Seismic Record: Shotpoint Traces, Common-Midpoint Gathers, and 3D Imaging in WCSB Exploration

A seismic record is the set of traces recorded from a single source activation, or shotpoint, in a reflection seismic survey. When an energy source such as a vibroseis truck or a small buried dynamite charge sends an acoustic wavefront into the subsurface, that wavefront reflects off boundaries where rock acoustic impedance changes (the product of density and velocity) and returns to an array of receivers laid out on surface. Each receiver, a geophone on land or a hydrophone at sea, records the arriving energy as a single time series called a trace, and the full collection of traces from that one shot, gathered side by side and plotted against two-way travel time, is the seismic record for that shotpoint. On a raw land record the first thing visible is the direct arrival and refracted first breaks angling across the near offsets, followed by reflection events that curve downward with offset in a hyperbolic pattern called normal moveout, and often coherent noise such as ground roll, air blast, and multiples. Individually a single record is noisy and hard to interpret, so seismic processing reorganizes traces from many shotpoints into common-midpoint (CMP) gathers, applies velocity analysis and normal-moveout correction to flatten the reflection hyperbolas, then sums or stacks the aligned traces to cancel random noise and reinforce genuine reflections. Thousands of processed and stacked records displayed together side by side form a seismic section, the familiar cross-sectional image of the subsurface that geoscientists interpret for structure and stratigraphy. In the Western Canadian Sedimentary Basin the seismic record underpins nearly every exploration and development decision. Two-dimensional lines shot across the Alberta foothills image the thrust-faulted structures that trap gas in the Turner Valley and Foothills belt, while dense three-dimensional surveys over the Montney, Duvernay, Cardium, and Viking define the geometry, faulting, and thickness variation that guide horizontal well placement and hydraulic-fracture design. Devonian reef plays such as Leduc, Nisku, Rainbow, and Swan Hills were historically found by recognizing the drape, velocity pull-up, and reflection character that reefs impose on the seismic record. Modern acquisition captures far more than structure: amplitude-versus-offset (AVO) behaviour preserved in the pre-stack records helps distinguish gas-charged from brine-filled sand, and time-lapse or 4D surveys repeat the same geometry over years to track fluid movement in floods such as Weyburn. Because the value of an interpretation is only as good as the data behind it, acquisition parameters recorded with every shot, source type, source and receiver spacing, fold, and sample rate, are documented so that the geophysical geology can be tied confidently to well control and depth.

Field Acquisition and Trace Geometry

On a WCSB 3D survey a vibroseis fleet sweeps a frequency range into the ground at each source point while a grid of geophone lines records the returning wavefield. A single shotpoint might feed several thousand live channels, so its seismic record is a large fan of traces spanning near and far offsets across the patch. The offset of each trace, the distance from source to receiver, controls the moveout and the AVO information it carries. Recording parameters such as a 2 ms sample rate, a listening time of 3 to 4 seconds, and receiver spacing of 20 to 40 m are logged with every record so processors can build accurate velocity models and honour the survey's designed fold and bin size during imaging.

Processing a Record into an Image

Turning field records into an interpretable volume runs through a defined sequence: geometry assignment, noise attenuation to suppress ground roll and multiples, deconvolution to sharpen the wavelet, CMP sorting, velocity analysis, normal-moveout and statics correction, stacking, and finally migration. Pre-stack time or depth migration is now standard over structurally complex WCSB foothills data because it collapses diffractions and places steep reflectors correctly. Each step is quality-controlled against the original records so that artifacts are not mistaken for geology, and the final migrated section is depth-tied to well logs and check-shot or vertical seismic profile data.

Related Terms

Many seismic records are processed and displayed together to build a Seismic Section, the interpreted cross-section of the subsurface, so the record is the atomic unit and the section is the finished picture. The receivers that convert ground motion into each trace are Geophones, and the vertical axis of every record is measured in Two-Way Time, the round-trip travel time of the reflected wavefront. Ultimately every interpretation serves the underlying Geology, tying seismic character back to the rocks, structures, and depositional systems that host oil and gas.

Duvernay 3D Survey Scenario near Fox Creek

Consider an operator shooting a 200 sq km 3D vibroseis survey over a Duvernay lease near Fox Creek, Alberta, at a cost on the order of CAD 6 to 10 million including permitting, acquisition, and processing. Each source point produces a rich seismic record across the live patch, and the stacked, migrated volume images Duvernay thickness, natural fracture corridors, and the offsetting basement-rooted faults that must be avoided to reduce induced-seismicity risk under AER Directive 083 protocols.

Interpreters use the volume to place 3,000 m horizontal laterals along the most brittle, best-developed Duvernay fairway and to steer fracture stages away from mapped faults. The seismic investment, a fraction of a single well's cost, routinely improves per-well economics by increasing the fraction of productive lateral and reducing the chance of a costly frac hit or a fault-related water problem.