Seismic Tomography for Subsurface Imaging in Manchester, NH

Manchester's subsurface is shaped by the Merrimack River and glacial Lake Hitchcock deposits. Thick sequences of varved silt and clay overlie fractured crystalline bedrock of the Nashoba Formation. Depth to bedrock varies sharply across the city. Less than 10 feet near Rock Rimmon. Over 100 feet in the old river terraces near Millyard. This contrast demands a geophysical method that images transitions without assumptions about layer geometry. We apply seismic tomography because it reconstructs velocity fields directly from first-arrival traveltimes. The result is a continuous 2D cross-section of P-wave or S-wave velocity. For site classification under IBC Chapter 16, the average shear-wave velocity in the upper 100 feet is what matters. A MASW survey complements this work by providing a 1D Vs profile. Together they satisfy the requirements of ASCE 7-22 for seismic site response analysis. Our field crew uses 24-channel seismographs with geophone spacings tuned to the target depth.

Velocity inversions in glacial lake clays can mask bedrock depth on conventional refraction spreads. Tomography resolves them because rays bend around the low-velocity zone rather than skipping it.

Methodology and scope

The freeze-thaw cycle in southern New Hampshire affects near-surface velocity by up to 15% between February and April. We schedule surveys when ground coupling is stable. The refraction tomography method records direct and critically refracted arrivals along a spread of vertical-component geophones. Shot points are placed at multiple offsets. The inversion algorithm iterates from an initial 1D gradient model to a final 2D tomogram. Reflection processing extracts the zero-offset section and stacks common-midpoint gathers. Vertical resolution depends on the dominant frequency of the source. A 40-lb accelerated weight drop typically yields 60-100 Hz in these soils. That resolves layers thicker than about 3 feet. The survey generates a raster of velocity cells. We interpret iso-velocity contours for rippability classification and bedrock surface mapping. In karst-prone zones near the Mammoth Road corridor, velocity anomalies below 4,000 ft/s in limestone stringers flag potential cavities. Combining the results with an in-situ permeability test helps assess whether those voids connect to the groundwater system.

Local geotechnical context

The Amoskeag Millyard redevelopment transformed nineteenth-century textile mills into office and residential space. Many foundations bear directly on granite ledges or shallow spread footings placed in the 1880s without subsurface investigation. Adjacent new construction must avoid undermining those historic structures. A single unmapped fracture zone within 15 feet of a footing can destabilize an excavation. Seismic tomography in Manchester detects those fracture zones before shoring design begins. The velocity gradient across a water-filled fracture drops 30 to 50 percent relative to intact rock. That signature is unmistakable on a tomogram. Omitting this survey risks triggering a collapse during drilling for deep excavations. The city's building department reviews geotechnical reports for compliance with the 2015 International Building Code and local amendments. Seismic velocity data also feeds into the site-specific ground motion analysis required for structures assigned to Seismic Design Category D.

Reference parameters

Parameter	Typical value
P-wave velocity in dry sand	1,500 - 3,000 ft/s
P-wave velocity in saturated till	5,000 - 7,500 ft/s
P-wave velocity in competent schist	12,000 - 16,000 ft/s
Typical geophone spacing	5 to 10 ft
Maximum imaging depth (refraction)	100 to 150 ft
Vertical resolution (reflection)	3 to 5 ft
Source type	Accelerated weight drop or sledgehammer

Complementary services

Seismic Refraction Tomography

P-wave and S-wave profiling for bedrock surface mapping, rippability analysis, and stratigraphic layering. Inversion uses traveltime tomography with curved-ray tracing.

High-Resolution Seismic Reflection

Zero-offset and CDP profiling for imaging stratigraphic detail below 50 feet. Suitable for mapping buried valleys and fault offsets in the Nashoba Formation.

Downhole and Crosshole Seismic

Direct measurement of compression and shear wave velocity versus depth in a single borehole or between borehole pairs. Used for Vs30 calculation.

MASW and Seismic Site Classification

Multichannel analysis of surface waves to obtain a 1D shear-wave velocity profile. Classifies the site per IBC Table 1613.2.2 for seismic design coefficients.

Frequently asked questions

What depth can seismic tomography reach in Manchester's glacial deposits?

Refraction tomography typically images to a depth of 100 to 150 feet with a 240-foot spread length in Manchester. The actual penetration depends on the velocity contrast between the varved clay and the underlying till or bedrock. In areas where the clay exceeds 100 feet in thickness, such as near the Merrimack River floodplain, we extend the spread to 360 feet or use a larger energy source. Reflection surveys can image deeper targets if the acoustic impedance contrast is sufficient.

How much does a seismic tomography survey cost for a typical lot in Manchester?

For a standard residential or light commercial lot in Manchester, a seismic refraction tomography survey with one to two profile lines ranges from US$2,540 to US$4,730. The cost varies with the number of geophone channels, total lineal feet surveyed, and whether both P-wave and S-wave data are collected. A site-specific quote is provided after reviewing the parcel geometry and the depth target.

Can seismic tomography detect voids in the limestone units around Manchester?

Yes, it can detect air-filled or water-filled cavities when the cavity diameter exceeds approximately one-fifth of the depth to the cavity. The velocity anomaly appears as a localized low-velocity zone on the tomogram. We correlate these anomalies with borehole control and other geophysical methods when the risk assessment requires it.

What is the difference between seismic refraction and MASW for site classification?

Seismic refraction tomography provides a 2D cross-section of P-wave or S-wave velocity, mapping lateral and vertical changes across the site. MASW produces a 1D shear-wave velocity profile averaged over the array length. For IBC site classification, MASW directly yields the Vs30 value used to assign Site Class A through F. We often run both methods on the same spread to combine the lateral resolution of tomography with the direct Vs measurement of MASW.