Skipping a seismic refraction line when you are dealing with the Glen Rose limestone is a gamble that backfires fast. In Austin, we have seen contractors punch through a hidden void or a dissolved cavity that a standard boring missed entirely. The Balcones Fault Zone does not give second chances. That is why our team runs both refraction and reflection tomography right from the first site visit. We do not guess at bedrock depth. We measure travel times with a 24-channel seismograph. For deeper targets, we combine the data with a MASW survey to verify the Vs30 profile. If the project requires pile support, the velocity cross-sections feed directly into the pile design parameters. In karst terrain, every foot of blind drilling is a risk you do not need.
In the Balcones Fault Zone, a 2D velocity cross-section often reveals a 50-foot lateral shift in bedrock that a grid of borings would miss.
Methodology and scope
Local considerations
The difference between West Lake Hills and the Colorado River floodplain is not just elevation. It is a completely different seismic velocity model. In the hills, limestone velocities exceed 12,000 ft/s within 10 feet of the surface. On the river terraces, unconsolidated gravel and clay show less than 3,000 ft/s down to 40 feet. A single refraction line across a site that straddles these two units captures the transition. Without it, the foundation design assumes uniformity that does not exist. The worst case we see is a structure bridging the contact between hard Edwards limestone and soft Taylor clay. Differential settlement cracks appear within two years. Seismic tomography maps the contact precisely. We also flag low-velocity zones that indicate solution cavities. In Austin, those cavities are common. A collapse during excavation is a safety incident. A velocity survey prevents it.
Explanatory video
Regulatory framework
The project adheres to ASTM D5777-18 for seismic refraction, ASCE 7-22 for site classification, and IBC 2024 Chapter 16.
Other technical services
P-Wave Refraction Tomography
2D profiling for bedrock depth, rippability, and fault mapping. Hammer or weight-drop source. 24- or 48-channel acquisition.
S-Wave Refraction Tomography
Vs profiling for site class determination per ASCE 7. Horizontal geophone array with polarized strike plate source.
Reflection Seismic Profiling
High-resolution CDP reflection surveys for deep stratigraphy and karst cavity detection beyond 200 ft depth.
Typical parameters
Frequently asked questions
How deep can a seismic refraction survey see in the Austin area?
With a standard 240-foot spread and a weight drop source, we typically image 60 to 100 feet of penetration in limestone. In softer alluvial soils along the Colorado River, the depth of investigation drops to 30 to 50 feet due to lower velocities and attenuation. For deeper targets we switch to reflection methods.
What is the difference between MASW and refraction tomography?
MASW uses surface waves to produce a 1D Vs profile directly below the array center. Refraction tomography uses body waves to produce a 2D cross-section of P-wave or S-wave velocity. We often run both on the same spread. MASW gives the site class. Refraction gives the geometry of the bedrock surface and lateral velocity changes.
Do you need a boring to calibrate the seismic data?
Yes. A velocity model alone gives you a geophysical boundary. We always recommend at least one borehole or test pit to tie the seismic velocity to a specific lithology. A limestone with 10,000 ft/s and a cemented gravel with the same velocity are different materials. The boring provides the ground truth.
What does a seismic tomography survey cost in Austin?
Can seismic reflection detect karst cavities under a building footprint?
Yes. High-resolution reflection with a close geophone spacing can image air-filled or clay-filled cavities as small as 3 feet in diameter at depths up to 100 feet. We see a polarity reversal and a velocity pull-down beneath the cavity. This is a standard method for pre-construction karst screening in Austin.