Push-Down (Seismic)
Push-down is a seismic interpretation phenomenon caused by relative seismic velocity differences between subsurface strata, where a shallow layer or feature with a low seismic velocity (such as a shale diapir, gas chimney, gas-bearing zone, or other low-velocity material) surrounded by rock with higher seismic velocity causes the underlying strata to appear deeper on a time-domain seismic display than they actually are — creating an apparent structural low directly beneath the low-velocity feature; the underlying physics is straightforward: seismic data is initially recorded and processed in the time domain (with vertical axis being two-way travel time rather than depth), and when seismic energy travels through a low-velocity material, the travel time is greater than it would be through equivalent thickness of higher-velocity material; the additional travel time appears as additional depth-equivalent in the time-domain display, with the resulting "push-down" of underlying reflectors creating the apparent structural low; after such features are converted from time to depth (through seismic depth conversion using the appropriate velocity model), the apparent structural low is generally reduced in magnitude or eliminated entirely, with the resulting depth display showing the actual structural geometry without the velocity-related distortion; hydrocarbon-related zones can display velocity push-downs because the velocity of hydrocarbons (particularly gas, with much lower velocity than oil and water) is slower than that of equivalent water-saturated rock, with the resulting push-down of underlying reflectors being recognized as a hydrocarbon indicator (HCI) on time-domain seismic data; gas chimneys (zones of upward gas migration through faults or other vertical pathways) particularly produce strong push-down effects because the gas-charged rocks have substantially reduced velocity compared to the surrounding unaltered rocks; recognition of push-down phenomena requires careful seismic interpretation that distinguishes velocity-related artifacts from actual structural features.
Key Takeaways
- Time-vs-depth interpretation distinction is fundamental to recognizing push-down phenomena — seismic data initially recorded in two-way travel time displays the structural geometry as the integrated effect of geological depth and the velocity profile through the overlying rocks; in areas of uniform velocity, the time-domain display closely reflects the structural geometry; in areas with significant velocity heterogeneity (particularly low-velocity features overlying deeper formations), the time-domain display includes velocity-related artifacts that distort the apparent structural geometry; modern seismic interpretation routinely uses both time-domain and depth-converted displays, with the comparison between the two supporting identification of velocity-related artifacts including push-downs and the equivalent pull-up phenomena (where higher-velocity features cause underlying reflectors to appear shallower than they actually are).
- Hydrocarbon indicator applications of push-down phenomena exploit the velocity-saturation relationship to identify potential hydrocarbon zones — gas-bearing rocks have substantially lower velocity than equivalent water-bearing rocks (typically 10-30 percent lower depending on saturation), with the resulting push-down of underlying reflectors providing a direct hydrocarbon indicator (DHI); oil-bearing rocks have only modestly lower velocity than water-bearing rocks (typically 1-5 percent lower), with weaker push-down effects that may or may not be reliably detected; modern seismic interpretation uses push-down analysis combined with other DHI methods (amplitude analysis, AVO analysis, frequency analysis) for the integrated hydrocarbon identification that drives exploration prospect risking.
- Gas chimney recognition through push-down patterns supports identification of vertical migration pathways — gas chimneys are zones of upward gas migration through faults or other vertical pathways, with the gas-charged rocks producing distinctive seismic signatures including chaotic reflection patterns within the chimney, push-down of reflectors beneath the chimney, and amplitude effects (typically dim reflections within the chimney with brighter reflections at the chimney boundaries); the integrated gas chimney signature supports identification of these features in seismic interpretation, with the implications including potential charge to overlying traps (gas chimneys may indicate active hydrocarbon migration) and potential drilling hazards (gas-charged shallow zones can pose well control challenges).
- Depth conversion methodology accounts for velocity heterogeneity to remove push-down artifacts — modern depth conversion uses velocity models built from various sources including check-shot surveys (direct velocity measurement at well locations), VSP data (vertical seismic profile data providing detailed velocity information), seismic stacking velocities (derived from seismic processing), and geological models (using the expected lithology distribution to constrain the velocity model); the resulting depth-converted seismic display shows the actual subsurface geometry without the time-domain velocity-related distortions; for areas with strong velocity heterogeneity, the depth conversion may include sophisticated layered velocity models or full pre-stack depth migration that accounts for the velocity variations during the imaging process.
- Operational implications of push-down recognition include exploration prospect evaluation (the apparent structural lows from push-down may not be actual structural features and should not be used as exploration targets), drilling hazard assessment (gas chimneys identified through push-down patterns may pose drilling hazards from shallow gas), and reservoir characterization (gas-charged zones identified through push-down support reservoir mapping); modern integrated seismic interpretation includes systematic push-down analysis as part of the broader hydrocarbon indicator and structural interpretation workflow that drives exploration and development decisions.
Fast Facts
Push-down recognition has been part of seismic interpretation since the development of modern seismic exploration techniques, with continuous refinement of recognition methods and integration with broader hydrocarbon indicator analysis over decades. Modern seismic interpretation routinely uses push-down analysis combined with other DHI methods to support exploration prospect evaluation across diverse petroleum basins worldwide.
What Is Push-Down?
Push-down is a seismic interpretation phenomenon where low-velocity features cause apparent structural lows in underlying reflectors on time-domain seismic data. Recognition of push-down patterns supports hydrocarbon identification, gas chimney mapping, and proper structural interpretation through depth conversion.
Synonyms and Related Terminology
Push-down is sometimes called velocity push-down or velocity sag. Related terms include pull-up (the opposite phenomenon), hydrocarbon indicator (related concept), gas chimney (typical cause), depth conversion (the correction), velocity model (the analysis tool), seismic interpretation (the application), check-shot survey (velocity calibration), VSP (related velocity measurement), and dim spot (related phenomenon).
Why Push-Down Matters in Seismic Interpretation
Push-down recognition supports proper seismic interpretation by distinguishing velocity-related artifacts from actual structural features, with the integrated analysis driving exploration prospect evaluation and drilling planning. The continued application of push-down analysis in modern seismic interpretation demonstrates the value of recognizing these velocity-related phenomena.