Enhancing Exploration Without Compromising Low‑Frequency Quality | TGS Case Study

The Location

Location

East Breaks area, Gulf of America

Survey

Amendment West 1 (field test)

Survey Year

Field data acquired in 2025

Date Processed

Dec. 2025 to Feb. 2026

Data acquisition (in this study)

Sparse Ultra-Long Offset OBN

Survey Size

Test Area: ~130 km²

Water depth

~900 m – 1200 m

The Amendment West 1 sparse nodal survey was acquired in the East Breaks area of the Gulf of America. The production program covered a 5,400 km² node patch with 3,742 nodes deployed at 1,200 by 1,200 m spacing and a 13,050 km² source patch acquired on a 100 m crossline by 50 m inline grid.

Within the western portion of the survey, TGS conducted a unique field test consisting of six Gemini Enhanced Frequency Sources (EFS) towed in a hexa-source acquisition configuration, supported by Apparition encoded shooting, over a 2.1 by 60 km source patch consisting of six sail lines. A comparison was enabled by retrieving one line of 19 co-located nodes early, allowing direct evaluation of dithered quad-source production data against encoded hexa-source test data in the data and image domains.

The Challenge

Balancing Efficiency and Data Quality

Exploration-scale OBN surveys require high source productivity to cover large areas efficiently while delivering data quality suitable for imaging and inversion. Conventional multi-source acquisition often creates a trade-off, where adding more sources leads to a more difficult source separation problem. Apparition encoded shooting is optimal in this respect, because of its strong-on-strong, weak-on-weak property. In addition, it enhances Signal-to-Noise Ratio (SNR) at ultra-low frequencies because of the small encoding delays between shots in an Apparition encoded salvo.

Amendment West 1 used a dual-vessel quad-source Gemini configuration with dithered shooting. The challenge was to test whether a more efficient, single-vessel hexa-source encoded configuration maintains signal fidelity and whether the expected uplift in ultra-low frequency SNR can be demonstrated.

Figure02_quad_vs_hexa

Comparison between quad- (top) and hexa-source (bottom) Gemini EFS configurations.

Recovering Ultra-Low Frequencies

Ultra-low frequency signal content below 2 Hz is essential for reliable Full Waveform Inversion (FWI). A low SNR in this band may hinder convergence of FWI toward an accurate long-wavelength velocity model. In complex geological settings, these frequencies are critical for stabilizing the inversion process and resolving reliable subsurface properties.

In dithered multi-source acquisition, weaker low-frequency arrivals can also be masked by stronger overlapping events, which further limits recoverability after deblending. The test aimed to validate whether deterministic Apparition encoding could improve signal coherence and separation in this band, enhancing SNR and making low-frequency energy more usable for inversion.

Demonstrating Imaging Value

Improved data-domain quality must translate into measurable gains in imaging and model building. The Amendment West 1 test needed to show that decoded hexa-source data not only appeared cleaner but could match production data in Reverse-Time Migration (RTM) imaging and deliver more accurate ultra-low-frequency elastic FWI updates from the same sparse receiver geometry.

Figure03_Challenges2and3_v2

The Solution

Test Design

Gemini Enhanced Frequency Source
Single vessel, hexa-tow configuration
Apparition multi-source encoding
Sparse ZXPLRe node patch

Pre-Processing Solution(s)

Deterministic decoding

VMB and Imaging

Elastic Dynamic Matching FWI
12Hz RTM Imaging

Data Management

Vessel-to-shore LEO satellite data transfer
Imaging AnyWare® processing platform

In the main production survey, TGS deployed an integrated acquisition-to-imaging approach that combined ZXPLRe ocean bottom nodes, Gemini Enhanced Frequency Source technology, quad-source dithered shooting scheme with deblending-based energy separation, vessel-to-shore LEO satellite data transfer and Elastic Dynamic Matching FWI (E-DMFWI) model building. The Amendment West 1 field trial tested a hexa-source encoded geometry against the quad-source dithered production baseline.

Decoded test data and deblended production data were compared through gathers in different frequency bands, RTM imaging and ultra-low frequency E-DMFWI updates. This end-to-end workflow linked broadband data, efficient acquisition, responsive delivery logistics and inversion-based model building within a single TGS solution.

Encoding and Decoding Maximize Gemini’s Low-Frequency Advantage

Gemini EFS provides stronger ultra-low-frequency energy than conventional airgun sources, giving the workflow a better signal foundation from the start. In dithered multi-source acquisition, deblending can struggle when weak low-frequency diving waves or deep reflections are masked by stronger overlapping signals, limiting the recoverable bandwidth.

Apparition encoded shooting is particularly effective for preserving ultra-low frequencies because its strong-on-strong, weak-on-weak property allows decoding to reinforce coherent signal while suppressing interference more predictably than conventional separation.

Source encoding enhances signal quality at the ultra-low frequencies, while inverse decoding provides built-in noise rejection. The result is higher SNR across the full bandwidth, especially in the 0-2 Hz range, improving recovery of weak events and preserving more coherent sub-2 Hz signal than deblended data, as seen in the Amendment West 1 field test.

Cleaner ultra-low-frequency input improves E-DMFWI results by providing more reliable long-wavelength information for model updates, maximizing the use of Gemini's enhanced bandwidth.

Explore OBN Solutions

The Results

Better SNR for the Ultra-Low Frequencies

The combined use of Gemini EFS and Apparition decoding delivered significantly improved SNR across all frequencies compared to deblended production data. Building on the enhanced low-frequency output of the Gemini source, Apparition encoding/decoding led to unprecedented SNR at ultra-low frequencies.

The improvement was most pronounced below 2 Hz, confirming better recoverability of inversion-critical low-frequency information.

Figure05_shots

Comparison of deblended (top row) and decoded (bottom row) OBN gathers.

Figure06_RTM

12 Hz QC RTM images along the node line. Left: from towed streamer WAZ data. Middle: from deblended quad-source production data. Right: from decoded hexa-source field test data.

RTM Imaging On Par

RTM imaging showed no material difference between decoded hexa-source data and deblended quad-source production data, confirming that increased source density did not degrade imaging quality.

Consistent acquisition quality from ZXPLRe nodes and stable source behavior from Gemini EFS contributed to reliable input data, while efficient data delivery through LEO satellite transfer supported timely processing and rapid quality assessment of results in the image domain.

Improved FWI Updates

Ultra-low frequency FWI updates derived from decoded Apparition data more closely approximated the target velocity perturbation than those from deblended production data. Enhanced low-frequency input from Gemini EFS and improved SNR on Apparition decoded data provided higher-quality inversion input. These gains were fully leveraged through E-DMFWI, which enabled improved long-wavelength velocity updates critical for complex subsurface imaging.

Top Left: 1.8 Hz E-DMFWI prod. velocity model and QC RTM. Top Right: 3% perturbation applied to prod. model. Bottom Left: E-DMFWI update using deblended data (starting from 3% perturbed model). Bottom Right: E-DMFWI update using decoded data (starting from 3% perturbed model).

Conclusion

The Amendment West 1 field test demonstrated an integrated workflow combining ZXPLRe nodes, Gemini Enhanced Frequency Source technology and Apparition deterministic encoding, delivering a step change in low-frequency data quality while improving acquisition efficiency. Enhancements in signal strength, coherence and recoverability directly address the traditional trade-off between productivity and data fidelity. These gains are preserved through decoding and fully exploited in imaging workflows using E-DMFWI, resulting in more stable inversion and improved long-wavelength velocity models.

Looking ahead, this integrated TGS workflow provides a scalable path for future exploration surveys that demand both efficiency and high-fidelity data. By combining advanced acquisition, near-real-time data delivery through LEO satellite transfer and inversion-driven imaging, the approach supports faster decision-making and enables more confident evaluation of complex subsurface targets in increasingly challenging environments.

Commercial impact

End-to-End Acquisition-to-Imaging Workflow

Combines ZXPLRe nodes, Gemini EFS, Apparition decoding, E-DMFWI, and LEO satellite transfer into a unified solution from acquisition through inversion.

Higher Efficiency Without Data Quality Trade-Off

Enables increased source productivity while preserving low-frequency signal quality required for modern inversion workflows.

Improved Inversion and Imaging Confidence

Delivers stronger low-frequency input and more stable E-DMFWI convergence, resulting in improved long-wavelength velocity models and subsurface clarity.

Faster Data Access and Decision Making

Uses vessel-to-shore LEO satellite transfer to accelerate data availability and support more responsive processing and interpretation.

Scalable Exploration Solution

Provides a repeatable and scalable approach for future surveys in complex settings that require both operational efficiency and high-fidelity data.