Fan Shooting

Key Takeaways

• The shadow zone mechanism makes fan shooting particularly diagnostic for detecting buried salt domes: refracted energy traveling through the slower surrounding sediments at the standard refraction velocity arrives at surface geophones at the expected travel time calculated from the shot-receiver offset divided by the formation velocity, but geophones positioned such that their raypath to the shot passes through or near the high-velocity salt body receive the refracted head wave significantly earlier than predicted because the salt conducts the energy faster than sediments; conversely, geophones at azimuths where the subsurface raypath would need to pass through the interior of the dome may receive late arrivals or no clear refraction arrival at all (the shadow zone) because the dome's velocity contrast deflects the head wave energy away from those directions; the fan array is designed to sample enough azimuths (typically every 5 to 15 degrees around the shot point, spanning a 180-degree or 360-degree fan) that the angular boundaries between the early-arrival zone and the shadow zone can be identified with sufficient precision to locate the salt dome's lateral position to within a few hundred feet.
• Fan shooting field operations in the 1920s and 1930s used early mechanical seismographs and required coordinated deployment of multiple recording crews around a central shot point, with the explosive charges (typically 50 to 500 pounds of dynamite in a shot hole) detonated at a precisely measured time recorded on both the shot and the receiver seismographs: the shot-to-receiver distances were several miles to allow the refracted head wave (which travels at the high velocity of the deep formation) to overtake the direct wave through the shallow slow sediments and arrive first at the surface geophones; the travel time differences across the fan were measured in milliseconds from the paper seismograph records, and these time differences were converted to subsurface velocity anomaly maps by comparing observed arrival times to the arrival times predicted from a reference sediment velocity; the equipment limitations of the era (no magnetic recording, analog galvanometer-based seismographs, manual timing from tuning-fork frequency standards) introduced measurement uncertainties of a few milliseconds that limited the resolution of the fan shooting maps, but were sufficient to identify salt dome anomalies at distances of miles from the dome center.
• Historical significance of fan shooting in opening the Gulf Coast petroleum province is substantial: the method's invention and application in the 1920s by the Geophysical Research Corporation (GRC) and by teams working for major oil companies enabled the systematic exploration of the Texas and Louisiana Gulf Coast, where hundreds of salt domes underlie the shallow-water continental shelf and onshore coastal plains; the first commercial use of fan shooting to successfully locate an oil-bearing salt dome is generally credited to the discovery of the Orchard field in Texas in 1924, and within a decade fan shooting and the contemporaneous torsion balance gravity survey method had identified most of the major structural salt dome features in the Gulf Coast region; without fan shooting, the discovery of major Gulf Coast fields including Spindletop-era domes and subsequent offshore salt structures would have required random drilling or much later discovery by gravity or magnetic methods that became practical only with improvements in instrumentation in the 1930s and 1940s.
• Fan shooting's replacement by reflection seismology beginning in the 1930s and its near-total obsolescence by the 1950s reflects both the limitations of refraction methods and the dramatically superior resolution of reflection data: fan shooting maps the top or flanks of a high-velocity anomaly in two dimensions on a rough plan map, but cannot determine the precise geometry of the salt dome or the detailed structure of the sedimentary layers draped around it that control the location of oil and gas traps; reflection seismology records the two-way travel time of reflected seismic energy from subsurface interfaces, creating a cross-sectional image of the subsurface stratigraphy and structure at much finer resolution than refraction methods; the development of the continuous seismic profiling method (shooting and recording while moving) in the 1930s and the advent of multi-channel reflection recording with common mid-point stacking in the 1960s progressively reduced fan shooting to a historical technique, though refraction methods still see application in near-surface geotechnical investigations and in seismic velocity model building for deep reflection processing.
• Modern applications of the fan shooting concept appear in seismic refraction tomography, which uses large numbers of shot-receiver pairs at varied azimuths to invert first-arrival travel times for a two-dimensional or three-dimensional velocity model of the shallow subsurface, with applications in geotechnical site characterization, groundwater exploration, and as a tool for improving the near-surface velocity model used to correct reflection seismic data for near-surface velocity variations: refraction tomography shares the fundamental principle of fan shooting (using travel time variations across many source-receiver directions to infer velocity structure) but uses computerized inversion algorithms to extract far more detailed velocity information from much larger datasets than the manual travel time comparison of historical fan shooting; the near-surface velocity models produced by refraction tomography in areas with complex weathering (thick low-velocity glacial sediments, laterite layers, or permafrost) are essential for applying the static corrections that improve reflection data quality and enable accurate structural mapping of the deeper exploration targets.