Seismic Refraction & Reflection Tomography in Arlington, TX

IBC Chapter 16 and ASCE 7-22 require site-specific shear wave velocity data when Site Class determination from SPT alone is insufficient. In Arlington, the interbedded limestone, marl, and shale of the Cretaceous Eagle Ford and Woodbine Formations produce sharp velocity contrasts that standard MASW surveys can miss. Seismic refraction and reflection tomography map these interfaces with precision—critical where karstic voids and solution cavities in the Austin Chalk correlate directly to differential settlement risk. The 32.7°N latitude and hot summers demand attention to trigger timing and geophone coupling in dry clays. We deploy 48-channel seismographs with vertical and horizontal geophones, processing first breaks and reflection hyperbolae through iterative tomographic inversion to deliver 2D velocity cross-sections calibrated against SPT drilling logs and local borehole data.

Seismic velocity in weathered Austin Chalk can drop from 2,800 m/s to 850 m/s within a 3-meter vertical window—miss that transition and you miss the settlement mechanism.

Methodology and scope

A common mistake in Arlington is running a single refraction line parallel to the road and assuming homogeneous bedrock. The Woodbine Formation weathers unevenly, producing sand-rich channels within shale that refract energy differently. We see it in the field: a contractor skips cross-line shooting, and three months later a footing differentials out because a low-velocity lens sat undetected. Our protocol includes orthogonal spreads with hammer and accelerated weight drop sources, capturing P-wave and S-wave arrivals simultaneously. Tomographic inversion resolves velocity gradients—not just layer boundaries—which matters when the weathered chalk transitions to competent limestone over less than 2 meters. For deeper targets exceeding 30 meters, reflection processing of the same dataset images the contact with the underlying Grayson Marl. Combining these results with grain-size analysis and Atterberg limits on cuttings from companion borings builds a defensible ground model that satisfies Arlington building officials on first submittal.

Local geotechnical context

The crew deploys a 48-channel Geometrics Geode seismograph with 4.5 Hz vertical and 14 Hz horizontal geophones, connected via takeout cables laid along a surveyed profile. In Arlington's August heat—surface temperatures regularly exceed 110°F on asphalt—geophone spikes loosen in desiccated clay, degrading coupling and introducing high-frequency noise. Our field lead checks each channel's impulse response before every shot. The real risk is misinterpretation of a low-velocity zone: the Woodbine sand channels can appear as voids on a refraction tomogram if the inversion is not constrained by borehole control. We tie every velocity boundary to actual lithology from test pit logs or SPT refusal depths. Without that ground truth, a refraction model is just colors on a screen. The other failure mode is assuming isotropic velocity in dipping beds—Arlington's regional dip of 1–2 degrees southeast is subtle but enough to skew reflection stacking velocities if not corrected during normal moveout.

Typical values

Parameter	Typical value
Method applied	P-wave refraction + SH-wave reflection
Channel count / spread	48 channels, 2–5 m geophone spacing
Source type	14 lb sledgehammer (refraction), 100 kg AWD (reflection)
Sampling interval / record length	0.125 ms / 1024 ms standard
Tomographic inversion algorithm	Wavepath eikonal traveltime (WET)
Typical investigation depth	15–60 m below ground surface
Output	2D Vp and Vs sections, Poisson's ratio, Gmax profiles
Applicable standard	ASTM D5777-18, ASCE 7-22 §20

Complementary services

2D P-Wave Refraction Tomography

First-arrival traveltime inversion for bedrock mapping and rippability assessment. Deployed for commercial pad sites and pipeline alignments where Woodbine shale depth controls excavation cost.

Combined Refraction + SH-Wave Reflection

Simultaneous P and SH acquisition on the same spread. Delivers Vs profiles for IBC site classification and Poisson's ratio for deformation modulus estimation—standard for Arlington school and hospital projects.

Cross-Hole & Downhole Seismic Verification

Borehole-to-borehole velocity measurements calibrated to surface tomography. Used when the Arlington building official requires direct Vs measurement per ASCE 7-22 §20.4 for Site Class B–D confirmation.

Quick answers

How much does a seismic refraction tomography survey cost in Arlington?

A typical 2D refraction line of 115–230 linear feet with 24–48 geophones runs between US$2,450 and US$4,870, depending on spread length, source type (sledgehammer vs. accelerated weight drop), and whether both P and S-wave acquisition are required. The cost includes field crew, equipment, tomographic processing, and a signed report with velocity sections.

What depth can seismic refraction reach in North Texas limestone?

With a 230-foot spread and an accelerated weight drop source, we routinely image to 50–60 feet in competent Austin Chalk. Penetration drops in dry, highly weathered chalk where attenuation is high; adding reflection processing of the same record extends usable depth by 20–30% without additional field time.

Can seismic tomography detect voids or cavities in Arlington's karstic limestone?

Yes—air-filled voids produce a sharp velocity drop surrounded by a diffraction pattern visible on raw shot gathers. Tomographic inversion resolves the low-velocity anomaly, but confirmation requires a borehole or test pit intersecting the feature. We flag anomalies and recommend targeted verification drilling.