Crosshole seismic testing measures how quickly selected compression and shear waves travel between boreholes at particular depths. To understand crosshole seismic P-wave and S-wave velocities, first-arrival picks, borehole spacing and maximum shear modulus Gmax, begin with the measured travel path and its timing. A reported velocity is a travel-time interpretation, not a direct strength test or an unrestricted prediction of soil behaviour under a foundation.
The practical reading sequence is straightforward: establish which source and receiver were used; confirm their three-dimensional separation at the measurement depth; distinguish a P-wave pick from an S-wave pick; calculate velocity from the appropriate difference in distances and arrival times; then convert a reliable small-strain shear-wave velocity to a shear modulus only when the relevant mass density is known. Crosshole data can be exceptionally useful, but borehole deviation, refraction and ambiguous arrivals change the inference.
This guide explains geotechnical crosshole seismic test reports, travel-time graphs, P and S arrivals, Vs profiles, Vp/Vs ratios, elastic constants, first-arrival ambiguity and the difference between an interval estimate and a resolved underground image. The worked numbers are hypothetical and independently checkable. A reader should finish able to identify a wrong unit, a misleading shortcut or an interpretation that claims more than the low-strain measurement supports.
Quick answer: what does a crosshole seismic test measure?
A crosshole survey positions a seismic source and receivers in separate boreholes and records wave arrival times at defined elevations or depths. The measured separation and wave timing are used to infer compression-wave velocity Vp and/or shear-wave velocity Vs across the interval sampled. The result describes low-strain wave propagation in the tested materials, subject to geometric and wave-path assumptions.
The current ASTM D4428/D4428M-25 crosshole seismic standard describes low-strain horizontal P- and S-wave testing, borehole surveys, instrumentation and effects of refraction and reflections. The standard is a method framework, not a guarantee that every field velocity is correct. Instrument operation, travel paths and interpretation still require professional review.
Why engineers measure waves between boreholes
A soil sample removed from the ground may experience disturbance, stress relief and changes in drainage or confinement. Crosshole testing observes wave propagation within the subsurface rather than relying solely on a specimen taken to a laboratory. It can support low-strain stiffness characterization and comparisons with borehole logs, other geophysical surveys and engineering analyses.
The test does not replace sampling, groundwater measurements or other in-situ investigations. Velocity alone cannot reliably identify the exact geological name of every layer, the complete stress–strain curve, bearing resistance or settlement of a future structure. Its strength lies in adding a measured dynamic property along identifiable travel paths, especially when the interpretation can be checked against direct ground information.
Begin with the geometry, not the plotted curve
A crosshole interpretation depends on the relative position of source and receiver points. Designers may show the boreholes as vertical parallel lines separated by a nominal distance, but real holes can deviate. Their separation at a depth may differ from their separation at the ground surface. A small geometric error propagates directly into an estimated velocity.
Read the borehole plan, surveyed locations and inclinometer or deviation records when available. Check whether the reported travel distance is straight-line, nominal horizontal spacing or a modeled refracted path. A distance without its measurement basis is not enough. An attractive Vs-versus-depth curve can conceal a poor geometric assumption that distorts every calculated point.
What is a P-wave?
A compression or primary wave involves particle motion with a component parallel to propagation. It commonly arrives earlier than an S-wave because Vp is usually higher in ordinary stable elastic earth materials. It can propagate through solids and fluids, but an observed P arrival in soil depends on the combined physical system and its saturation state.
Use Vp for a measured compression-wave velocity, generally in metres per second. Do not confuse it with peak particle velocity, which is a different vibration quantity, often abbreviated PPV. The letter P does not mean pressure measured in kilopascals. Read axis units and the instrument channel labels before comparing P-wave data with another dynamic or seismic report.
What is an S-wave?
A shear or secondary wave is associated with transverse particle motion and the material’s resistance to shear deformation. In geotechnical crosshole work, SH and SV describe different polarizations of shear-wave particle motion relative to the propagation and observation geometry. Such distinctions can matter in anisotropic or complex ground.
Shear-wave velocity Vs is often central to small-strain soil stiffness. A faster Vs, all else equal under the relevant model, corresponds to a larger small-strain shear modulus for a given mass density. However, Vs by itself is not a measured ultimate shear strength. The shear stiffness mobilised by tiny seismic strains differs from stiffness and resistance under large engineering deformations.
Do not identify waves by their arrival order alone
In a clean simple waveform, the P arrival often precedes the S arrival. Yet real recordings contain source vibrations, converted modes, electronic artefacts, tube waves, reflections and ambient noise. A small early feature need not be a reliable P pick, and a larger subsequent peak need not be the first S motion.
Look for source polarization reversals, multiple traces and consistent timing across neighbouring depths. A careful field report explains which features were selected and why. A velocity calculated from the wrong phase may be mathematically tidy while physically wrong. The first job is identifying the right propagating wave, not finding any clear peak.
The origin time matters
Arrival time means time relative to a defined source event. If source activation is not properly recorded, the measured absolute time may include an unknown trigger delay. Crosshole configurations with multiple receivers can sometimes use differential arrivals, reducing dependence on some common timing offsets.
A report should identify the source trigger, receiver channels and instrument sampling interval. Do not compare times taken from traces with different uncorrected time origins as though they were on one clock. An error of a fraction of a millisecond may matter when travel times are only tens of milliseconds. Timing precision should be interpreted relative to the size of the interval being measured.
First arrivals are not necessarily direct arrivals
A first arrival is the earliest identifiable wave energy reaching a receiver. In layered ground, wave energy can travel through a faster layer along a refracted path and reach the receiver before a direct wave through the layer of interest. The first arrival therefore may not have traversed the apparently obvious straight line.
ASTM’s crosshole framework warns of wave refraction near high-velocity contrasts and strong total internal reflections that may distort the direct wave. A report that divides nominal spacing by an unexamined first arrival may assign the speed of a nearby layer to the wrong depth. Wave-path interpretation is a scientific step, not optional polishing.
Horizontal layering is a model assumption
Many crosshole analyses use a local approximation of horizontally layered ground. Real sites can include lenses, sloping interfaces, buried channels, rock fractures and lateral changes. A result at one test depth may sample more than one geotechnical material if the path crosses a boundary or refracts.
The reader should compare the velocity section with nearby borehole stratigraphy and note any unresolved inconsistencies. Agreement with one log does not prove lateral uniformity along the path. A crosshole interval velocity is a path-sensitive estimate, whereas a geological layer assignment requires additional evidence. The distinction becomes especially important in uneven fill or complex natural deposits.
Three boreholes and two receivers can improve an estimate
Some crosshole configurations use one source borehole and two receiver boreholes aligned along a test direction. If the source time or near-source response is uncertain, a difference between two receiver arrival times can estimate velocity over the receiver-to-receiver interval, provided the wave follows the assumed path and is the same identifiable phase in both records.
This approach does not eliminate borehole deviation, refraction or picking errors. It changes the measured interval and some cancellation properties. State which source and receiver pair produced the reported value. A figure labelled simply Vs at five metres depth may hide whether the calculation used one travel time from a source or a difference between two receivers.
Worked example: travel-time differences
Imagine a P- or S-wave identifiable on two receiver traces at depths matched to a source. Receiver A lies 3.0 metres from the source, and receiver B lies 9.0 metres away along an idealised straight path. The corresponding S arrivals are 12 milliseconds and 36 milliseconds after a common source trigger.
The difference in path distances is 6.0 metres, while the difference in arrival times is 24 milliseconds, or 0.024 seconds. The interval speed equals 6.0 divided by 0.024, giving Vs = 250 metres per second. The result refers to the modelled receiver-to-receiver travel interval; it does not prove that the entire 9-metre path has uniform speed.
Why milliseconds must become seconds
A metre-per-second velocity formula requires time in seconds. The example’s 24 milliseconds are 0.024 seconds, not 24 seconds. Dividing six metres by 24 produces 0.25 metres per second, a thousandfold error. Dividing by 0.024 gives 250 metres per second.
This error is easy to introduce when a spreadsheet imports an arrival column measured in milliseconds but the velocity cell assumes seconds. Every worked report should display units alongside the input distance and time difference. A reviewer should be able to reproduce the calculated value without guessing the instrument’s storage convention.
Timing errors can dominate short intervals
If the measured differential arrival is 24 milliseconds, uncertainty of 0.5 milliseconds is already about two percent of the interval. The corresponding approximate relative uncertainty in velocity, before considering distance error and path complexity, is of a similar order because velocity is inversely related to time.
Shorter receiver spacings can produce even smaller time differences, making the same picking uncertainty relatively more important. Increasing spacing can help timing resolution but may cross more complicated geology or strengthen refracted arrivals. Survey design is therefore a trade-off between measurable travel times and the representativeness of the assumed path.
Borehole deviation changes measured distance
Assume the nominal receiver spacing is 6.00 metres, but deviation surveys show a 6.10-metre separation at the depth of interest. Using 24 milliseconds gives a corrected idealised velocity of 6.10 divided by 0.024, or approximately 254.2 metres per second. The nominal calculation gave 250 metres per second.
The difference of about 1.7 percent comes entirely from geometry, before considering picking or material heterogeneity. Deviation correction should therefore not be dismissed as an administrative footnote. The nominal plan distance and the surveyed downhole distance are different measurements, and only one may correspond to the relevant travel path.
Boreholes must have compatible depth references
A depth of ten metres below ground in one borehole is not necessarily the same elevation as ten metres below ground in another if the collar elevations differ. Local ground levels, survey datum and drilling records must be reconciled. Inclinometer deviations also change the positions of the source and receivers in three-dimensional space.
Read whether the plotted depth is below each collar, below a common datum or a corrected elevation. A crosshole profile assembled from inconsistent references could pair wave paths at different geological horizons. Similar-looking numbers do not guarantee that the instruments faced the same layer. Correct elevation matching is part of a correct seismic interpretation.
Low-strain stiffness is not foundation failure strength
Seismic tests operate at very small dynamic strains compared with many geotechnical failure and settlement processes. A modulus inferred from Vs describes an initial or maximum shear-stiffness regime under simplifying assumptions. Soil stiffness generally changes with strain amplitude, stress state and loading history.
Using Gmax directly as a constant stiffness for large settlement or earth-pressure movements may overestimate resistance. Designers often require appropriate stiffness degradation relations or calibrated constitutive models. The geophysical report should state the strain regime; an engineering model should then justify how that measurement informs its own loading scenario.
From Vs to small-strain shear modulus Gmax
For a material adequately approximated by elastic wave theory, small-strain shear modulus is related to mass density and shear-wave velocity by Gmax = rho multiplied by Vs squared. Rho is mass density in kilograms per cubic metre and Vs is metres per second, so Gmax is in pascals, equivalent to newtons per square metre.
Suppose the hypothetical soil has rho = 1,850 kilograms per cubic metre and Vs = 250 metres per second. Gmax = 1,850 multiplied by 250 squared = 115,625,000 pascals, approximately 115.6 megapascals. This is a modulus estimate for the stated density and wave velocity, not a test showing the soil resists 115.6 MPa of shear before failure.
Why density is not a harmless input
If Vs is measured but density is assumed, the modulus inherits uncertainty in the density assumption. Bulk soil density depends on water content, void ratio and composition. A value that would be plausible for one soil may be inappropriate for another, particularly for very loose, organic or heavily compacted material.
A report should identify whether mass density came from a measured field or laboratory property, an accepted engineering estimate or a generic assumption. Do not confuse dry density with total bulk mass density under the relevant wave-propagation conditions. A table that reports Gmax to four decimal places while assigning density only approximately can overstate precision.
Vs has a squared influence on Gmax
Because Gmax is proportional to Vs squared, a ten percent increase in shear-wave speed, with density held constant, increases inferred modulus by a factor of 1.1 squared, or 1.21. That is a 21 percent increase. Conversely, a ten percent decrease yields 0.9 squared, or 0.81, an approximately 19 percent decrease.
These nonlinear effects matter when comparing noisy velocity picks or selecting design values. A small systematic bias in arrival time or distance can have a larger percentage effect on modulus. Readers should inspect measured Vs uncertainty before accepting narrow differences in derived stiffness as evidence of different ground conditions.
Vp and Vs together can inform elastic ratios
For an ideal isotropic linear elastic continuum, the ratio of compression- to shear-wave velocity relates to Poisson’s ratio. If r = Vp/Vs, a common relationship is nu = (r squared minus two) divided by twice (r squared minus one). This formula is valid only within its theoretical assumptions and for suitable measurements of the same effective material condition.
Take Vp = 800 metres per second and Vs = 250 metres per second as a teaching example. The ratio is 3.2 and its square is 10.24. The inferred nu is 8.24 divided by 18.48, approximately 0.446. It would be incorrect to call that value a direct measurement of long-term drained soil Poisson’s ratio without considering pore fluid, frequency and the assumed elastic response.
Water affects P-wave interpretation
Compression waves in saturated soil can be strongly influenced by pore-fluid compressibility and soil–fluid interaction. The observed Vp may be much less diagnostic of dry soil skeleton stiffness than a reader assumes. The same material can show different P-wave behaviour with saturation and measurement conditions.
A high Vp should not automatically be translated into a hard rock identification or a high large-strain bearing capacity. Compare water conditions and borehole logs. Dynamic elastic parameters inferred from both Vp and Vs need an understanding of the material model; a simple formula is not a substitute for checking saturation and the relevant engineering loading regime.
The limitations of dynamic Young’s modulus
Within the same ideal linear elastic approximation, Young’s modulus E relates to G and Poisson’s ratio through E = 2G(1 + nu). Using the hypothetical Gmax of 115.6 MPa and nu of about 0.446 gives a dynamic E of roughly 334 MPa.
That computed quantity describes an idealised low-strain elastic parameter. It should not be substituted directly for a static load-test modulus or a settlement design parameter without adequate correlation and strain-level adjustment. The equations are mathematically consistent under their assumptions but do not establish that those assumptions hold for a heterogeneous, saturated and strain-dependent soil mass.
Why a shear-wave velocity profile can vary with depth
Confining stress usually changes with depth, and different geological materials have different elastic properties. A measured Vs profile may rise gradually, jump across a boundary or show a low-velocity layer below faster ground. Such variations are evidence of wave propagation differences, not automatically a complete identification of soil type.
A lower Vs at one depth might reflect less stiff material, a different effective stress state, disturbance or anisotropy. A high value may occur in dense soil, cemented material or rock. A profile should be interpreted alongside drilling descriptions, groundwater observations and test limitations. Avoid labelling every change in velocity as an exact geological contact unless independent evidence supports that interpretation.
A wave path does not sample one mathematical point
The source and receiver may be nominally placed at the same depth in separate boreholes, but seismic energy travels through a finite region and may encounter layered interfaces. The resulting path can be influenced by material above or below the nominal test elevation. Depending on wavelength and source characteristics, the measurement represents a wave-propagation response rather than an infinitesimally small voxel of earth.
When an engineering plot shows one velocity at each depth, treat the markers as observations tied to test geometry and analysis rather than point-scale perfect measurements. The vertical spacing between readings also limits the detail that can be resolved. A sharp boundary drawn between widely spaced measurements may be an interpolation, not a newly observed physical surface.
Refraction near a fast layer can distort interpretation
Consider slower soil adjacent to a much faster layer. Wave energy can enter the faster material, travel along a different route and return to a receiver sooner than a simple straight-path model predicts. Dividing horizontal spacing by that earlier travel time could produce an apparent velocity greater than the speed of the target soil.
To investigate this, compare adjacent depths, arrival shapes, geologic logs and alternative ray paths. A report should explain whether Snell’s law was applied and how potential refracted first arrivals were handled. The absence of that discussion is particularly important where boreholes cross strong stiffness contrasts. Data reduction must not make a complicated physical arrival appear deceptively simple.
Total internal reflections can complicate waveforms
Strong velocity contrasts can produce reflected energy and effects associated with total internal reflection under applicable geometries. These signals can overlap the intended direct arrivals and make picking more uncertain. An unexpectedly large wave packet may not represent stronger local material; it may be the result of focusing or boundary interactions.
The relevant standard calls for attention to such phenomena in crosshole testing. Readers should look for waveform overlays or a technical explanation rather than treating every amplitude change as a change in shear-wave speed. Velocity is inferred from timing and a path model; amplitude includes source coupling, attenuation, geometry and interference effects.
Source polarization and shear arrival identification
Some crosshole surveys generate shear waves using source motions designed to emphasise a particular polarization. Reversing the source direction can help identify consistent shear arrivals by checking associated waveform behaviour. This is especially important when earlier compressional arrivals or noise obscure the S-wave onset.
A report should state the source and receiver orientation and show how the S pick was distinguished. If only a single trace is supplied, confidence may be limited by ambiguous onsets. A reliable S-wave velocity depends as much on choosing the right arrival phase as on dividing distance by time.
Signal-to-noise quality affects the picked time
Background vibration, poor borehole coupling and instrument interference can obscure weak arrivals. Multiple repeat records and coherent waveform alignment may help distinguish stable arrivals from random noise. Nevertheless, stacking can improve visibility only when the data are aligned appropriately; it cannot correct an inherently wrong wave-path model.
Readers should check the sampling frequency and displayed time scale. If two traces are shifted using a processing procedure, the recorded correction must be documented before arrival times are compared. A seismic trace is evidence after acquisition and processing, not a magic picture whose prominent peak directly identifies the correct material property.
Borehole casing and grouting affect coupling
A receiver depends on physical connection to the surrounding ground, and the source must transmit useful energy into the subsurface. Borehole construction, casing, grout and sensor orientation can affect recorded waveforms. Gaps or poorly bonded sections may complicate identification of arrival phases, especially across depth.
The field record should describe the borehole installation and quality checks. If a suspicious velocity step coincides with a known construction problem, it merits scrutiny before an engineering interpretation is assigned. A detailed site investigation report does not need to hide these caveats; they help distinguish actual subsurface variation from measurement artefacts.
What a three-borehole arrangement can reveal
With a source and two spaced receivers, a practitioner may compare arrivals over two path intervals. If the inner and outer travel differences produce inconsistent apparent speeds, this could reflect lateral variation, timing differences, refraction or receiver coupling. Such inconsistency is useful diagnostic information rather than merely a reason to average the numbers.
A robust interpretation examines whether the assumed material along both paths is similar. Averaging two incompatible velocities can erase evidence of actual heterogeneity or a test problem. The data support a stronger conclusion when the route between each instrument is made explicit.
Crosshole velocity is not a tomography image
A conventional crosshole velocity calculation uses selected wave arrivals over limited paths at specified depths. Crosshole seismic tomography is a more elaborate inversion using multiple travel paths and ray coverage to reconstruct spatial velocity variations under a model. These are not identical measurements and do not automatically have equal spatial resolution.
An existing crosshole tomography and inversion guide explains issues such as ray coverage and smearing in a different application. This article focuses instead on straightforward soil test reporting, wave identification and low-strain dynamic properties. A path-derived average cannot be presented as though every centimetre of the route was separately resolved.
How crosshole differs from downhole testing
Crosshole surveying uses wave travel between boreholes and often emphasises horizontal propagation at selected depths. A downhole survey generally uses a near-surface seismic source and receiver instruments at different depths inside one borehole. Their ray geometry, corrections and sensitivity to lateral heterogeneity differ. Both can estimate Vs, but they do not test identical paths.
The earlier downhole seismic first-arrival guide explains how travel times are used to infer vertical velocity intervals and Vs30 under appropriate assumptions. Compare the measurement geometries before declaring a disagreement between downhole and crosshole values to be an instrument defect.
How crosshole differs from SCPTu
The seismic cone penetration test combines cone measurements with seismic arrivals collected during a penetration survey. It can relate dynamic Vs information to penetration resistance and pore-pressure observations at a location. Crosshole tests measure propagation between boreholes with their own geometry and installation requirements. A similar Vs value does not mean that the methods sampled exactly the same volume of ground.
See the SCPTu seismic cone interpretation guide. It separates Vs measurements from cone resistance, pore-pressure response and empirical soil-behaviour indices. Different testing systems can complement one another without one becoming a direct substitution for all the others.
How crosshole differs from laboratory bender elements
Bender element testing uses wave propagation within a prepared soil specimen under laboratory conditions. It requires interpretation of transducer coupling, travel path and the picked shear arrival. Crosshole measurements use an in-situ path through natural or engineered ground. Both can investigate small-strain shear-wave velocity, but different stress states and geometries may produce different results.
The bender element travel-time guide is a useful comparison. A disagreement does not prove that one test is wrong. Examine specimen disturbance, stress, saturation, path length, density and picking criteria before reconciling laboratory and field estimates.
Use Vs30 only after defining intervals
Vs30 is a harmonic-average measure of shear-wave velocity over the upper 30 metres, commonly calculated as 30 divided by the sum of layer thickness divided by layer Vs. It is a property of a defined vertical profile and depends on sufficient depth coverage. A crosshole result at one or two depths cannot by itself determine Vs30.
For a hypothetical profile with 10 metres at 200 m/s and 20 metres at 400 m/s, total shear travel time is 10/200 plus 20/400, equal to 0.10 second. Vs30 equals 30/0.10 = 300 m/s. This calculation is vertical travel-time averaging; it does not simply average 200 and 400, and it does not prove the accuracy of the two layer velocities.
Code-specific site classes require current rules
Engineering design codes may use Vs30 or other geotechnical descriptors to help assign seismic site categories. Exact thresholds and required supporting tests depend on the applicable jurisdiction, code edition and project. A generic article should not pretend that one fixed global table is legally universal.
A crosshole report may contribute reliable Vs measurements to that evaluation, but the project designer must verify the code and site conditions. If a borehole covers less than 30 metres, filling the missing interval with an arbitrary value can distort Vs30 and any category based on it. Distinguish measured intervals, interpreted gaps and independent code determinations.
Comparing two sites with the same Vs
Two soils may have the same measured Vs but different mass densities, plasticities, strengths, stress histories and groundwater conditions. Their Gmax estimates also differ if densities differ. Even matching Gmax at small strain does not guarantee the same large-strain modulus degradation or cyclic response under an earthquake.
A helpful report combines wave velocity with other measurements rather than compressing the site into one headline number. If a designer compares alternatives, the supporting evidence should include stratigraphy, depth, stress and the relevant engineering question. A numerical velocity is part of a model input, not an all-purpose soil quality score.
A worked uncertainty comparison
Return to the fictional 6.10-metre path and 0.024-second differential arrival. The estimated speed is about 254.2 m/s. If the time pick could reasonably vary by plus or minus 0.0005 second while distance is fixed, the speed ranges from about 248.98 m/s at 0.0245 second to about 259.57 m/s at 0.0235 second.
This range describes one simplified timing sensitivity, not a full confidence interval. Additional uncertainty from distance surveys, path refraction, instrument alignment and material variation remains. Calling the result exactly 254.1667 m/s hides the practical accuracy of the test. Round the reported result to a level supported by actual measurement uncertainty.
A worked modulus-sensitivity comparison
Using a hypothetical bulk mass density of 1,850 kg/m³ and the velocities from the timing check, Gmax varies approximately from 114.7 MPa to 124.6 MPa if density is held constant. That is a notable range caused by just one source of uncertainty in the recorded arrival interval.
The squared dependence of Gmax on Vs explains the amplification. In practice, the density may also have uncertainty, so a complete assessment might be wider. This numerical check is useful when reviewing reports that present finely differentiated stiffness layers without enough measurements to support such resolution.
What a good crosshole report should disclose
At minimum, the reader needs the geometry and surveyed borehole positions, source and receiver types, depths or elevations, acquisition settings, arrival-picking criteria, measured times and the equations used for velocity. It should distinguish Vp and Vs, state how refraction and reflections were treated and explain derived modulus assumptions.
A site interpretation should compare the results with available ground logs and note poor-quality intervals. Calculated properties such as Gmax, dynamic Young’s modulus and Poisson’s ratio should carry assumptions about density, saturation and elastic behaviour. A complete report helps a reviewer reconstruct the measurement chain rather than asking them to trust an unexplained coloured profile.
Common mistakes and how to repair them
Mistake one is using surface borehole spacing at depth without deviation correction. Repair it with surveyed downhole coordinates. Mistake two is dividing a distance in metres by milliseconds as though the result were metres per second. Repair it by converting time to seconds. Mistake three is selecting a P arrival and calling its velocity Vs. Repair it with waveform and polarization checks.
Mistake four is treating the earliest arrival as a direct wave through the target layer. Repair it by considering refraction and velocity contrasts. Mistake five is presenting Gmax as the soil’s ultimate shear resistance. Repair it by distinguishing small-strain stiffness from large-strain strength. Mistake six is treating a path average as a finely resolved tomography image. Repair it by identifying the actual measurement geometry.
Specialist vocabulary is part of the measurement chain. A first arrival is not necessarily a direct arrival, maximum shear modulus is not ultimate strength, and interval does not mean a separately imaged block of ground. Readers explaining the report across languages can use cross-linguistic transfer to compare terms while retaining these technical distinctions. Translating a label without preserving its operational meaning can turn a sound seismic observation into a misleading engineering promise.
Independent transfer exercise with answers
A source and receiver pair have a measured downhole separation of 8.4 metres. The identified shear arrival interval is 0.028 second. Under the simple straight-path assumption, what is Vs? Divide 8.4 by 0.028 to obtain 300 m/s. If an analyst instead uses a nominal 8.0-metre separation, the answer becomes about 286 m/s. The difference shows why the surveyed geometry matters.
Now suppose the soil mass density is 1,900 kg/m³. The simplified Gmax is 1,900 times 300 squared = 171,000,000 pascals, or 171 MPa. Does this establish a static foundation bearing capacity of 171 MPa? No. It is a low-strain shear modulus, not an ultimate resistance. Does the result prove every layer along the path has Vs of 300? No; the measurement represents the assumed path and interval.
Questions readers often ask
Does crosshole testing directly measure shear strength? No; it measures wave propagation used to infer low-strain stiffness. Is the first visible pulse always a direct P arrival? No. Is Gmax computed from velocity without knowing density? Not reliably. Can saturated soil influence Vp? Yes. Are crosshole and downhole values always the same? No; their paths differ.
Does an interval velocity identify an exact soil type? No. Must borehole deviations be considered? Yes when they materially affect geometry. Can a stronger waveform mean a stiffer layer? Not by itself. Does one Vs reading determine Vs30? No. Are standard methods guaranteed to remove every uncertainty? No; competence, site conditions and correct interpretation remain essential.
Primary standards and connected reading
The active ASTM D4428/D4428M-25 crosshole seismic standard defines the scope and methodological assumptions. The 2026 ASTM downhole seismic standard offers a separate geometry for comparison. These sources describe actual methods; access to a purchased standard may be required for complete normative instructions, and this teaching article does not replace the licensed text.
Read the related downhole seismic, SCPTu, bender element and crosshole tomography guides to compare complementary measurement approaches.
The final reading habit: follow the path
A crosshole seismic velocity is best understood as a claim about a selected wave, travelling along an interpreted route, with a measured distance and time difference. Every later stiffness number relies on that original measurement and the material model added to it. A reader who can trace that chain can distinguish a useful engineered estimate from an overconfident colour map.
Good technical interpretation does not make the subsurface falsely simple. It explains what the signal showed, which assumptions gave the velocity meaning and what additional information is needed for design. That is how a geotechnical report becomes accessible without losing the science on which its decisions depend.