Deep Seismic Sounding | Oil Authority

Deep seismic sounding (DSS) is a seismic technique that uses very long source-receiver offsets (typically 100 to 500 kilometers or more), powerful energy sources (large explosive charges, arrays of vibrators, or airgun arrays), and long recording times (10 to 30 seconds or more of two-way travel time) to image the entire crust and upper mantle of the earth, including the Mohorovicic discontinuity (the Moho, the boundary between the crust and mantle at depths of 25-60 km under continental crust), deep crustal faults, intracrustal reflectors, and the lithospheric structure that controls the tectonic setting and thermal evolution of sedimentary basins; in petroleum exploration, deep seismic sounding is used in frontier basin evaluation and regional basin framework studies to understand the basement architecture, crustal thickness variations, and deep-seated structural elements that control the overall geometry of sedimentary basins, the location of rift systems and passive margins that host petroleum-bearing sedimentary wedges, and the thermal history of the basin (which controls source rock maturation and the timing of petroleum generation) through the relationship between crustal type and thickness and the geothermal gradient; deep seismic surveys provide the regional structural context within which conventional petroleum exploration seismic surveys (which typically record 5-15 seconds of two-way travel time and image only the upper few kilometers of sedimentary rock) are interpreted, answering fundamental geological questions about basin origin, subsidence mechanisms, and the deep crustal architecture that commercial exploration seismic cannot image.

Key Takeaways

The Moho reflection and refraction arrivals recorded on deep seismic sounding profiles provide the most direct measurement of crustal thickness available from surface geophysical methods, and crustal thickness is a first-order control on the thermal evolution of sedimentary basins and their petroleum potential: continental crust (average thickness 35-40 km) has lower heat flow than oceanic crust (average thickness 7 km) because the radioactive heat-producing elements (U, Th, K) are concentrated in the granitic continental crust; thin continental crust (less than 30 km, formed during rifting) has higher heat flow than normal continental crust because the thinning brings the hot mantle closer to the surface; the McKenzie uniform stretching model (published 1978) uses the stretching factor beta (the ratio of original to thinned crustal thickness measured from Moho reflection depths on DSS profiles) to calculate the subsidence history and thermal maturity of rift basin sediments, with higher beta values indicating more stretching, higher initial heat flow, and faster source rock maturation; DSS-determined Moho depths are thus a fundamental input to basin modeling workflows that reconstruct the burial and temperature history of source rocks to predict where oil generation occurred and where the oil migrated; continental margins where DSS reveals extreme crustal thinning (beta greater than 4-5) have the structural and thermal conditions for deep-water carbonate bank formation and for rapid source rock maturation that are characteristic of prolific petroleum systems like the Brazilian pre-salt and the North Sea.
Deep crustal reflectors imaged on DSS profiles reveal the structural architecture of sutures, ancient collision zones, detachment faults, and lower crustal flow zones that influence the geometry of overlying sedimentary basins and the location of structural traps: the middle and lower continental crust, when imaged by DSS, frequently shows bright, sub-horizontal to gently dipping reflectors interpreted as ductile shear zones, mafic intrusions (sills and lenses of denser rock injected into the lower crust during rifting or magmatic events), or ancient nappe and thrust structures from past orogenic episodes; these deep reflectors are invisible to commercial exploration seismic because their great depth (15-40 km) places them far below the recording time and source energy of standard petroleum seismic surveys; the relationship between deep crustal structures and the overlying basin geometry has direct petroleum exploration implications: Paleozoic sutures and thrust sheets imaged by DSS beneath the Precambrian basement of North American platforms influenced the geometry of Phanerozoic sedimentary cover and the location of structural traps; ancient rifts imaged in the lower crust beneath passive margins control the alignment of failed rift basins and the sediment supply patterns that determine the distribution of reservoir facies; mafic intrusions in the lower crust seen on DSS profiles identify regions where magmatic underplating has reset the thermal history of the basin, potentially reburying source rocks into the oil or gas window after an earlier period of cooling.
Wide-angle refraction surveys (WARR), a component of deep seismic sounding programs, record refracted arrivals at very large offsets from powerful sources and use the travel times of these first arrivals to build velocity models of the entire crust, complementing the reflectivity image provided by near-vertical incidence reflection recording: refracted P-wave arrivals from the Moho (Pn arrivals, traveling along the top of the mantle at approximately 8 km/s) can be recorded at offsets of 200-600 km from large explosive sources or arrays of airgun or vibrator sources, and their travel times define the depth to the Moho along the refraction profile; refraction velocity models resolve the average velocity in each crustal layer (upper crust at 5.8-6.2 km/s, middle crust at 6.4-6.8 km/s, lower crust at 6.8-7.3 km/s, upper mantle at 7.8-8.2 km/s) that is used to distinguish crustal type (continental versus oceanic versus transitional) along passive margins, to calibrate gravity anomaly models (since crustal density is related to seismic velocity through empirical relationships), and to improve the accuracy of migration of DSS reflection data by providing a better velocity model for the uppermost mantle and lower crust; the combination of reflection (vertical-incidence) and refraction (wide-angle) recording on the same deep seismic profile provides both the impedance image (from reflections) and the velocity model (from refractions) needed for comprehensive crustal characterization.
National and international deep seismic sounding programs have produced a global database of crustal structure profiles that form the regional geological framework for petroleum basin assessment: major programs include COCORP (Consortium for Continental Reflection Profiling, USA, active from 1975 to the mid-1990s, recording over 35,000 km of deep reflection profiles across the conterminous United States and revealing the geometry of major crustal provinces, sutures, and rift basins), BIRPS (British Institutions Reflection Profiling Syndicate, recording deep profiles around the UK continental shelf that imaged the Caledonian and Variscan sutures controlling North Sea basin geometry), ECORS (Etude de la Croute Continentale et Oceanique par Reflexion et Refraction Sismiques, France), Dekorp (Germany), and ANCORP/COCORP-South America programs; these profiles, when combined with regional potential field data (gravity and magnetics) and the emerging global seismic tomography models from earthquake seismology, provide the backbone for understanding how continental collision, rifting, and passive margin evolution created the sedimentary basin template within which petroleum exploration occurs; in frontier regions (Arctic shelf, East African rift lakes, deep-water passive margins of West Africa and South America), new deep seismic surveys are specifically acquired as part of pre-competitive government geological surveys to attract exploration investment by demonstrating the basin architecture and petroleum potential of the frontier area.
Deep seismic sounding data quality challenges arise from the great travel distances involved, the low frequency content of deep reflections, and the variable coupling of seismic energy into the earth at very long offsets: deep reflectors from the lower crust and Moho are recorded at very low frequencies (2-10 Hz for deep reflections, compared to 10-80 Hz for commercial seismic), because high-frequency energy is absorbed by anelastic attenuation as it travels through 50+ km of rock (intrinsic attenuation factor Q is typically 100-300 in the crust, causing a loss of approximately 1/Q of the energy per seismic wavelength); the low frequency and long travel path of DSS arrivals make them difficult to separate from surface wave noise and from the air wave (direct acoustic wave through the atmosphere from large explosive sources); recording very long two-way times (up to 30 seconds) requires specialized long-record seismographs and data acquisition systems not used in commercial exploration; near-surface velocity heterogeneity and topography introduce statics (time shifts) on DSS records that must be corrected before coherent reflections from deep crustal structures can be identified and stacked; the combination of these technical challenges means that DSS data quality is variable and requires specialized processing (including frequency-wavenumber filtering to remove coherent noise, very long operator deconvolution to compress the low-frequency source wavelet, and careful statics correction using shallow refraction data) before deep reflectors can be confidently identified and correlated along profiles.

Fast Facts

The systematic study of crustal structure using seismic refraction at long offsets began in the 1940s and 1950s, with major contributions from researchers at university and government geophysical institutes who used large chemical explosions as seismic sources and portable seismographs deployed along profiles hundreds of kilometers long to record the refracted arrivals from the Moho. The transition to deep seismic reflection profiling using commercial-style acquisition with Vibroseis sources and multi-channel recording was pioneered in the United States by the COCORP project beginning in 1975, led by geophysicists Jack Oliver and Sidney Kaufman at Cornell University, who demonstrated that the commercial reflection seismic technique could image the deep crust with unprecedented clarity by extending the recording time and using iterative processing methods to enhance deep, low-amplitude reflections. The COCORP profiles across the Appalachians, the Great Plains, the Basin and Range province, and the Gulf Coast transformed understanding of North American crustal structure and directly influenced petroleum exploration strategies by revealing the extent and geometry of Proterozoic rifts, sutures, and intracratonic basins beneath the sedimentary cover.

What Is Deep Seismic Sounding?

Deep seismic sounding is seismic profiling at the scale of the entire crust. Where commercial petroleum seismic surveys record reflections from depths of 1 to 10 kilometers over recording times of 5 to 15 seconds, deep seismic sounding extends the recording to 30 seconds (reaching the mantle at 60-90 km depth) and uses source-receiver offsets of hundreds of kilometers to capture refracted arrivals that map the velocity structure of the entire crust. The result is a view of the earth from its surface down to the base of the lithosphere — the Moho, the lower crustal flow zones, the ancient sutures and rifts that were the geological infrastructure on which sedimentary basins were later built. For petroleum exploration, the direct product of deep seismic sounding is the regional basin framework: where did rifting occur, how much did the crust thin, what is the thermal history implied by that thinning, and where are the deep basement highs and lows that guided the accumulation of sediment that later became reservoir and source rock. These are the first-order questions of any frontier exploration program, and deep seismic sounding is one of the few geophysical tools that can answer them from surface measurements without drilling a well.