High-resolution Seismic Reflection Techniques Research Articles

Sizing the carbon sink associated with Posidonia oceanica seagrass meadows using very high-resolution seismic reflection imaging

Among blue carbon ecosystems, seagrass meadows have been highlighted for their contribution to the ocean carbon cycle and climate change mitigation derived from their capacity to store large amounts of carbon over long periods of time in their sediments. Most of the available estimates of carbon stocks beneath seagrass meadows are based on the analysis of short sediment cores in very limited numbers. In this study, high-resolution seismic reflection techniques were applied to obtain an accurate estimate of the potential size of the organic deposit underlying the meadows of the Mediterranean seagrass Posidonia oceanica (known as ‘matte’). Seismic profiles were collected over 1380 km of the eastern continental shelf of Corsica (France, Mediterranean Sea) to perform a large-scale inventory of the carbon stock stored in sediments. The seismic data were ground-truthed by sampling sediment cores and using calibrated seismo-acoustic surveys. The data interpolation map highlighted a strong spatial heterogeneity of the matte thickness. The height of the matte at the site was estimated at 251.9 cm, being maximum in shallow waters (10–20 m depth), near river mouths and lagoon outlets, where the thickness reached up to 867 cm. Radiocarbon dates revealed the presence of seagrass meadows since the mid-Holocene (7000–9000 cal yr BP). Through the top meter of soil, the matte age was estimated at 1656 ± 528 cal yr BP. The accretion rate showed a high variability resulting from the interplay of multiple factors. Based on the surface area occupied by the meadows, the average matte thickness underneath them and the carbon content, the matte volume and total Corg stock were estimated at 403.5 ± 49.4 million m3 and 15.6 ± 2.2 million t Corg, respectively. These results confirm the need for the application of large-scale methods to estimate the size of the carbon sink associated with seagrass meadows worldwide.

Shallow water acoustic techniques to investigate transitional environments: A case study over Boka Kotorska Bay

The eastern border of the Adriatic Sea shows several examples of transitional areas marked by fragmented coastlines, islands and coastal bays. Bays and estuaries interact with the main basin influencing it and being influenced by it in terms of circulation patterns and freshwater supply. Coastal and transitional areas represent highly dynamic, rapidly changing environments subjected to multiple interactions with the marine-land-fluvial systems related processes. Processes such as coastal erosion, seawater intrusion, pollution and sediment transport and deposition affect these areas. One of the most important transitional areas along the entire Adriatic coast, from both an environmental and a socio-economic viewpoint, is the bay of Boka Kotorska, where the sea enters inland for over 20 km. The Bay is located along the Montenegro margin and is part of a ria coastal system surrounded by high mountains part of the Dinaric range. Boka Kotorska Bay includes three major basins, connected by two narrow straits with a maximum depth of 67 m. Despite its historical and geostrategic role the morphology and geology of the Bay is poorly known. Coastal areas are among the most urbanized and populated regions; for these reasons the application of geophysical minimally invasive methods are required in order to collect geological and morphological data. High resolution seismic reflection techniques such as CHIRP systems, adopting a non-impulsive source wavelet, are ideal to carry out seismic surveys in transitional environments. Here, we present a detailed description of this system in order to furnish the mathematical basis for forward modelling. Moreover, we outline a procedure to estimate the earth reflectivity by inversion of the recorded signal envelope (reflection strength), i.e., the standard way to store CHIRP data. Reflection strength data alone, characterizing the signal power as a function of time, do not retain all the characteristics of the recorded signal they represent and thus, cannot be further processed and hardly be compared with results from forward modeling. New high resolution multibeam and seismic reflection data collected in the Boka Kotorska Bay reveal unknown details of present-day morphologies and sedimentary infilling geometries. Several processes are involved in shaping the bay and its seafloor. Our results show that the observed morphologies are due to the interaction at different timescales of climate, water circulation, sealevel changes, erosion, sedimentation and tectonics constrained by the geological and structural setting of the area.

Submerged beachrock preservation in the context of wave ravinement

This study examines a Holocene-aged submerged shoreline, Limestone Reef, located in the shallow subtidal zone of South Africa’s east coast. It comprises an elongate, coast-oblique, slab-like outcrop of beachrock situated above the contemporary fair-weather wave base. It is currently undergoing mechanical disintegration. Its unique and rare preservation in a high-energy setting affords an opportunity to examine the mechanical processes occurring during wave ravinement associated with rising sea level. The submerged shoreline and the adjacent shoreface were examined using high-resolution seismic reflection, side-scan sonar and shallow-water multibeam echosounding techniques. Limestone Reef rests on top of unconsolidated Holocene deposits. The structure’s surface is characterised by reef-perpendicular gullies with rubble derived from the slab fringing its seaward edge. Limestone Reef slopes gently seawards and has a steep landward-facing edge where gullies are most prominently developed. Teardrop-shaped rippled scour depressions, marked by high backscatter, are located seawards of the submerged shoreline. These elongate in a seaward direction and are filled with bioclastic gravels and residual rubble from Limestone Reef. The gullies in the upstanding structure are indicative of wave plucking and abrasion of the shoreline. The material exposed by the rippled scour depressions is identical to that comprising the postglacial ravinement surface identified in the offshore stratigraphy. These deposits are considered to represent the contemporary, actively forming wave ravinement surface. The results suggest that wave ravinement of submerged shorelines is a discontinuous process dominated by the seaward entrainment of material from its landward edge controlled by high-energy drawback during storm surges. The ravinement process appears to operate at the seasonal scale and averages out over the long-term millennial scale for the continuous surface.

High resolution seismic reflection studies in Godavari coal fields: Mapping of coal seams and associated structural features

High Resolution Seismic Reflection technique is one of the most suitable geophysical methods for mapping the shallow coal seams and associated structural features, which will provide essential input for adopting proper methodology for exploitation and mine planning of coal reserves. In the present study, depth of the coal horizons were determined and the geometry of the faults in Ramagundam area, Gondwana basin is illustrated which is located at the junction of the Bastar and Dharwar cratons. The High Resolution Seismic Surveys conducted along eight parallel profiles with coverage of surface area one km2 in the study region. The study reveals (1) seismic sections up to 1000m depth with thick, gently dipping Barakar Formation, (2) thick coal horizons at a depth range of 200 to 500m, and (3) NW-SE striking and gently dipping normal faults are observed in coal horizons. The obtained results will provide better knowledge of the coal reserves in the study area.

Mapping shallow groundwater aquifer by performing high-resolution seismic reflection technique in Wadi Nisah

SUMMARY This study was carried out in Wadi Nisah to the southwest of Riyadh, Saudi Arabia, at latitude of 24° 14' 28 N and longitude of 46° 29' 59 E. The main purpose of this study is to investigate the depth of the shallow groundwater aquifer and the geological structures that could affect the ground water bearing layer in the area. For this purpose, the high-resolution seismic reflection technique was performed along Six 2D seismic lines and geophone spacing of one meter. The constructed seismic sections revealed that the depth of the water bearing formation lies in the range of 100m to about 240m and normal faulting affect the surface of this formation. The interpreted water-bearing formation is correlated with Biyadh Formation as confirmed by the drilled near water wells. The output of this research proved that the high resolution seismic reflection technique can be an effective method for determining water bearing layer depth in such arid area.

Mapping of Bedrock Using the High-Resolution Seismic Reflection Technique at Wadi Al Dawasir Region, Saudi Arabia

This paper critically evaluates the utility of seismic data to assist in the interpretation of bedrock. Four high-resolution seismic reflection profiles were carried out to provide estimates of the depth to the bedrock and to detect any geological faults present in the area, which may affect the hydro-geological system at Wadi Al Dawasir region, 690 km south of Riyadh city. The bedrock reflection is clearly present in all sites. The depth of the bedrock at site 1 was approximately 750 m, 800 m at site 2, 700 m at site 3, and 950 m at site 4. The bedrock depth obtained from the seismic agreed with the well information. We developed a clear understanding of the bedrock in the study area and mapped its depth as well as mapped some of the clear fault locations.

Amplitude Versus Offset (AVO) An Emerging Tools for Mineral Exploration in the Basement Complex: Case Study of Zaria, North Central Nigeria.

The mining industry has traditionally used geologic field mapping, electromagnetic and potential field techniques, and drilling to explore for new mineral deposits, but with new discoveries of large near-surface deposits becoming increasingly rare and the known reserves of most economic minerals in decline, it is clear that new deep exploration techniques are required to meet the future needs of industry and society. With gravity and magnetic methods unable to resolve targets beyond about 500 m, high-resolution seismic reflection techniques similar to those used by the petroleum industry, but modified for the hardrock environment, show the greatest potential for extending exploration to depths of 3 km, the current maximum depth of mining. A high resolution seismic technique has recently been carried out in Zaria to delineate and characterise the structures and lithologies within the batholith using AVO analysis. The geology of Zaria revealed that it is made up of Precambrian rocks that consist of granite that form the major part of the batholiths along with low grade meta-sediments and gneisses that constitute the country rock into which the granitic batholiths intruded. The results of the AVO analysis showed a general trend of increase of amplitude with offset. Three AVO types were identified, which include type -3 that corresponds with occurrence of sills, type -4 that corresponds with the occurrence of granite and gneisses and type -5 that identified the occurrence of fracture basement. The effectiveness in the identification of the various lithologies and structures showed that AVO will serve as an effective tool for mineral exploration in the nearest future.

Mapping of bedrock using the high-resolution seismic reflection technique at Wadi Al Dawasir Region, Saudi Arabia

This paper critically evaluates the utility of seismic data to assist in the interpretation of bedrock. Four high-resolution seismic reflection profiles were carried out to provide estimates of the depth to the bedrock and to detect any geological faults present in the area, which may affect the hydro-geological system at Wadi Al Dawasir region, 690 km south of Riyadh city. The bedrock reflection is clearly present in all sites. The depth of the bedrock at site 1 was approximately 750 m, 800 m at site 2, 700 m at site 3, and 950 m at site 4. The bedrock depth obtained from the seismic agreed with the well information. We developed a clear understanding of the bedrock in the study area and mapped its depth as well as mapped some of the clear fault locations.

Integration of P- and SH-wave high-resolution seismic reflection and micro-gravity techniques to improve interpretation of shallow subsurface structure: New Madrid seismic zone

Shallow high-resolution seismic reflection surveys have traditionally been restricted to either compressional (P) or horizontally polarized shear (SH) waves in order to produce 2-D images of subsurface structure. The northernmost Mississippi embayment and coincident New Madrid seismic zone (NMSZ) provide an ideal laboratory to study the experimental use of integrating P- and SH-wave seismic profiles, integrated, where practicable, with micro-gravity data. In this area, the relation between “deeper” deformation of Paleozoic bedrock associated with the formation of the Reelfoot rift and NMSZ seismicity and “shallower” deformation of overlying sediments has remained elusive, but could be revealed using integrated P- and SH-wave reflection. Surface expressions of deformation are almost non-existent in this region, which makes seismic reflection surveying the only means of detecting structures that are possibly pertinent to seismic hazard assessment. Since P- and SH-waves respond differently to the rock and fluid properties and travel at dissimilar speeds, the resulting seismic profiles provide complementary views of the subsurface based on different levels of resolution and imaging capability. P-wave profiles acquired in southwestern Illinois and western Kentucky (USA) detect faulting of deep, Paleozoic bedrock and Cretaceous reflectors while coincident SH-wave surveys show that this deformation propagates higher into overlying Tertiary and Quaternary strata. Forward modeling of micro-gravity data acquired along one of the seismic profiles further supports an interpretation of faulting of bedrock and Cretaceous strata. The integration of the two seismic and the micro-gravity methods therefore increases the scope for investigating the relation between the older and younger deformation in an area of critical seismic hazard.

Imaging wooden artefacts using Chirp sources

This paper investigates the potential for imaging submerged and buried wooden artefacts in the marine environment using high-resolution seismic reflection techniques. Models to calculate theoretical reflection coefficients of 11 wood species buried in shallow marine sediments are developed. The models use estimates of acoustic impedance contrasts to predict the strength of reflections resulting from buried wooden artefacts. Traditionally, wooden wrecks are composed of oak, with lesser components of mahogany, pine and elm. Experimental results on wood samples from a sixteenth century oak wreck (the Mary Rose) are presented as a control on theory. Results indicate that wooden wrecks may be readily imaged by a suitable sub-bottom profiler operating under appropriate survey conditions. Particular reference is made to Chirp sonar, which transmits a frequency modulated pulse, providing high-resolution digital seismic data. Chirp data acquired over an eighteenth century buried oak wreck (the Invincible, Horsetail, East Solent) is presented. Calculations of reflection coefficients from reflection data acquired over the site support both the theoretically and experimentally derived data. © 1997 John Wiley & Sons, Ltd.

Submerged sand ridges on the western continental shelf off Bombay, India: evidence for Late Pleistocene-Holocene sea-level changes

The morphology, distribution and probable origin of submerged sand ridges on the western continental shelf off Bombay, India have been studied using echosounding, side-scan sonar and high-resolution seismic reflection techniques supplemented by seabed sampling. 7–8 rows of well defined, broad, symmetrical submerged sand ridges were found on the middle and the outer shelf 100–200 km offshore and between 75 and 100 m water depth, the ridges being more prominent between 85 and 90 m. These ridges are 0.5 to 10 km wide, 1.5 to 18 m high, several tens of km long, with a few kilometers spacing and a trend of NNW-SSE parallel to the shelf edge. The inter-ridge areas are characterised by long, narrow palaeo-lagoonal-like depressions. Some rounded/smooth ridges appear to be moribund while others have undergone reworking as indicated by superimposed bedforms/sand waves and reefs. Ridges also occurred on the inner shelf where they were either eroded during the Holocene transgression or covered by Recent sediments. The sand ridges originated as dunes which were submerged and reformed into very large sand bars. Further drowning and lithification favoured the development of organic growth and sand wave formation which must have taken place during a period of stillstand or slow sea level rise in the early to mid-Holocene. The superimposed bedforms reflect the prevailing flow regime on the shelf.

Seismic imaging of massive sulfide deposits; Part I, Rock properties

Laboratory measurements of the density (rho ) and compressional wave velocity (v P ) of massive sulfides and host rocks at elevated pressures show that ores occupy a V P - rho field which is distinct from that of common silicate rocks and governed by simple mixing rules between the properties of host rocks and pure sulfide minerals. The velocities of the most common sulfides are extremely variable, ranging from 3.7 km/s for galena, through 4.7 km/s for pyrrhotite, 5.6 km/s for sphalerite and chalcopyrite, to 8.0 km/s for pyrite, but their densities are always high (7.6, 4.6, 4.1 - 4.2, and 5.0 g/cc, respectively), causing ores of even intermediate grade to have high acoustic impedances. This suggests that large massive sulfide bodies should make strong to spectacular seismic reflectors, depending on their grade, mineralogy, size, and setting. In principle, it should thus be possible to use high-resolution seismic reflection techniques for base metal exploration and delineation.

Application of high resolution seismic reflection techniques for fracture mapping in crystalline rocks

Surface reflection seismic techniques have the capability of mapping subsurface geological features without disturbing the rock mass. They also have an added capability of penetrating to a much deeper depth than any other geophysical technique, including the ground probing radar. However, the successful application of reflection seismic techniques in crystalline rocks has in general been more difficult than in sedimentary basins, because of the irregular geometry and low acoustic impedance contrasts across geological boundaries. In this paper, we describe the imaging of fracture zones in crystalline rocks. Effective data processing, carefully modified from the conventional approaches, was applied on two high-resolution field data previously collected by different contractors. The strategy included enhancement of the signal hidden under the large-amplitude scattering noise, through pre- and post-stack processing such as shot f-k filtering, residual statics and careful muting after NMO correction. Two sets of low S/N test data from Canada and Sweden are analyzed in this research. The reflected energy in these data sets appeared to be more closely related to fracturing than to lithologic boundaries. The major fracture zones at shallow depth have been mapped with the desired resolution and can be correlated to the available well-log and seismic crosshole tomographic data. Once the surface waves were removed, shallow reflectors in the fracture zones could be identified and analyzed on the field records. Focusing analysis of the seismic image was performed in the constant-offset section to investigate the trends of major fracture zones. The complex attributes were also analyzed to determine whether they could be applied to the shallow fracture zones. Instantaneous frequency plots outline the intense fracturing zone and instantaneous phase plots identify the major and minor fractures, and other coherent events with different dip attitudes which interfere with each other.

Imaging a buried river channel in an intertidal area of South Wales using high-resolution seismic techniques : Brabham, P J; McDonald, R J Q J Engng GeolV25, N3, 1992, P227–238

A buried river channel in an intertidal area at Barry Old Harbour, South Wales, UK, is investigated using high-resolution seismic refraction and reflection techniques. The refraction technique is able to profile the base of the 20 m deep buried valley because the Carboniferous Limestone bedrock has a significant velocity contrast with overlying water- saturated unconsolidated sediments. Common midpoint (CMP) reflection profiling, using a single detonator as the seismic source, images the base of the river channel and the internal sedimentary sequences. Seismic reflections with frequencies in the 400 to 600 Hz range are observed from as shallow as 5 m depth (6 ms two-way time). The bedrock profiles derived from the refraction and reflection experiments are in precise agreement. (A)

Imaging a buried river channel in an intertidal area of South Wales using high-resolution seismic techniques

Abstract A buried river channel in an intertidal area at Barry Old Harbour, South Wales, UK, is investigated using high-resolution seismic refraction and reflection techniques. The refraction technique is able to profile the base of the 20m deep buried valley because the Carboniferous Limestone bedrock has a significant velocity contrast with overlying water-saturated unconsolidated sediments. Common midpoint (CMP) reflection profiling, using a single detonator as the seismic source, images the base of the river channel and the internal sedimentary sequences. Seismic reflections with frequencies in the 400 to 600 Hz range are observed from as shallow as 5 m depth (6 ms two-way time). The bedrock profiles derived from the refraction and reflection experiments are in precise agreement.

Mapping of quaternary deposits and bedrock topography using high-resolution seismic reflection techniques

Marine normal-incidence reflection and P- and S-wave refraction/wide-angle reflection data from the BABEL project, line 1, in the Bothnian Sea have been used for investigating the nature of the reflectivity in the area. The data are available along the same line, and therefore the same piece of crust is probed by two seismic methods. We use this favourable circumstance for model calculations concerning both data sets by analysing the waveforms of reflections from normal-incidence traveltimes at approximately 4, 12 and 16 s TWT. Strong sub-horizontal reflections are observed on the normal-incidence reflection section at about 4 s TWT. These reflections have been modelled using the reflectivity method (1-D modelling) utilising the water bottom reflection as an estimate of the source wavelet and show a positive polarity. Thickness and P-wave velocity consistent with these reflections are 50 m and 7 km/s, respectively, with a host rock velocity of 6 km/s. From the polarity, velocity and thickness, these reflections are attributed to dolerite sills in the upper crust. Strong lower-crustal reflections are observed on the wide-angle and normal-incidence reflection sections. However, the corresponding S-wave reflections from the lower crust are considerably weaker. The implied high positive impedance contrast for the P-wave lower-crustal reflection and the weak S-wave reflectivity suggests the presence of a highly mafic sheet at about 40 km depth. The wide-angle P- and S-wave sections show strong reflections at the Moho. Coincident normal incidence data show only weak Moho reflectivity indicating a gradient in velocity at the crust/mantle boundary. Dipping events onlap the strong sub-horizontal lower crustal and the P-wave Moho reflections on the wide-angle data. These are explained by dipping structures above the lower crustal reflector and below the Moho, with the latter also consisting of a gradient in velocity.

High-resolution seismic reflection survey of a massive sulfide orebody in Sardinia, Italy — An experiment

A first attempt to investigate the application of the high-resolution seismic reflection technique in massive ore exploration was carried out in Sardinia (Italy). A mixed sulfide mineralized orebody (pyrite and sphalerite) within Cambrian carbonate rocks in the Campo Pisano mining area (southwest Sardinia) was investigated. The seismic profile data, obtained using a Geometrics ES-2420 seismograph, were subsequently processed on a Digital VAX/VMS computer with the CogniSeis software seismic package DISCO. On the basis of this analysis, it is possible to delineate with the seismic reflection technique, the contact surfaces between the mineralized orebody and surrounding rock, on condition that an accurate selection of the acquisition parameters and appropriate data processing be made.

Locating faults in underground coal mines using high-resolution seismic reflection techniques : Gochioco, L M; Cotten, S A GeophysicsV54, N12, Dec 1989, P1521–1527

Locating faults in underground coal mines using high-resolution seismic reflection techniques : Gochioco, L M; Cotten, S A GeophysicsV54, N12, Dec 1989, P1521–1527

Direct detection of shallow surface voids using high-resolution seismic-reflection techniques : Steeples, D W; Miller, R D Proc 2nd Multidisciplinary Conference on Sinkholes and the Environmental Impacts of Karst, Orlando, 9–11 February 1987P179–183. Publ Rotterdam: A A Balkema, 1987

Direct detection of shallow surface voids using high-resolution seismic-reflection techniques : Steeples, D W; Miller, R D Proc 2nd Multidisciplinary Conference on Sinkholes and the Environmental Impacts of Karst, Orlando, 9–11 February 1987P179–183. Publ Rotterdam: A A Balkema, 1987

High resolution seismic reflection, an exploration tool within an underground environment (example from Zimbabwe)

Metallurgical grade chromite ore in Zimbabwe is mined from two underground mines, Peak Mine and Railway Block Mine, in Shurugwi. Peak Mine is at present just over 800 m deep. In the search for new chromite ore bodies, such a depth limits the application of the conventional geophysical exploration tools. Exploration diamond drilling is becoming more and more an expensive resort. Alternative and effective geophysical techniques are therefore being actively sought after. The high resolution seismic reflection technique, carried out right within Peak Mine, has the potential to become a useful exploration tool.