Single-Hole Wave Velocity Testing (Downhole Seismic Method): Principles, Procedures and Applications

Single-hole wave velocity testing, also known as the downhole seismic method, is an in-situ geotechnical test that measures compression-wave velocity (Vp) and shear-wave velocity (Vs) in soil and rock strata through a single borehole. The resulting velocity profile is essential for seismic site classification, dynamic modulus determination, and vibration-sensitive foundation design, and is widely performed in accordance with ASTM D7400 and GB 50021 by accredited third-party laboratories. This article explains the working principle, equipment, test procedure, data interpretation, and typical engineering applications of the method.

Technical Overview

Single-hole wave velocity testing, commonly referred to as the downhole seismic method, is an in-situ geophysical testing technique in which seismic waves are generated at the ground surface and recorded by a sensor lowered to successive depths within a single borehole. Because the receiver is positioned below the ground surface at known depths, the method delivers a direct, depth-resolved profile of compression-wave velocity (Vp) and shear-wave velocity (Vs) along the borehole, which surface-based geophysical methods can only estimate indirectly.

The fundamental principle is travel-time measurement. A seismic source at the surface emits a wave train that propagates through the strata to a three-component geophone fixed against the borehole wall. By recording arrival times at closely spaced depths and correcting for the inclined ray path between the offset source and the receiver, the interval wave velocity of each soil or rock layer can be calculated. Shear-wave velocity is the most valuable output, as it is a direct index of soil stiffness at very small strains and forms the basis of seismic site classification in most design codes.

The test is generally performed in accordance with ASTM D7400, Standard Test Method for Downhole Seismic Testing, and in China under GB 50021, Code for Investigation of Geotechnical Engineering, with site classification criteria defined in GB 50011, Code for Seismic Design of Buildings. Key technical characteristics include:

• Only one borehole is required, making the method faster and more economical than crosshole seismic testing (ASTM D4428).
• A three-component borehole geophone (one vertical and two horizontal sensors) is used to capture both P-wave and S-wave signals.
• Depth intervals of typically 1 m to 2 m provide a detailed velocity profile, with additional measurement points at strata boundaries.
• Derived parameters include Poisson's ratio, dynamic shear modulus, and dynamic Young's modulus when soil density is known.
• The method is applicable to soils, weathered rock, and intact rock, in both cased and uncased boreholes.

Testing Samples

Unlike laboratory testing, single-hole wave velocity testing is performed on the ground itself, with the "sample" being the in-situ soil and rock mass intersected by the borehole. Typical testing environments include:

Soil Strata (Clay, Silt, Sand and Gravel)

Most tests are conducted in soil boreholes advanced through soft to stiff clays, silts, and loose to dense sands. In unstable formations, boreholes are supported with casing or drilling mud, and the annular space between the casing and the formation is grouted to ensure good seismic coupling and prevent signal attenuation.

Rock and Weathered Rock

In boreholes penetrating weathered crust, fractured zones, and intact bedrock, the method yields Vp and Vs values that reflect rock mass quality, degree of weathering, and fracturing. The velocity profile obtained in rock is frequently used to verify bearing strata and to correlate with rock mass classification indices.

Engineered Fill and Improved Ground

Boreholes through engineered fill, dynamically compacted ground, stone column treatments, or grouted zones can be tested to verify the effectiveness of ground improvement by comparing velocity profiles before and after treatment.

Testing Items

The primary and derived measurement results of single-hole wave velocity testing include:

• Shear-wave velocity (Vs) at each measurement depth and interval shear-wave velocity for each soil or rock layer.
• Compression-wave velocity (Vp) at each measurement depth and corresponding layer interval values.
• Layer-averaged Vs values and the equivalent shear-wave velocity over the upper 20 m of the profile (or over the actual depth of overburden), as required for code-based site classification.
• Vp/Vs ratio and dynamically computed Poisson's ratio.
• Dynamic shear modulus (Gd = ρVs²) and dynamic Young's modulus, calculated using the measured density of each layer.
• Identification of anomalous zones such as low-velocity interlayers, cavities, and highly fractured intervals.
• Site category classification for seismic design in accordance with the applicable building code.

Testing Methods

Borehole Preparation

The borehole is drilled vertically, and its deviation is surveyed for deep or inclined holes so that true receiver depths can be corrected. Where casing is installed, the annulus is backfilled with grout or sand to ensure firm coupling between the formation, the casing, and the borehole fluid, because loose or washed-out annular material severely attenuates high-frequency signals.

Field Test Procedure

A shear-wave source, typically a wooden plank loaded with a heavy weight or a vehicle, is placed on the ground surface at a fixed offset of approximately 1 m to 3 m from the borehole collar. Horizontal hammer blows on opposite ends of the plank generate SH-waves with reversed polarity, which is the key to unambiguous shear-wave first-arrival identification. A vertical hammer blow on a steel plate generates the P-wave. The three-component geophone is lowered to each measurement depth, clamped firmly against the borehole wall using a pneumatic or mechanical arm, and the waveforms are recorded by a digital seismograph triggered by a switch on the hammer. At least three consistent records per depth are collected to verify repeatability.

Data Processing and Interpretation

Arrival times of the P-wave and S-wave are picked from the recorded traces, with shear-wave arrivals confirmed by the polarity-reversal technique. Because the source is offset from the borehole, the travel path is inclined; the measured travel times are corrected to equivalent vertical paths using the geometric relationship between source offset and receiver depth. Interval velocities are then computed from the corrected travel-time differences between adjacent depths, producing a continuous velocity-versus-depth profile. From this profile, layer velocities, the equivalent shear-wave velocity for site classification, and dynamic elastic parameters are derived.

Quality Control

Reliable results depend on clear first arrivals, accurate depth control, verified polarity reversal of shear-wave signals, consistent repeatability of repeated records at each depth, proper grouting of cased intervals, and correction for borehole deviation and cable stretch. Sampling intervals of the recording system are typically 0.1 ms or finer to resolve near-surface arrivals precisely.

Applications

Single-hole wave velocity testing is a routine component of geotechnical investigation for seismic design and vibration engineering. Principal applications include:

• Seismic site classification and determination of the design ground motion for buildings and structures under GB 50011 and equivalent international seismic codes.
• One-dimensional site response analysis and seismic microzonation studies for urban planning and critical facilities.
• Shear-wave-based evaluation of liquefaction potential in saturated sand deposits.
• Determination of dynamic shear modulus and damping characteristics for the design of machine foundations, compressor bases, and vibration-isolation systems.
• Dynamic stiffness parameters for foundation design under cyclic and impact loads, including rail transit, wind turbine, and dam projects.
• Verification of ground improvement works such as dynamic compaction, vibro-replacement, and grouting.
• Assessment of rock mass integrity, weathering grade, and depth to competent bearing strata.
• Complementary input for settlement and deformation analysis where small-strain stiffness is required.

FAQ

What is the difference between the single-hole method and the crosshole method?

The single-hole (downhole) method uses one borehole, with the source at the surface and the receiver at depth, measuring waves along a predominantly vertical path. The crosshole method (ASTM D4428) uses two or more boreholes, with the source and receivers at the same depth, measuring along horizontal paths. The crosshole method provides higher accuracy for thin horizontal layers but requires additional boreholes and therefore higher cost.

How deep can the test be performed?

Typical test depths range from 20 m to 100 m, depending on borehole condition, receiver clamping force, and source energy. Deeper profiles are achievable with powerful sources, careful grouting, and borehole deviation corrections.

Is casing required for the test?

Casing is required in collapsible or caving formations. For reliable results, the annular space between the casing and the borehole wall must be completely grouted, otherwise the recorded signals may be severely attenuated or dominated by pipe arrivals.

How is the equivalent shear-wave velocity calculated?

The equivalent shear-wave velocity is computed as the total overburden thickness (usually 20 m, or the actual depth of soil overlying bedrock) divided by the sum of the travel times through each layer, which equals the thickness of each layer divided by its shear-wave velocity. This value determines the site category for seismic design.

What factors affect the accuracy of the results?

Accuracy depends on the quality of borehole preparation and grouting, correct identification of shear-wave first arrivals, precise depth measurement and deviation correction, source offset geometry, ambient vibration levels, and the assumption that strata are horizontally layered and that ray paths can be approximated as straight lines.