crosswell seismic tomography, Oil & Gas Glossary

What Is Crosswell Seismic Tomography?

Crosswell seismic tomography is a borehole geophysics technique that generates seismic waves in one wellbore and records them in a second parallel wellbore, using the travel times and amplitudes of the recorded waves to construct a high-resolution velocity model and attenuation image of the formation between the two wells, providing resolution of 1-5 metres at interwell distances of 100-1,000 metres — far exceeding surface seismic resolution — for reservoir monitoring, steam front tracking, and interwell heterogeneity characterisation.

Key Takeaways

Crosswell seismic images the formation between two wells with resolution of 1-5 metres, versus 20-50 metres for surface 3D seismic.
The seismic source is deployed at multiple depths in one well; receivers at multiple depths in the second well record the transmitted waves.
Travel time tomography maps P-wave velocity; attenuation tomography maps Q (quality factor) variations.
Steam front and CO2 flood front tracking is a major application because thermal and gas effects strongly change seismic velocity.
Processing requires ray-path inversion or full waveform inversion to convert travel time data to velocity images.

How Crosswell Seismic Tomography Works

A crosswell seismic survey requires two boreholes separated by 100-1,000 metres in the reservoir formation of interest. A seismic source tool is suspended in the source well on a wireline and fired at multiple depth positions (typically one metre apart) to generate an acoustic pulse that propagates through the formation to the receiver well. In the receiver well, an array of geophones or hydrophones records the seismic waveforms as the source is moved through its depth positions. The result is a full data set of seismic wave travel times and amplitudes from every source depth to every receiver depth, typically providing thousands of independent ray paths through the interwell formation.

This travel time data is processed by seismic tomographic inversion — the same mathematical approach used in medical CT scanning but applied to seismic waves in rock rather than X-rays in tissue. The inversion finds the velocity model that best explains the observed travel times: high-velocity zones (dense carbonates, cemented sands, or formations from which steam has removed hot water) slow travel times less than low-velocity zones (porous gas-saturated sands, steam-heated zones, or fractured intervals). The spatial resolution of the resulting tomogram is approximately 1/2 to 1/3 of the wavelength of the seismic waves used, which for crosswell tools operating at 100-1,000 Hz in formations with velocities of 2,000-4,000 m/s gives wavelengths of 2-40 metres and resolution of 1-20 metres — dramatically better than surface seismic at comparable frequencies.

Crosswell Seismic Tomography Applications Across International Jurisdictions

In Canada, crosswell seismic tomography has been used in WCSB SAGD operations to image steam chamber development between injector-producer well pairs in Athabasca oil sands operations. The propagation of steam through the oil sands changes the acoustic velocity from the cold, bitumen-saturated baseline value (approximately 2,200-2,400 m/s) to the heated, steam-saturated value (approximately 1,600-1,800 m/s), creating a velocity contrast that time-lapse crosswell tomography tracks as the steam chamber grows. Suncor, Cenovus, and Canadian Natural Resources have used 4D crosswell monitoring at their Cold Lake and Athabasca SAGD facilities to optimise steam injection rate and direction. AER oil sands scheme approval submissions sometimes include geophysical monitoring programme descriptions that incorporate crosswell seismic as a steam front monitoring tool.

In the United States, crosswell seismic has been used for CO2 flood monitoring in the Permian Basin and at the Weyburn CO2 project (which straddles the Canada-US border in Saskatchewan and North Dakota) to track the advancing CO2 saturation front between injection and production wells. The DOE's National Energy Technology Laboratory (NETL) has funded crosswell seismic research for carbon capture and storage (CCS) site monitoring, where the technique's high resolution relative to surface seismic provides better spatial characterisation of CO2 plume geometry. In Norway, Equinor has evaluated crosswell seismic for monitoring waterflooding in chalk reservoirs at Ekofisk, where the high resolution could detect thin chalk layers and fractures that control waterflood conformance. In the Middle East, Saudi Aramco's EXPEC ARC research has investigated crosswell seismic for monitoring waterflood displacement efficiency in the heterogeneous Arab Formation carbonates at Ghawar.

Fast Facts

The theoretical maximum crosswell seismic frequency is limited by borehole resonance (the borehole acts as a waveguide that cuts off high frequencies) and formation attenuation (higher frequencies are absorbed more rapidly with distance). In practice, useful crosswell seismic frequencies of 100-3,000 Hz are achievable at interwell spacings of 100-500 metres. At the high end of this frequency range (1,000-3,000 Hz) and short interwell spacings (100-200 m), crosswell seismic achieves vertical resolution of 0.5-1.5 metres in moderate-velocity formations — resolution sufficient to identify individual bed boundaries and image thin reservoir layers that surface seismic cannot resolve. This capability makes crosswell seismic the highest-resolution interwell imaging tool available in the oil industry.

4D (Time-Lapse) Crosswell Seismic for Reservoir Monitoring

The most commercially successful application of crosswell seismic is 4D monitoring — repeating the tomographic survey at intervals during field production to track fluid or thermal changes in the reservoir. Steam flooding in heavy oil and oil sands produces dramatic velocity changes (15-30% velocity reduction in steam-saturated zones) that are easily detectable in crosswell travel time differences between surveys. CO2 flooding reduces P-wave velocity in zones where CO2 displaces brine (gas compressibility lowers bulk modulus). Waterflood conformance produces smaller velocity changes (brine replacing oil) but still detectable at high data quality. The time-lapse crosswell velocity change is mapped to fluid saturation change using the Gassmann equation, providing a quantitative estimate of fluid displacement between surveys. This 4D crosswell monitoring data can be used to adjust injection rates, identify bypassed zones, and validate dynamic reservoir simulation models against observed fluid movement.

Tip: When designing a crosswell seismic survey for reservoir monitoring, ensure the source and receiver depth ranges extend beyond the target formation by at least 20-30% of the interwell spacing. Rays that travel at steep angles (nearly horizontal) have limited resolving power for vertical velocity variations because they spend little time traversing thin beds at different depths. The tomographic inversion requires a range of ray angles — including upward-traveling rays from shallow source positions to deep receiver positions and vice versa — to constrain both horizontal and vertical velocity variations. A survey limited to source and receiver depths within the target formation will underestimate vertical velocity resolution due to the lack of steep-angle ray coverage at the formation top and bottom.

Crosswell seismic tomography is also referenced as:

Interwell seismic tomography — used in academic literature to distinguish from VSP (vertical seismic profile) and surface seismic; "interwell" emphasises the well-to-well measurement geometry
Cross-borehole seismic — an alternate phrasing used in some service company and regulatory documentation; mathematically and physically identical to crosswell seismic tomography
Borehole-to-borehole seismic — descriptive term used when explaining the technique to non-specialist audiences; emphasises the physical geometry without requiring familiarity with tomography terminology

Frequently Asked Questions

What is the main limitation of crosswell seismic tomography compared to surface 3D seismic?

The main limitation of crosswell seismic is its requirement for two appropriately spaced boreholes — it cannot image where wells do not exist. Surface seismic covers large areas from a single acquisition campaign without any well access requirement; crosswell seismic is limited to the formation volume between specific existing or purpose-drilled wellbore pairs. This makes crosswell seismic a well-spacing and economics-constrained technique: it is cost-effective for monitoring specific injection-production well pairs or for detailed characterisation between key appraisal wells, but impractical for regional exploration or field-wide imaging. Additionally, crosswell data acquisition requires source and receiver tools to be deployed simultaneously in two live wells, creating operational complexity and potential production downtime that surface seismic avoids entirely. For most exploration applications, surface 3D seismic provides adequate resolution at far lower cost per unit area; crosswell seismic adds value only in the specific development and monitoring contexts where its higher resolution justifies the operational complexity.

How does crosswell seismic resolution compare to well log resolution?

Well logs provide the highest vertical resolution of any subsurface measurement — typically 0.1-0.6 metres for most wireline tools and 0.15-0.3 metres for LWD — but they sample only the immediate vicinity of the wellbore (0.1-2 metres radially for most tools). Crosswell seismic has much lower vertical resolution than logs (1-20 metres depending on frequency and spacing) but images the full volume between two wells at distances of 100-1,000 metres from either borehole. The two methods are therefore complementary rather than competing: logs provide fine-scale characterisation at the wellbore while crosswell seismic provides coarser-scale but spatially comprehensive characterisation in the interwell volume. Integrating both data types — using logs to calibrate the crosswell velocity model and using crosswell tomography to extrapolate log-calibrated rock properties between wells — is the standard approach in detailed reservoir characterisation projects.

Why Crosswell Seismic Tomography Matters in Oil and Gas

The fundamental challenge of reservoir engineering is that wells provide detailed information only at their specific locations, while the economic behaviour of a field is determined by the heterogeneous properties of the reservoir between wells — properties that must be inferred rather than measured directly. Crosswell seismic tomography is one of the few technologies that directly images the interwell reservoir volume at a resolution high enough to detect the thin beds, fracture zones, and fluid-saturation variations that control sweep efficiency and production performance. For EOR and thermal recovery projects where the economic return depends on accurate knowledge of how injected fluids are moving through the formation, time-lapse crosswell seismic provides the monitoring capability that validates the recovery process efficiency and enables adaptive management of injection patterns to optimise recovery factor.

Crosswell Seismic Tomography: Definition, Interwell Imaging, and Reservoir Characterisation