Downhole & Crosshole Seismic Survey Uganda

What Are Downhole and Crosshole Seismic Surveys?

A downhole seismic survey measures seismic wave velocities at depth inside a borehole. A triaxial geophone probe (measuring vertical and two horizontal components) is lowered down a borehole, and a seismic source — a sledgehammer struck on a horizontal plate or a weight drop — is triggered at the surface at a known offset and azimuth. By recording the arrival times of the compressional (P) and shear (S) waves at successive depths, Georesolve derives the in-situ Vp and Vs velocity profile of the ground.

A crosshole seismic survey measures seismic velocities between two or more boreholes. A source is fired in one borehole at known depths while geophones in an adjacent borehole record the arrivals. Because the wave travels horizontally between the holes, crosshole surveys provide high-resolution velocity tomography of the ground between boreholes — ideal for characterising foundation conditions and detecting anomalies within a specific depth interval.

These borehole seismic methods are the most reliable way to measure the dynamic properties of soil and rock. Georesolve Africa has delivered downhole seismic surveys for major infrastructure including the Standard Gauge Railway (SGR) corridor in Eastern Uganda and dam foundation investigations such as the MWOGO-RUKARARA Multipurpose Dam in Rwanda. The resulting Vs profiles are used to determine Seismic Site Class, compute the low-strain shear modulus, and assess liquefaction and dynamic foundation response.

How a Downhole Seismic Survey Works

Borehole preparation. A borehole is drilled to the target depth (typically 30 to 100 m for geotechnical work). The hole must be straight and stable, ideally cased or grouted for good geophone coupling.
Source setup. A sledgehammer and metal plate (for shear waves) or weight drop are positioned at the surface at a known offset from the borehole, oriented to generate horizontally polarised shear waves in the plane of the geophone.
Depth profiling. The triaxial geophone probe is lowered to the base of the borehole and raised in regular depth intervals (e.g. every 1 m). At each depth a source is triggered and the P- and S-wave arrivals are recorded.
First-arrival picking. The arrival times of the compressional (Vp) and shear (Vs) waves are picked from each record. The shear wave is identified by its polarity reversal under reversed source orientation.
Velocity computation. Wave travel time versus depth (after correcting for the surface-offset geometry) is differentiated to produce interval Vp and Vs with depth, and the average velocities (including Vs30) are computed.
Dynamic properties. Vs is converted to low-strain shear modulus Gmax and, combined with density, to Poisson's ratio. The Vs profile is used to assign Seismic Site Class.
Interpretation & reporting. A technical report with velocity-depth profiles, site class, and dynamic property tables is delivered, integrated with the borehole log and laboratory test results.

Equipment

Component	Specification
Downhole probe	Triaxial (3-component) geophone probe with centraliser and clamp, measuring vertical, radial, and transverse motion for simultaneous Vp and Vs detection
Shear source	Sledgehammer and steel shear plate (horizontal polarisation) plus weight-drop source for deeper or noisier sites
Compressional source	Vertical hammer strike on a base plate for P-wave generation
Seismograph	Multi-channel engineering seismograph with high sample rate (kHz) for accurate first-arrival picking of both P and S waves
Cabling	Depth-marked geophone cable with winch for controlled, repeatable depth positioning
Crosshole kit	Downhole source (spark or mechanical) and receiver probes for tomography between adjacent boreholes, with survey horns for accurate depth control

Applications

Seismic Site Class

Determine Vs30 and assign Seismic Site Class (A–E) for building code compliance (Eurocode 8 / Ugandan regulations).

Dynamic Modulus

Compute low-strain shear modulus Gmax and Poisson's ratio for foundation and earthquake engineering.

Liquefaction Assessment

Use Vs profiles to evaluate liquefaction susceptibility of sandy soils for seismic design.

Dam Foundations

Characterise foundation dynamic response, zoning, and treatment depth for dams and embankments.

Crosshole Tomography

High-resolution imaging of velocity anomalies between boreholes for foundation and landslide studies.

Transport Corridors

Dynamic site characterisation for railways, bridges, and highways including the SGR corridor.

Deliverables

Vp and Vs velocity-depth profiles (interval and average)
Vs30 value and assigned Seismic Site Class
Low-strain shear modulus (Gmax) and Poisson's ratio profile
Crosshole velocity tomography (where performed)
First-arrival picks and raw shot records for QC
Integration with borehole logs and lab test data
GIS-ready data layers
Comprehensive technical report with methodology, QC, and interpretation

Case Study: Downhole Seismic for MWOGO-RUKARARA Multipurpose Dam, Rwanda

Downhole seismic survey at MWOGO-RUKARARA Dam, Rwanda

Geotechnical Investigations for MWOGO-RUKARARA Multipurpose Dam — Rwanda

Location: Ruhango District, Rwanda Year: 2026 Client: Confidential infrastructure client

Georesolve delivered a comprehensive geotechnical investigation for the MWOGO-RUKARARA Multipurpose Dam, including downhole seismic surveys for in-situ Vp and Vs profiling. The borehole seismic data characterised the dynamic properties of the foundation materials, supporting seismic site classification and dynamic response analysis of the dam.

The downhole velocity profiles were integrated with borehole logging, SPT, and laboratory testing to define foundation zoning, treatment depth, and seismic design parameters for this multipurpose water infrastructure project in the Ruhango District of Rwanda.

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Frequently Asked Questions

What is a downhole seismic survey?

A downhole seismic survey measures seismic wave velocities at depth inside a borehole. A geophone is lowered down a borehole and a seismic source (sledgehammer or weight drop) is triggered at the surface at a known offset. By measuring the arrival time of the wave at successive depths, the seismic velocity profile of the subsurface is derived. It is one of the most reliable methods for measuring in-situ shear-wave (Vs) and compressional-wave (Vp) velocities.

What is a crosshole seismic survey?

A crosshole seismic survey measures seismic velocities between two or more boreholes. A source is fired in one borehole at known depths while geophones in an adjacent borehole record the arrivals. Because the wave travels horizontally between holes, crosshole surveys provide high-resolution velocity tomography of the ground between boreholes — ideal for characterising foundation conditions and detecting anomalies within a specific depth interval.

Why measure shear-wave velocity (Vs)?

Shear-wave velocity (Vs) is the primary parameter for evaluating the dynamic behaviour of soil and rock. It is used to determine Seismic Site Class (Vs30), compute the low-strain shear modulus (Gmax) for earthquake engineering, assess liquefaction susceptibility, and characterise foundation stiffness. Because shear waves do not travel through water, Vs is sensitive to the soil/rock skeleton rather than pore fluid — making it far more diagnostic than compressional velocity alone.

What is Vs30 and seismic site class?

Vs30 is the average shear-wave velocity in the uppermost 30 metres of the ground. It is the standard parameter used by building codes (including Eurocode 8 and the Ugandan building regulations) to assign a Seismic Site Class — typically classes A (hard rock) through E (soft soil). Downhole seismic surveys are the direct method for measuring the Vs profile from which Vs30 and site class are computed.

How deep can downhole seismic survey profile?

A downhole seismic survey can profile to the full depth of the borehole — commonly 30 to 100 metres for geotechnical investigations, and deeper for dam or mining projects. The method requires a borehole of sufficient quality (cased or open-hole) and good coupling. Crosshole surveys require two or more boreholes spaced 5 to 30 metres apart, depending on the target depth and resolution.

How much does a downhole seismic survey cost in Uganda?

The cost of a downhole or crosshole seismic survey depends on borehole depth, number of boreholes, source type, and the level of processing and interpretation. These surveys are typically performed in conjunction with geotechnical boring or drilling campaigns. Georesolve Africa provides tailored quotes integrated with your drilling programme. Contact us for a detailed proposal.