVE project, glacial sediments, Moho, geophysics, borehole characterization, Western Alps
News Source: Violet Maxwell. (October 9, 2026). Seismic Surveys Map Hidden Valley Floor Before Drilling Into Earth’s Deep Crust. Scienmag.
