Transition-zone (TZ) acquisition

The shoreline is the hardest seismic environment in the industry. Conventional marine streamers need at least 15 m of water to operate; conventional land crews can’t work in saturated marsh or knee-deep water. A single shoreline line often crosses dry land, marsh, shallow water, and open sea within 3–5 km — and the client wants one continuous image. Transition-zone (TZ) crews blend land, marsh, shallow-boat, and full source-vessel operations daily.

The four zones and their equipment

• Dry land: buried geophones (or nodes) + vibroseis or small dynamite. Standard land practice.
• Marsh / wetland: pole-mounted geophones held above the water table + airboats fitted with air-blast guns or small dynamite. Wet, muddy, slow.
• Shallow water (0–5 m): buried hydrophones or hybrid phone/hydrophone nodes at the sea-floor + shallow-draft shot boats. The shot boats operate at low tide; at high tide they’re sometimes replaced by airboats.
• Deep water (>5 m): OBN/OBC on the sea floor + a full marine source vessel. Ordinary marine practice.

Fold continuity

A well-designed TZ survey delivers a near-uniform fold across the whole transition. The main source of fold holes is hand-off at zone boundaries: a land receiver can’t record a marine shot perfectly, and vice versa. Best practice: overlap 2–4 stations of each receiver type across every boundary, so every subsurface bin has contributions from both sides. Budget 5–10% extra equipment to make this overlap work.

Daily logistics

A TZ day coordinates four parallel crews. Tide is the master schedule. Low tide opens the shallow-water window for shot boats and dry-land work on exposed mudflats; high tide lets airboats over the marsh. A typical shift runs 12–18 hours around tide peaks, with a 6-hour turnaround between shifts. Coordinating hand-off at each zone boundary — source timing, receiver pickup schedule, crew safety zone — is where most TZ operational cost hides.

When TZ is worth it

TZ day-rates run 2–4× a pure-land or pure-marine crew because three-to-four teams must coordinate daily. That only pencils out for surveys where the shoreline strip is geologically critical: Nile delta nearshore (BG, Shell — marsh + shallow gas plays), Gulf of Mexico shelf-edge, Lagos Bight (ExxonMobil), Niger Delta, offshore Abu Dhabi. Where the shoreline is geologically unimportant, surveys are planned to avoid it.

References

• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.
• Cordsen, A., Galbraith, M., Peirce, J. (2000). Planning Land 3-D Seismic Surveys. SEG Geophysical Developments 9.
• Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.