Seismogram: Shotpoint Traces, Trace Gathers, and WCSB Seismic Section Building
A seismogram is the record of ground motion produced by a single seismic energy source, consisting of the set of traces recorded by the receivers that listened to one shotpoint, and it is the raw building block from which every processed seismic image is assembled. When a source fires, whether a Vibroseis sweep, a dynamite charge in a shothole, or a marine air-gun pulse, elastic waves radiate through the subsurface, reflect and refract at boundaries between rock layers of differing acoustic impedance, and return to a spread of geophones or hydrophones, each of which converts the arriving vibration into a time series of amplitude that is one seismic trace. The collection of traces from that one shot, plotted side by side against a common two-way travel-time axis, is the shot gather or field seismogram, and on it an interpreter can see the direct arrival, refracted head waves, ground roll, and the hyperbolic reflection events whose curvature (moveout) encodes the velocity of the overlying rock. A single seismogram alone does not make an image; instead, numerous seismograms from many shotpoints along a line or across a 3D grid are corrected, sorted, and combined so that the many traces sampling each subsurface point are summed together. That processing sequence includes deconvolution to sharpen the wavelet, static corrections to remove near-surface time distortions, normal-moveout correction and stacking to boost the signal-to-noise ratio, and migration to move reflections to their true subsurface positions, after which the traces are displayed together as a continuous seismic section. Each vertical strip of that section traces its lineage back to the field seismograms recorded in the survey. In the Western Canadian Sedimentary Basin, seismograms underpin exploration and development across the full stratigraphic column, from mapping Devonian Leduc reef edges and Nisku pinnacle reefs to delineating Montney and Duvernay unconventional targets and steering the placement of horizontal wells. The quality of the individual seismograms sets a hard ceiling on what any later processing can recover, so field parameters such as source effort, receiver-array geometry, and fold are chosen to give clean shot records over the target depth, and quality control in the field often involves inspecting raw seismograms shot by shot to confirm the first breaks, reflection continuity, and noise levels are acceptable before the crew moves on. The seismogram is therefore both the literal data a seismic crew acquires and the unit of evidence that, multiplied thousands of times over and rigorously combined, becomes the depth image a WCSB geoscientist interprets to site a multi-million-dollar well.
Key Takeaways
- One shot, many traces: A seismogram is the full set of traces recorded from a single shotpoint, each trace being the amplitude-versus-time record of one geophone or hydrophone. Together they form the shot gather, the raw field record on which direct arrivals, refractions, ground roll, and hyperbolic reflection events are all visible before any processing.
- Building block of the section: No single seismogram is an image; many seismograms from many shotpoints are corrected, sorted into common-midpoint gathers, and stacked so that multiple traces sampling each subsurface point sum together. The result is displayed as a continuous seismic section, every column of which derives from the field seismograms.
- Moveout carries velocity: The hyperbolic curvature of a reflection across a shot seismogram, its normal moveout, encodes the velocity of the rock above the reflector. Processors flatten that curvature with an NMO correction before stacking, and the velocities extracted double as a tool for depth conversion and pore-pressure prediction in WCSB wells.
- Quality gates the image: The signal content of the raw seismograms sets the ceiling on the final image, so field crews inspect shot records for first-break clarity, reflection continuity, and noise before advancing. Source effort, array geometry, and fold are tuned to produce clean seismograms over the target depth, since no processing recovers signal the field record never captured.
- Foundation of WCSB targeting: Seismograms acquired across Alberta and northeast British Columbia image Devonian Leduc and Nisku reefs, Montney and Duvernay unconventional intervals, and shallow gas, and the stacked, migrated product guides where operators land vertical and horizontal wells that cost several million CAD each.
From Field Record to Common-Midpoint Gather
The path from a field seismogram to an interpretable section runs through re-sorting the data. Traces first recorded organized by shotpoint are regrouped into common-midpoint gathers, so that all traces reflecting from the same subsurface point, but recorded at different source-receiver offsets, sit together. Applying the normal-moveout correction flattens the reflection hyperbola in that gather, and summing the flattened traces produces a single high-fold stacked trace with far better signal-to-noise than any one field trace. Repeat across every midpoint and the stacked traces line up into the section. This is why fold, the number of traces contributing to each stacked point, is a headline survey design parameter in the WCSB.
Vibroseis and the Correlated Seismogram
Most onshore WCSB seismic uses Vibroseis rather than dynamite, and that changes what the raw seismogram looks like. A vibrator truck emits a long frequency sweep rather than an impulse, so the field record is initially a smeared, uninterpretable signal until it is cross-correlated with the known pilot sweep, which collapses each reflection back to a compact wavelet and produces the correlated seismogram an interpreter recognizes. Vibroseis lets crews work in settled agricultural and near-urban areas of central Alberta where shothole dynamite is impractical, and stacking several vibrator sweeps per point builds the source energy needed to image deep Devonian targets without a single large impulsive shot.
Fast Facts
The word seismogram predates petroleum exploration entirely, coming from earthquake science where a seismograph traced a single squiggle of ground shaking on a rotating paper drum. Reflection seismology borrowed the term but inverted the economics: instead of passively waiting for one earthquake, an exploration crew fires thousands of controlled shotpoints and records a seismogram at each, then combines them, so a modern 3D survey over a WCSB township can generate tens of millions of individual traces that collectively resolve rock features only a few metres thick at several kilometres depth.
Related Terms
A seismogram is the raw record that, once corrected and stacked, becomes a seismic section, and each trace within it is the response of a geophone to arriving ground motion. The reflection events on the record include both compressional P-wave arrivals and, on multicomponent surveys, converted shear energy, so the seismogram is the common medium in which every wave mode a survey captures is first written down before interpretation begins.
Real-World WCSB Scenario: QC on a Montney 3D
A contractor shooting a Vibroseis 3D over a Montney block near Dawson Creek, British Columbia, records field seismograms shot by shot, with the crew's observer inspecting each correlated record for first-break quality and reflection continuity down to the roughly 2,500 m target. On one receiver line the shot seismograms show badly degraded reflections traced to a buried gravel channel scattering the source energy, a near-surface problem that no amount of later processing would fully repair.
Catching it in the field, the crew increases sweep effort and adjusts the receiver array over the problem zone, restoring usable reflection signal on the reshot seismograms. The fix costs a day of crew time against a multi-million CAD survey, but it preserves image quality over the pad location where the operator will later land horizontal wells worth far more than the acquisition itself.