Profile
In geophysics, to profile means to measure how a property changes along a line on the surface or down a borehole. A gravity profile records gravity readings at stations spaced along a road or a survey line. A magnetic profile records the magnetic field along the same kind of traverse. A seismic profile is a 2D vertical slice through the earth assembled from a line of sources and receivers. The profile shows the lateral or vertical variation of the property as a single curve or section. Profiling is one of the oldest geophysical techniques, dating to the early 20th century, and it remains in use today as a quick, low-cost way to map subsurface variations before committing to a more expensive 3D survey.
Key Takeaways
- To profile a property in geophysics is to measure that property along a line, producing a 1D curve or 2D section that shows how the property changes with position. Profiling contrasts with probing, which measures variation with depth at a single point.
- Common surface profiles include gravity, magnetic, electromagnetic, and seismic. Each profile uses different physical instrumentation but follows the same basic geometry: stations or sources spaced along a line, with measurements recorded at each station and plotted as a function of distance.
- A 2D seismic profile is a vertical cross-section through the earth assembled from sources and receivers spaced along a straight line. The output is a depth or time section that shows reflectors as wavy lines stacked above one another, allowing geologists to map structural and stratigraphic features along the line.
- Surface profiles were the dominant geophysical exploration tool from the 1920s through the 1970s before 3D seismic became economical. Many of the world's giant oil fields were originally identified by gravity or magnetic anomalies on regional profiles, then confirmed by 2D seismic profiles, and only later imaged in 3D after they were already producing.
- Profiles still play a major role in regional reconnaissance, in low-budget exploration, and in monitoring applications where 3D coverage is not justified. Time-lapse gravity profiles are used to monitor reservoir compaction in some Norwegian Continental Shelf fields. Borehole gravity profiles are used to detect bypassed pay behind casing in mature fields worldwide.
Fast Facts
The first systematic geophysical profiling for petroleum exploration was the 1920s gravity surveys over Gulf of Mexico salt domes. Crews dragged torsion balance instruments along survey lines, taking hours per station, and built up gravity profiles that revealed the dense rock surrounding salt diapirs as small but measurable anomalies. The Spindletop and Damon Mound salt domes were among the first oil fields confirmed by gravity profiling. The same physics applies today, but a modern airborne gravity gradiometer can collect more data in one hour than the entire 1920s industry collected in a year.
What Profiling Means in Geophysical Practice
Imagine measuring the temperature in your house by walking from room to room with a thermometer. You write down the reading at each location: 19 degrees in the kitchen, 22 in the living room, 17 in the basement, 23 in the upstairs bedroom. Plot those numbers against position and you have a temperature profile of your house. The profile shows you which rooms are warm and which are cold without telling you why.
Geophysical profiling does the same thing but for properties of the earth that cannot be measured directly with a thermometer. A gravity profile measures the local gravitational acceleration at stations spaced along a road. The readings vary by a tiny fraction of a percent depending on what is below the surface: dense rock makes gravity slightly stronger, low-density salt or sediment makes it slightly weaker. The pattern in the profile reveals subsurface structure that would otherwise be invisible.
A magnetic profile does the same with the earth's magnetic field. Rocks with high iron content (basalt, gabbro, certain volcanic rocks) produce magnetic anomalies. Sedimentary rocks generally do not. A magnetic profile across a sedimentary basin can map the depth to magnetic basement, which controls the basin geometry and the volume of sedimentary rock available for petroleum systems.
Where Profiles Show Up in Modern Exploration
Seismic profiles are the most common type in modern oil and gas exploration. A 2D seismic profile is built by laying out a line of geophones along a survey traverse, firing a source at one end, and recording the seismic waves that travel through the earth and reflect off subsurface boundaries. The recorded data is processed into a vertical cross-section that shows reflectors stacked from the surface down to several kilometres depth. Geologists interpret the section by tracing reflectors, identifying structural features (folds, faults, salt bodies), and mapping stratigraphic units.
Gravity and magnetic profiling remain in use for regional-scale work, particularly in frontier basins where seismic coverage is sparse. The Trinidad and Tobago offshore margin, the Falkland Islands shelf, parts of the Carnarvon Basin in Australia, and some areas of the East African Rift have been mapped extensively with gravity and magnetic profiles before any seismic was acquired. These methods are cheaper than seismic per kilometre and can be flown by aircraft, giving rapid coverage of large areas at modest cost.
Specialized profiling techniques fill specific niches. Vertical seismic profiling (VSP) is a borehole method where receivers are placed in a well and surface sources fire from various offsets, building up a high-resolution image of the formation around the well. Time-lapse 4D seismic is essentially a series of 3D surveys repeated over time, with each survey functioning as a profile of the reservoir state at that point in its production life.
Synonyms and Related Terminology
Profiling is sometimes called surveying along a line, traversing, or running a section. A 2D seismic profile is also called a seismic line or 2D line. The contrasting term is "probing," used for measurements that vary with depth at a fixed surface location (such as a vertical electrical sounding). Related terms include 2D seismic (a seismic survey acquired along a single line of sources and receivers, producing a vertical cross-section through the earth; the original form of seismic exploration before 3D surveys became standard), 3D seismic (a volumetric seismic survey acquired with sources and receivers distributed over an area; produces a 3D image of the subsurface that can be sliced in any direction; the modern standard for detailed exploration and development), gravity survey (a geophysical method that measures small variations in the earth's gravitational field along profiles or grids; used to map subsurface density variations including salt bodies, basement structure, and basin geometry), magnetic survey (a geophysical method that measures variations in the earth's magnetic field; used primarily to map magnetic basement depth and to identify mafic intrusive rocks; commonly flown by aircraft for regional coverage), and vertical seismic profile (VSP, a borehole seismic method that places receivers in a wellbore and records waves from surface sources at various offsets; produces a high-resolution image of the formation immediately around the well).
Why a Single Line of Data Can Find a Field That a Map Cannot
An exploration team is evaluating a frontier basin off the coast of West Africa. They have regional gravity data acquired by aircraft over the past five years, but no seismic. The gravity coverage is dense enough to produce a contour map of the area, but most of the variation in the map is from regional crustal effects, not from the local prospects the team cares about.
The team commissions a 2D seismic profile across one promising trend identified on the gravity map. The profile takes three weeks to acquire and another two months to process. The result is a 28-kilometre cross-section through the suspected prospect, showing a clear three-way structural closure against a basin-bounding fault, with multiple stacked reservoir-quality reflectors above the closure.
That single 2D line is the data that converts a regional gravity hint into a drillable prospect. The map showed where to look. The line confirmed what was actually there. Two years later, the team drills a wildcat on the closure and discovers 320 million barrels of recoverable oil. The total cost of the gravity survey was about USD 1.1 million. The total cost of the 2D line was about USD 800,000. The total cost of the wildcat well was USD 95 million, and the field is on track to generate billions in cumulative revenue. Most of the value of geophysical profiling is in those early-stage decisions: where to look, where to drill, and which prospects to walk away from. The line of data does what no map can do alone.