Refraction Static Corrections Research Articles

Seismic imaging of the gold deposit and geological structures through reprocessing of legacy seismic profiles near Kloof–Driefontein Complex East Mine, South Africa

AbstractTwo legacy reflection seismic profiles were acquired in 1988, north of the Kloof–Driefontein Complex East Mine in the West Rand goldfield (South Africa), for the purpose of gold exploration and mine planning. These legacy 2D seismic data have been reprocessed using the latest processing tools to improve imaging. Special interest is given to the Black Reef Formation, which hosts a known gold orebody. The original legacy data are of poor quality, especially in areas that are dominated by dolomitic outcrops. To improve the quality of the data, special attention was given to the refraction static correction to enhance the continuity of the reflections below dolomitic rocks. Refraction seismic tomograms from both profiles exhibit three‐layer P‐wave velocity models: (1) topsoil (1000–2000 m/s), (2) a weathered layer ranging from ca. 100 to 300 m in thickness (2000–5000 m/s) and (3) bedrock (> 5000 m/s). Seismic profile OK‐212 shows poor imaging of the Black Reef Formation because of the scattering of seismic energy in the near‐surface due to dolomites from the Transvaal Supergroup, while seismic profile OK‐213 exhibits south‐dipping reflections that are associated with the Black Reef Formation. To improve the structural imaging resolution, we tried pre‐stack time migration, pre‐stack depth migration and post‐stack time migration using the Kirchhoff algorithm. PreSDM most improved the imaging of deeper reflections due to its ability to honour complex lateral variations in the velocity field. Both pre‐stack time migration and post‐stack time migration enhanced the continuity of the near‐surface reflections below the dolomitic rocks.

3D reflection seismic imaging of the Zinkgruvan mineral‐bearing structures in the south‐eastern Bergslagen mineral district (Sweden)

AbstractMineral exploration is facing greater challenges nowadays because of the increasing demand for raw materials and the lesser chance of finding large deposits at shallow depths. To be efficient and address new exploration challenges, high‐resolution and sensitive methods that are cost‐effective and environmentally friendly are required. In this work, we present the results of a sparse 3D seismic survey that was conducted in the Zinkgruvan mining area, in the Bergslagen mineral district of central Sweden. The survey covers an area of 10.5 km2 for deep targeting of massive sulphides in a polyphasic tectonic setting. A total of 1311 receivers and 950 shot points in a fixed 3D geometry setup were employed for the survey. Nine 2D profiles and a smaller 3D mesh were used. Shots were generated at every 10 m, and receivers were placed at every 10–20 m, along the 2D profiles, and 40–80 m in the mesh area. An analysis of the seismic fold coverage at depth was used to determine the potential resolving power of this sparse 3D setup. The data processing had to account for cultural noise from the operating mine and strong source‐generated surface waves, which were attenuated during both pre‐ and post‐stack processing steps. The processing workflow employed a combination of 2D and 3D refraction static corrections, and post‐stack FK filters along inlines and crosslines. The resulting 3D seismic volume is correlated with downhole data (density and P‐wave, acoustic impedance, reflection coefficient), synthetic seismograms, surface geology and a 3D model of mineral‐bearing horizons in order to suggest new exploration targets at depth. The overall geological architecture at Zinkgruvan is interpreted as two EW overturn folds, an antiform and a synform, affected by later NS‐trending folding. Two strong sets of shallow reflections, associated with the Zn–Pb mineralization, are located at the hinge of an EW‐trending antiform, while a strong set of reflections, associated with the main mineralization, is located at the overturned apex of the EW synform. The NS Knalla fault that crosses the study area terminates the continuation of the mineral‐bearing deposits at depth towards the west, a conclusion solely based on the reflectivity character of the seismic volume. This study illustrates that sparse 3D data acquisition, while it has its own challenges, can be a suitable replacement for 2D profiles while line cutting, and environmental footprints can totally be avoided.

Joint consistent static correction of PP- and PS-waves: A case history of Basin A

The shallow S-wave velocity model is difficult to accurately obtain in seismic exploration, resulting in poor imaging quality when traditional static correction methods are applied to PS-waves. Moreover, unlike for PP-waves, identifying an interface near the water table to use as the datum for PS-waves is almost impossible in many cases. We have developed a case history of joint consistent static correction for PP- and PS-waves of Basin A in Sichuan province, China. In our workflow, crosscorrelation calculation is performed on PP-wave common shotpoint and common receiver-point stacked sections and PS-wave common receiver-point stacked sections to jointly obtain stable shot and receiver statics of PP- and PS-waves. Based on the similarity between the PP-wave and the PS-wave events from the same interface, P-wave statics are used to constrain the calculation of S-wave statics. For comparison, conventional refraction static correction is implemented on the research data. Results show that both approaches produce comparable results for PP-waves, but the PS-wave results from the conventional approach indicate apparent residual statics. This case history demonstrates that joint consistent static correction can effectively solve the static correction problems of multicomponent seismic data and significantly improve the imaging quality.

Sparse 3D reflection seismic survey for deep-targeting iron oxide deposits and their host rocks, Ludvika Mines, Sweden

Abstract. Many metallic mineral deposits have sufficient physical property contrasts, particularly density, to be detectable using seismic methods. These deposits are sometimes significant for our society and economic growth and can help to accelerate the energy transition towards decarbonization. However, their exploration at depth requires high-resolution and sensitive methods. Following a series of 2D seismic trials, a sparse, narrow source–receiver azimuth, 3D seismic survey was conducted in the Blötberget mine, in central Sweden, covering an area of approximately 6 km2 for deep-targeting iron oxide deposits and their host rock structures. The survey benefited from a collaborative work by putting together 1266 seismic recorders and a 32 t vibrator, generating 1056 shot points in a fixed geometry setup. Shots were fired at every 10 m where possible, and receivers were placed at every 10–20 m. Notable quality data were acquired despite the area being dominated by swampy places as well as by built-up roads and historical tailings. The data processing had to overcome these challenges for the static corrections and strong surface waves in particular. A tailored for hardrock setting and processing workflow was developed for handling such a dataset, where the use of mixed 2D and 3D refraction static corrections was relevant. The resulting seismic volume is rich in terms of reflectivity, with clear southeast-dipping reflections originating from the iron oxide deposits extending vertically and laterally at least 300 m beyond what was known from available boreholes. As a result, we estimate potential additional resources from the 3D reflection seismic experiment on the order of 10 Mt to be worth drilling for detailed assessments. The mineralization is crosscut by at least two major sets of northwest-dipping reflections interpreted to dominantly be normal faults and to be responsible for much of the lowland in the Blötberget area. Moreover, these post-mineralization faults likely control the current 3D geometry of the deposits. Curved and submerged reflections interpreted from folds or later intrusions are also observed, showing the geological complexity of the study area. The seismic survey also delineates the near-surface expression of a historical tailing as a by-product of refraction static corrections, demonstrating why 3D seismic data are so valuable for both mineral exploration and mine planning applications.

Identifying mineral prospectivity using seismic and potential field data in the Hongniangyu district, Inner Mongolia, China

Since the discovery of several large porphyry Mo deposits (>0.1 Mt Mo) in the Jining region of Inner Mongolia, China, during the past decades, the area has been receiving wide interest for mining activities. However, mineral exploration in this area is challenging because most of it is thickly covered by Cenozoic sediments. Geophysical surveys, especially the seismic method, are becoming increasingly important for investigating deep-seated ore-controlling structures (e.g., porphyry granites, faults and even orebodies), supporting the discovery of buried ore deposits. Recently, potential field data and seismic reflection data have been acquired for the Hongniangyu area, south of Jining region. In this study, we mainly use seismic reflection and potential field data to obtain subsurface geological model in support of mineral exploration targeting. Gravity and magnetic data were firstly used for spatial analysis of mineral system by mapping the distribution of the faults, concealed granites, and the sedimentary basin (through wavelet analysis, tilt angle, and Euler deconvolution). To address the common problem of low signal-to-noise ratio of seismic reflection data obtained in hard-rock terranes, we applied a sophisticated workflow including refraction static correction, noise filtering, surface-consistent deconvolution, and velocity analysis. The obtained post-stack migrated seismic profile clearly imaged the geometry of the concealed graben located at the north end of the Datong Basin, and the South-East dipping Kouquan normal fault controls the depression of the graben and offsets Cenozoic sediments by several hectometers up to ~500 m. Joint modeling of seismic, gravity and magnetic data further provided evidence of the three buried belts of granites. Intriguingly, one of the concealed granitic belts located in the north of Hongniangyu area was found to have both significant chargeability and Mo geochemical anomalies, indicating a high potential of hosting porphyry-type Mo ore deposits. As expected, Mo and W mineralized cores were discovered in this target area but limited to only one deep drilling hole. The presented results provide the geological framework of Hongniangyu district and guidelines for delineating new potential Mo mineralization in the Jining region.

Time-lapse difference static correction using prestack crosscorrelations: 4D seismic image enhancement case from Ketzin

A method for static correction of time-lapse differences in reflection arrival times of time-lapse prestack seismic data is presented. These arrival-time differences are typically caused by changes in the near-surface velocities between the acquisitions and had a detrimental impact on time-lapse seismic imaging. Trace-to-trace time shifts of the data sets from different vintages are determined by crosscorrelations. The time shifts are decomposed in a surface-consistent manner, which yields static corrections that tie the repeat data to the baseline data. Hence, this approach implies that new refraction static corrections for the repeat data sets are unnecessary. The approach is demonstrated on a 4D seismic data set from the Ketzin [Formula: see text] pilot storage site, Germany, and is compared with the result of an initial processing that was based on separate refraction static corrections. It is shown that the time-lapse difference static correction approach reduces 4D noise more effectively than separate refraction static corrections and is significantly less labor intensive.

Research and Applicability of Refraction Static Correction and Tomographic Inversion Static Correction

Refraction static correction and tomographic inversion static correction are commonly used in seismic data processing. It is often believed that the effect of tomographic inversion static correction is better than the effect of refraction static correction. But it is not true. This paper summarized the applicable conditions, advantages and disadvantages of these two methods. In low mountains areas, the effect of tomographic inversion static correction is better than the effect of refraction static correction. However, in loess tableland areas, the effect of refraction static correction is almost the same as the effect of tomographic inversion static correction. It can’t reflect the advantages of tomographic inversion static correction. All of the above is proved from models and sections. So each technique must meet its applicable conditions to reflect its superiorities. Integrated application of static correction methods is effective to solve the complex surface statics problems.

Re-processing and interpretation of 2D seismic data from the Kristineberg mining area, northern Sweden

The Kristineberg mining area in the western part of the Skellefte ore district, northern Sweden, contains the largest massive sulphide deposit in the district. In 2003, two parallel seismic lines, Profiles 1 and 5, each about 25 km long and about 8 km apart were acquired in the Kristineberg area. The initial processing results were successful in imaging the large-scale structures of the area down to 12 km of the crust, but resulted in relatively poor seismic image near the mine. In this paper, we re-processed the seismic data along Profile 1 that crosses the mine. The main objective was to improve the seismic section near the mine for further correlation with new seismic data recently acquired in the area. The crooked-line acquisition geometry, very low fold coverage of less than 17, complex geology and sparse outcrops in the area made the data re-processing and interpretation challenging. Despite these challenges, significant improvement is observed in the seismic data, in terms of event continuity and resolution. Refraction static corrections allowed high frequencies to be retained, which improved the seismic section. The refraction static solution was manually checked and adjusted at every iteration to avoid unstable solutions. 3D visualization of the re-processed data with other seismic profiles recently acquired in the area allowed the seismic reflections to be correlated. The majority of the reflections are interpreted to originate from either fault zones or lithological contacts. A very shallow reflection correlates well with the location of the Kristineberg mineralized horizon. ► Reflection data re-processing of a low fold data near the Kristineberg mine was conducted. ► The section is significantly improved and contains higher frequencies and more reflections. ► Important steps were field static corrections as well as band-pass and deconvolution filters. ► 3D visualization allowed us to correlate shallow reflections with the known mineralization horizons.

3D imaging challenges in steeply dipping mining structures: New lights on acquisition geometry and processing from the Brunswick no. 6 seismic data, Canada

We have analyzed and processed a [Formula: see text] nonorthogonal 3D surface reflection seismic data in the Brunswick no. 6 area to better understand the effect of acquisition geometry on the resultant image and to provide 3D information about the main geologic structures hosting the mineralization. The 3D data were processed using a conventional prestack dip moveout (DMO) and poststack migration algorithm with special focus on refraction static corrections, velocity analysis, and DMO corrections that are important for the data recorded in crystalline environment. However, the nonorthogonal nature of the 3D data combined with its narrow-azimuth, irregular offset distributions, and 2D nature of midpoint distribution in common depth point bins resulted in a lower quality seismic image than those observed on a series of 2D seismic profiles collected in the area prior to the 3D data acquisition. 2D wavenumber spectrum of the data suggests acquisition footprint associated with the data. Most of the noise associated with the acquisition footprint manifested itself as short-length, high-amplitude shallow reflections but largely were attenuated using a dip filter running in the wavenumber domain. Various bin size and geometries were tested, and the best result was obtained using rectangular bins aligned in the orientation of the shot lines. The processing results indicated that the highly prospective and mineralized Brunswick horizon is part of a continuous reflective package that could guide future deep mineral exploration in this mining camp.

3D seismic survey at the Millennium uranium deposit, Saskatchewan, Canada: Mapping depth to basement and imaging post-Athabasca structure near the orebody

Three-dimensional seismic reflection measurements have been used to assist mine planning at the Millennium uranium deposit, Canada. The deposit is located within the crystalline basement, separated from the overlying Athabasca Basin sediments by an unconformity potentially associated with significant fluid flow. The primary objective of the [Formula: see text] survey was to image the unconformity and possible post-Athabasca deformation structures in and around the deposit. Clear unconformity reflections are observed within most of the survey area, although there are amplitude variations due to complex geology, including intense hydrothermal clay alteration around the deposit. Finite-difference modeling indicates that the wide-angle character of the unconformity reflections is due to a gradual velocity increase at the unconformity. The reflections are obscured by large time delays, due to Quaternary sediments covering the area, making refraction static corrections crucial. The seismic interpretation shows large variations in the unconformity depth (from approximately 430 to 650 m), indicating a pronounced basement depression that coincides with a gravity low. Reflections from the unconformity are vague within the depression, especially in the vicinity of the deposit. Although the orebody is not directly visible in the seismic image, there is a lack of reflectivity coincident with the alteration surrounding the mineralization. We also observed reflections which likely originate at the contact between the altered and fresh basement rock located beneath the deposit. The seismic data further indicate post-Athabasca faults in the vicinity of the orebody. Based on the initial seismic interpretation, the depth of the crown pillar was adjusted and the mine infrastructure moved away from areas interpreted to be affected by the intense hydrothermal alteration surrounding the deposit. The capability to image the unconformity, post-Athabasca structure, and hydrothermal alteration also highlights the potential use of seismic surveys in uranium exploration.

Detailed images of the shallow Alpine Fault Zone, New Zealand, determined from narrow-azimuth 3D seismic reflection data

Previous high-resolution seismic reflection investigations of active faults have been based on 2D profiles. Unfortunately, 2D data may be contaminated by out-of-the-plane reflections and diffractions that may be difficult to identify and eliminate. Although full 3D seismic reflection methods allow out-of-the-plane events to be recognized and provide superior resolution to 2D methods, they are only rarely applied in environmental and engineering studies because of high costs. A narrow-azimuth 3D acquisition and processing strategy is introduced to produce a high-resolution seismic reflection volume centered on the Alpine Fault Zone (New Zealand). The shallow 3D images reveal late Quaternary deformation structures associated with this major transpressional plate-boundary fault. The relatively inexpensive narrow-azimuth 3D acquisition pattern consisting of inline source and receiver lines was easily implemented in the field to provide 2- by [Formula: see text] CMP coverage over an approximately 500- by [Formula: see text] area.The narrow-azimuth acquisition strategy was well suited for resolving complex structures within the fault zone. Challenges in processing the data were amplified by the effects of strong velocity heterogeneity in the near surface and the presence of complex dipping, diffracted, and truncated events. A carefully tailored processing scheme including surface-consistent deconvolution, refraction static corrections, noise reduction, dip moveout (DMO) corrections, and 3D depth migration greatly improved the appearance of the final stacks. The 3D images reveal strong reflections from the faulted and folded late Pleistocene erosional basement surface. A steeply dipping planar main (dominant) fault strand can be inferred from the geometry and truncations of the overlying postglacial sediments. The 3D images reveal that the average apparent vertical displacement [Formula: see text] of the basement surface across the dominant fault strand at this location is somewhat less than that estimated from a pilot 2D seismic reflection profile, suggesting that the provisional dip-slip rate based on the 2D data is a maximum.

Refraction Static Correction without Picking First Arrival Times

Refraction Static Correction without Picking First Arrival Times

3D seismic reflection surveying at the CO2SINK project site, Ketzin, Germany: a study for extracting shallow subsurface information

ABSTRACTThree‐dimensional (3D) reflection seismic data were recorded as part of a pilot scale carbon dioxide (CO2) geological storage project (CO2SINK) with the aim of mapping the structural geometry of the site and providing a 3D baseline prior to CO2 injection. Standard processing originally focused on the storage target, a saline aquifer at 500–700 m depth and successful imaged coherent reflections from 150 ms to 900 ms in the seismic volume. However, the relatively sparse distribution of sources and receivers, the frequency content of the data and artefacts of the processing resulted in the uppermost 150 ms being poorly resolved. This depth range contains caprock, shallow faults and an aquifer system. Thus, characterizing the shallow subsurface is important in terms of site delineation of potential leakage paths and monitoring after CO2 injection. In order to study the potential of mapping the uppermost reflectors and shallow structures associated with major fault zones, a comprehensive reprocessing effort on a subset of the 3D data was performed. The challenge in imaging shallow reflections is dependent upon the separation of ground roll and refracted energy from the reflected energy, as well as compensation for time shifts due to statics. Among the processing sequences, refraction static corrections, careful muting and filtering, velocity analysis and 3D time migration were key steps for enhancing the resolution and coherency of shallow seismic reflections. This study images a previously unmapped horizon, close to the Quaternary‐Tertiary boundary, at about 95–120 ms (~65–90 m depth). Correlation of lateral variations in reflectivity along this boundary, lateral velocity variations in the tomographic image and the seismic signature in modelling studies suggest an aquifer/aquitard complex and variable lithology with associated localized silty or clayey sediments, overlying the Tertiary Rupelton clay unit. In the previous processing it was not clear if the deeper faults imaged on the 3D seismic survey extended to shallower levels than the base Tertiary. Thus, a comprehensive fault detection technique, multi‐attributes and neural networks analysis, was employed in this study to allow a more reliable fault geometry to be interpreted. Tracking of faults in the seismic image and comparisons with a tomography study indicate that some deeper faults may penetrate into the overlying Tertiary unit. These findings are important for understanding potentially risky areas and can be used as a database for future monitoring programmes at the site.

2D seismic data processing for straight lines in the loess plateaus in Fuxian of Shanbei

The crooked seismic lines along valleys were irregular previously in Fuxian of Shanbei, showing an irregular branch in plane, and hard to complete close grids. Therefore, it’s difficult to conduct reservoir inversion of 2D seismic data. In 2001, Zhongyuan Oilfield Company carried out the study on field acquisition methods and seismic processing technology in Fuxian. Straight lines were passing through plateaus and formed seismic grids by using flexible geometry with variable linear bins. Data processing involved model-inversion based refraction static correction, surface consistent amplitude compensation, deconvolution, and pre-stack noise attenuation. As the result, seismic data with a high fidelity was provided for the subsequent reservoir predictions, small-amplitude structure interpretation and integrative geologic study. Because all lines were jointed to form grids, comprehensive interpretation of reservoir inversion could be finally implemented by using the pseudo logging method to control lines without wells.

High‐resolution seismic traveltime tomography incorporating static corrections applied to a till‐covered bedrock environment

A major obstacle in tomographic inversion is near‐surface velocity variations. Such shallow velocity variations need to be known and correctly accounted for to obtain images of deeper structures with high resolution and quality. Bedrock cover in many areas consists of unconsolidated sediments and glacial till. To handle the problems associated with this cover, we present a tomographic method that solves for the 3D velocity structure and receiver static corrections simultaneously. We test the method on first‐arrival picks from deep seismic reflection data acquired in the mid‐ late to 1980s in the Siljan Ring area, central Sweden. To use this data set successfully, one needs to handle a number of problems, including time‐varying, near‐surface velocities from data recorded in winter and summer, several sources and receivers within each inversion cell, varying thickness of the cover layer in each inversion cell, and complex 3D geology. Simultaneous inversion for static corrections and velocity produces a much better image than standard tomography without statics. The velocity model from the simultaneous inversion is superior to the velocity model produced using refraction statics obtained from standard reflection seismic processing prior to inversion. Best results using the simultaneous inversion are obtained when the initial top velocity layer is set to the near‐surface bedrock velocity rather than the velocity of the cover. The resulting static calculations may, in the future, be compared to refraction static corrections in standard reflection seismic processing. The preferred final model shows a good correlation with the mapped geology and the airborne magneticmap.

Design and application of a towed land‐streamer system for cost‐effective 2-D and pseudo-3-D shallow seismic data acquisition

To reduce the field effort required for 2-D and 3-D shallow seismic surveying, we have developed a towed land‐streamer system. It was constructed with self‐orienting gimbal‐mounted geophones housed in heavy (1 kg) cylindrical casings, sturdy seismic cables with reinforced kevlar sheathing, robust waterproof connectors, and a reinforced rubber sheet that helped prevent cable snagging, maintained geophone alignment, and provided additional hold‐down weight for the geophones. Each cable had takeouts for 12 geophones at 1 m or 2 m intervals. By eliminating the need for manual geophone planting and cable laying, acquisition of 2-D profiles with this system proved to be 50–100% faster with 30–40% fewer personnel than conventional procedures. Costs of the land‐streamer system and total weight to be pulled could be minimized by employing nonuniform receiver configurations. Short receiver intervals (e.g., 1 m) at near offsets were necessary for identifying and mapping shallow (<50 m) reflections, whereas larger receiver intervals (e.g., 2 m) at far offsets were sufficient for imaging deeper (>50 m) reflections and estimating velocity‐depth functions. Our land‐streamer system has been tested successfully on a variety of recording surfaces (e.g., meadow, asphalt road, and compact gravel track). The heavy weight of the geophone casings and rubber sheet ensured good geophone‐to‐ground coupling. On the asphalt surface, a greater proportion of high‐frequency (above 300–350 Hz) energy was recorded by the land streamer than by standard baseplate‐mounted geophones. The land‐streamer system is a practical and efficient means for surveying in urbanized areas. Acquisition and processing of 3-D shallow seismic data with the land‐streamer system was simulated by appropriately decimating and reprocessing an existing 3-D shallow seismic data set. Average subsurface coverage of the original data was ∼50 fold, whereas that of the simulated data was ∼5 fold. The effort required to collect the simulated pseudo-3-D data set would have been approximately 7% of that needed for the original field campaign. Application of important data‐dependent processing procedures (e.g., refraction static corrections and velocity analyses) to the simulated data set produced surprisingly good results. Because receiver spacing along simulated cross‐lines (6 m) was double that along in‐lines (3 m), a pattern of high and low amplitudes was observed on cross‐sections and time slices at early traveltimes (⩽50 ms). At greater traveltimes, all major reflections could be identified and mapped on the land‐streamer data set. With this cost‐effective approach to pseudo-3-D seismic data acquisition, it is expected that shallow 3-D seismic reflection surveying will become attractive for a broader range of engineering and environmental applications.

The seismic velocity distribution in the vicinity of a mine tunnel at Thabazimbi, South Africa

Analysis of the refracted arrivals on a seismic reflection profile recorded along the wall of a tunnel at an iron mine near Thabazimbi, South Africa, shows variations in P-wave velocity in dolomite away from the de-stressed zone that vary between 4.4 and 7.2 km/s, though values greater than 5.8 km/s predominate along most of the profile. The seismic velocities at the tunnel wall, however, vary between 4.2 and 5.2 km/s. Time–depth terms are in the range from 0.1 to 0.9 ms, and yield thicknesses of the zone disturbed by the tunnel excavations of between 2 and 9 m. The very low seismic velocities away from the tunnel wall in two regions are associated with alcoves or ‘cubbies’ involving offsets in the wall of up to 10 m. The large variations in seismic velocity resolved over distances less than 15 m with signals of wavelength around 6–9 m are attributed to variations in the sizes and concentrations of fracture systems and cracks, and in the degree of groundwater saturation of the fracture systems. The results suggest that seismic velocity variations from reflection surveys may also assist modelling studies of the stress regime in deep mines, particularly if both P and S wave velocity variations can be determined. The seismic velocity variations inferred also show that application of refraction static corrections in the processing of ‘in-mine’ seismic reflection profiles is as important as in surface surveys, because of the higher frequencies of the seismic energy recorded in the deep mine environment.

Seismic reflection and borehole geophysical investigations in the Matagami mining camp

A high-frequency vibroseis seismic survey was conducted by Lithoprobe across a well-known section of the Matagami mining camp to assess the usefulness of seismic methods in lithological mapping. The data have been reprocessed to enhance shallow reflections. Critical processing steps for the preservation of the shallow features include refraction static corrections, cross-dip corrections, and first break mutes. The seismic data were calibrated using geophysical logs from two boreholes adjacent to the profile. Physical rock property measurements suggest that strong reflectivity within the volcanic sequence occurs at rhyolite-gabbro contacts. The Lower Wabassee Group, a sequence of gabbros and basalts with interlayered rhyolite horizons, has been identified as a seismic marker. This reflective package overlays the horizon where most sulphide deposits are known to be located. In the Matagami mining camp, seismic reflection methods can be used to map the deep volcanic stratigraphy and structures.

2D and 3D refraction statics via tomographic inversion with under-relaxation

The computation of refraction static corrections is a difficult but important step towards obtaining reliable seismic stacked sections, particularly with land data. The difficulty lies in the high degree of variability of thickness and velocity of near surface layers. The importance lies in the fact that if subsequent processing tools such as velocity analysis, residual reflection statics and migration are to perform optimally it is imperative to first resolve the major sta tics problems using refraction techniques capable of broadband solutions.

Seismic reflection techniques for base metal exploration in eastern Canada: examples from Buchans, Newfoundland

In 1989, as part of the Lithoprobe East program, high-resolution 60-fold seismic reflection profiles were recorded using Vibroseis sources in the vicinity of the Buchans mine. The area is considered to be a fold and thrust belt in which the massive sulphide-barite orebodies occurring within volcanic and sedimentary units of the Buchans Group are repeated in a large number of thrust systems. The main objective was to image the individual faults, generally narrow zones of brittle shear, some of which have been intruded by diabase sills. In 1991, Memorial University of Newfoundland recorded a 24-fold line using explosives as sources, coincident with one of the Lithoprobe lines, in order to compare the resolution of shallow structures of economic importance with that obtained using Vibroseis. The explosive sources provided reflections richer in high frequencies than the Vibroseis sources for depths less than 1 km. The resolution of the reflections is greatly improved by two processes: (1) the application of refraction static corrections, and (2) spectral balancing of the NMO-corrected CMP gathers over two octaves prior to stacking. Compared with Vibroseis, the approach using small explosive sources is considered preferable for future work in mineral exploration and mine development because of lower costs and better resolution of shallow moderately-dipping faults.