Following ASCE 7-22 site classification requirements, seismic tomography has become the go-to method for mapping subsurface velocity profiles in Fort Wayne’s complex glacial terrain. The city sits on a thick sequence of Pleistocene tills and outwash deposits left by the Wisconsin glaciation, with the underlying Silurian and Devonian carbonate bedrock often riddled with solution features. When you combine these karst-prone carbonates with the variable thickness of unconsolidated overburden, standard drilling alone rarely gives the full picture. Our team applies both refraction and reflection tomography to image the contact between the glacial drift and the top of rock, which in Fort Wayne can vary from less than 10 feet near the St. Joseph River bluffs to over 100 feet in the buried valleys that crisscross the county. For projects requiring shear-wave velocity profiles, we also integrate MASW surveys directly into the same survey line, giving you a complete Vs30 dataset for seismic site classification without mobilizing a second crew.
Seismic velocity contrasts in Fort Wayne’s glacial terrain often reveal paleochannels and solution features that no amount of conventional drilling would detect in isolation.
Our approach and scope
Local geotechnical context
The glacial stratigraphy beneath Fort Wayne presents a specific risk that seismic tomography is uniquely positioned to address: abrupt lateral changes in stiffness across short distances. You can have dense lodgement till on one side of a property and loose outwash sand on the other, separated by a near-vertical contact that a drilling program on 50-foot centers might completely miss. In the carbonate bedrock, solution cavities and enlarged joints—some exceeding 10 feet in height—create velocity inversions that standard refraction interpretation can overlook if the analyst assumes a simple layered earth model. We routinely run both forward and reverse shots and apply tomographic inversion algorithms that do not require the assumption of increasing velocity with depth, which is critical for detecting these hidden voids. For sites near the Maumee River floodplain, where the water table sits within 5 to 10 feet of the surface, saturated loose sands also raise liquefaction concerns under the design earthquake for the region. Seismic shear-wave velocities below about 600 fps in these sands correlate with a high liquefaction potential index, and we often recommend pairing the tomographic survey with a liquefaction assessment based on the Youd-Idriss framework to quantify the factor of safety for proposed foundations.
Reference standards
ASTM D4428-18, ASTM D7400-17, ASCE/SEI 7-22 Chapter 20, and IBC 2024 Section 1613 are applicable standards for site classification and seismic design procedures.
Complementary services
Refraction tomography for bedrock mapping
Designed for Fort Wayne’s variable drift thickness, this survey deploys 24 to 48 geophone arrays to map the top of the Silurian carbonate bedrock and identify buried valleys or paleochannels that could affect foundation performance. We use tomographic inversion rather than simple delay-time methods, which handles the velocity inversions common in the weathered shale interbeds of the Wabash Formation.
Vs30 profiling and IBC site classification
Using a combination of MASW and refraction microtremor techniques along the same survey line, we deliver shear-wave velocity profiles to 100 feet depth for ASCE 7 site class determination (A through F). This is particularly relevant in Fort Wayne, where soft lake clays and loose outwash sands frequently place sites in Site Class D or E, triggering higher design spectral accelerations than the default Site Class C assumed in the USGS hazard maps.
Typical parameters
Quick answers
How much does a seismic refraction survey cost in Fort Wayne?
How deep can seismic tomography see beneath Fort Wayne’s glacial deposits?
With a standard sledgehammer source and 24-channel array, refraction tomography typically reaches 40 to 60 feet in the dense tills common around Fort Wayne. For deeper targets—such as mapping bedrock at 80 to 120 feet in the buried valleys—we switch to a weight-drop or accelerated projectile source and longer geophone spreads. Reflection tomography can image horizons below 150 feet when the acoustic impedance contrast between the glacial drift and the underlying carbonate bedrock is strong, which it usually is in this part of the Maumee basin.
What’s the difference between refraction and reflection tomography for my Fort Wayne project?
Refraction tomography excels at mapping the top of bedrock and lateral velocity variations in the overburden, making it the workhorse for foundation design and site classification in Fort Wayne’s glacial terrain. It requires that seismic velocity generally increases with depth, though tomographic inversion can handle some velocity reversals. Reflection tomography images actual stratigraphic boundaries—clay-sand contacts, bedrock surface, even solution cavities—and does not require increasing velocity with depth. We often run both on the same spread: refraction for the near-surface velocity model and reflection for deeper structural detail, especially on karst-prone sites near the St. Marys River where voids in the Wabash Formation carbonates are a known hazard.