Seismic Tomography for Site Investigation in Mildura

Q: What is the typical cost for a seismic refraction survey in Mildura?

Longer lines or multi-spread roll-along surveys scale accordingly, and we provide a fixed-price quote after a site walk.

In Mildura, the contrast between the Murray River floodplain and the underlying Blanchetown Clay creates a tricky target for any geophysical survey. We often see projects where standard drilling misses lateral changes—a sand channel that will daylight into an excavation, or a calcrete layer that looks solid on the log but is actually only 300 mm thick. That is exactly where seismic tomography pays off. By shooting a spread of geophones across the site and picking first arrivals from multiple shot points, we build a velocity model that shows you the real geometry: where the clay thickens, where the sand lenses pinch out, and how deep you need to go to find competent material. For deeper targets, seismic reflection gives us a continuous image of stratigraphy well below 30 metres, something that is critical when you are designing deep foundations or assessing basin structure.

A velocity model from seismic tomography gives you a cross-section you can hand directly to the geotechnical engineer—no interpolation guessing between boreholes.

Our approach and scope

Mildura sits on the Murray Basin, a sequence of unconsolidated Tertiary and Quaternary sediments that can vary from loose aeolian sand to stiff, over-consolidated clay over just a few hundred metres. The semi-arid climate means the near-surface is often dry and cracked, which complicates geophone coupling—our field crew uses plastered spikes and checks coupling with tap tests on every spread before shooting. We typically run both P-wave and S-wave lines: P-waves for water table and bedrock mapping, S-waves for stiffness profiles that feed directly into site classification per AS 1170.4. The data processing pipeline includes delay-time analysis for statics correction, tomographic inversion with a starting model built from borehole control, and ray-coverage diagnostics so you know exactly which parts of the model are well-constrained. When the survey is adjacent to existing structures, we combine the velocity model with MASW surface-wave data to extract Vs30 without the depth limitations of a single-method approach.

Site-specific factors

Mildura's older commercial buildings sit on shallow footings that were designed without any subsurface velocity data. When a new structure goes up next door, vibration from excavation and compaction can reactivate old paleochannel margins—we have seen differential settlement of 15 mm in a single week on a site where a buried sand lens was missed by borehole-only investigation. Seismic tomography is the tool that catches those lenses before the excavator does. The Murray Basin also contains zones of dispersive clay; identifying them spatially matters for earthworks planning, and the velocity contrast between dispersive and non-dispersive clay is often detectable when we cross-plot P-wave and S-wave tomograms. For any project within 200 metres of the river, we also map the fresh-saline groundwater interface by its seismic velocity signature—a parameter that influences both excavation stability and long-term concrete durability.

Technical parameters

Parameter	Typical value
Method	Seismic refraction and reflection tomography (P- and S-wave)
Typical line length	46–230 m (single spread); longer lines by roll-along
Geophone spacing	1–5 m depending on target resolution
Depth of investigation (refraction)	Approximately 1/5 to 1/4 of spread length
Depth of investigation (reflection)	50–300 m with accelerated weight drop; deeper with larger source
Source type	Sledgehammer on plate, accelerated weight drop (ESS100), buffalo gun for S-waves
Data format deliverables	SEG-2 raw files, DMT TomoLab or Res2DInv inversion models, PDF cross-sections, CSV velocity picks
Reporting standard	AS 1726 geotechnical investigation format with tomographic cross-sections appended

Complementary services

Refraction Microtremor (ReMi) integration

We extract a 1D Vs profile from the same refraction spread using surface-wave analysis, giving you a Vs30 estimate without a separate MASW line—useful on tight urban sites in Mildura's CBD.

Rippability assessment

Seismic P-wave velocity is correlated with rock mass rippability per Caterpillar D10R charts; we deliver a rippability contour map for your earthworks contractor, showing where to switch from ripper to hammer.

Void and cavity detection

Tomographic inversion highlights low-velocity anomalies that may indicate solution cavities in the Murray Basin limestone or old collapsed drains—we flag these with XYZ coordinates for targeted drilling follow-up.

Crosshole seismic tomography

For critical structures, we run crosshole surveys between pairs of cased boreholes, achieving resolution down to 0.5 m—ideal for checking grout curtain integrity or pile shaft condition below the water table.

Quick answers

Can seismic tomography replace boreholes on a Mildura site?

No, and we never recommend it. Tomography gives you a continuous velocity cross-section, but you still need at least one borehole to calibrate the velocity-to-material conversion. We use borehole logs to assign lithology to velocity ranges—without ground truth, the model is just velocities. The smart approach is to drill two or three boreholes along the seismic line and use them as control points for the inversion.

What is the typical cost for a seismic refraction survey in Mildura?

How long does a seismic tomography survey take from start to final report?

Field acquisition for one or two spreads is usually completed in a single day. Processing and preliminary interpretation take three to four working days. If you need the velocity cross-section urgently for an excavation decision, we can send a draft section within 24 hours of demobilisation. Final signed reports with all deliverables typically go out within seven working days of the field work.

What depth can seismic reflection reach in the Murray Basin sediments?

With our accelerated weight drop, we typically image reflectors down to 80–120 metres in the Murray Basin, depending on the clay-sand layering. Where the target is deeper—say the top of the Renmark Group—we bring in a larger source and can extend the section to 250 metres. The key limitation is the seismic attenuation in dry, loose near-surface sand; we mitigate that with closely spaced shots and careful static corrections.