Potential Field Interpretation Research Articles

Комплекс интерпретации потенциальных полей для геологии

Комплекс интерпретации потенциальных полей для геологии

Integrated interpretation of magnetotelluric and potential field data: assessing the northeast Kimberley region

An integrated interpretation of potential field and magnetotelluric (MT) data was performed in the east Kimberley, northern Western Australia Structural interpretation of potential field data was constrained by geological field observations, petrophysics, remote-sensing and an understanding of the tectonic history of the region. Forward modelling of the potential field data located along the same survey traverse as the magnetotelluric data allowed comparison between the two datasets to assess complementarity of images and assist interpretation. Interpreted features include the presence of large-scale structures and associated electrical anomalies that indicate the presence of mineralisation deep in the crust, and guide prediction of mineralisation at or near the surface. The King River Fault is revealed to be a crustal-scale, west-dipping structure, which footwall bounds the western side of a large resistive body. A conductive anomaly is also located on the hanging wall of the King River Fault. A number of scenarios are discussed to the source of conductivity, including the presence of sulphides, saline water and graphite. Our assessment suggests that graphitic rocks, most likely with some sulphide content, contribute to the strength of this anomaly, and highlights the known potential of the east Kimberley to host graphite deposits. The conductive anomaly has a spatial and geometric correlation to Speewah Dome, a known prospective region. The depth of the conductor (c. 5km) precludes mining, but does indicate King River Fault is likely to form a mineralising conduit, and may contribute to possible Pb-Zn mineralisation where the fault reaches the surface.

Understanding the 3D structure of the Gilmore Fault Zone through geophysical modelling: implications for Lachlan tectonic reconstructions

This study aims to clarify the tectonic evolution of the Lachlan Orogen by modelling the subsurface morphology of the Gilmore Fault Zone (GFZ). The GFZ marks a distinct geophysical contrast between (high gravity, low magnetic intensity) high-grade metamorphic rocks found in the Wagga metamorphic belt (WMB), to the west, and the (low gravity, uniformly high magnetic intensity) low-grade volcanic rocks found in the Macquarie Arc and Silurian rift basins to the east. Understanding the structure of this fault at depth should provide constraints on existing models for the tectonic evolution of the Lachlan Orogen. Subsurface structure around the GFZ in the vicinity of Barmedman has been inverted by iterative 2.5D potential-field modelling of gravity and magnetics, constrained by pre-existing reflection seismic profiles, potential-field interpretations by previous workers, and physical properties data collected on representative lithologies. Preliminary findings show that the surface structure mapped as the Gilmore Fault is an east-dipping, shallow thrust fault, and doesn’t correspond to the major crustal ‘suture’ envisaged in regional tectonic studies. It is a secondary antithetic structure off the main west-dipping, crustal penetrating fault that separates the Macquarie arc and the WMB. Another, steeper west-dipping fault cuts these structures and separates the WMB from the Silurian Tumut Trough. This larger structure defines the regionally extensive tectonic feature of the GFZ. The trace of the mappable Gilmore Fault (as opposed to the GFZ) is curved, and terminates abruptly to the north, indicating the Gilmore Fault is the base of a series of thrust flakes imposed on the pre-existing main fault in the GFZ. West-dipping crustal-penetrating thrust faults east of the GFZ are indicative of successive collision, accretion and extension events. Thus far the modelled structure of the GFZ is not consistent with the terrane accretion model but is consistent with the accretionary orogen and orocline model.

Stable methods of interpretation of gravimetric data

A method for interpretation of potential geophysical fields, based on a stable inversion algorithm, is proposed. The stability of the algorithm is provided by an original choice of the zero approximation model and stepwise solution of the inverse problem by a correctness set. The three-dimensional density distribution of local structures as grid functions is reconstructed by layer-wise anomalies of a spilt field. Examples of interpretation of the practical gravimetric data illustrating the efficiency of the method are given.

A new source parameters estimation method of airborne gravity gradient tensor data

One of the important tasks of potential field interpretation is source parameters estimation of the geological structures, including the horizontal position and buried depth. A new method to interpret airborne gravity tensor data is proposed in this paper based on Normalized Downward Continuation (NDC) of the tensor data directional total horizontal derivatives. The NDC method was introduced for source depth estimation, which can be applied to analytical signal modulus and potential fields themselves. And the maxima of the NDC map mainly correspond to the centre location of the geologic source. We applied the NDC method to the directional total horizontal derivatives which can be used for delineating the sources’ horizontal edges. The maximum values of the result indicate the source edges horizontal position and buried depth simultaneously. During the calculation, the iteration method of the downward continuation is used in the NDC calculation process to improve the stability. The new method was tested on synthetic models and obtained satisfactory results. Compared with previous work, this new approach has a better lateral resolution.

An empirical law for wavelet maxima interpretation of potential fields: Application to the Uinta Mountains range

Wavelet methods have been used in potential fields study to estimate source properties such as depth or structural index, through the analysis of Wavelet Transform Modulus Maxima Lines (WTMML) intersections and slopes at high scales. Little has been done on the study of maximum points of the wavelet diagram, that we call here Maximum Wavelet Coefficient Scales (MWCS). Previous works have shown interesting correlations between MWCS and source depths, depending on the wavelet used in regards to the source nature and the data derivative order. In this paper, we introduce an empirical law involving spectral parameters that have not been studied so far, which allows analytical calculation of the MWCS, knowing the source characteristics and using certain wavelets. In return, the study of MWCS allows recovering source characteristics from the use of a single wavelet, without prior knowledge on the source.We demonstrate through synthetic models that the new capability of predicting the source type and depth according to the wavelet coefficient behaviour allows new ways of potential fields' sources characterization and identification. We show an application of the formula on a real case example in the Uinta Mountains (Utah, USA).

Total horizontal derivatives of potential field three-dimensional structure tensor and their application to detect source edges

Edge detection of the geological structures horizontal position is the vital task of the potential field interpretation, and some traditional edge detectors have poor ability in delineating edges clearly and accurately. Some people made efforts to purpose new methods which can balance the weak and strong anomalies simultaneously. In this paper, we purposed new edge detection methods based on directional total horizontal derivatives of three-dimensional structure tensor for the causative sources. In order to balance weak and strong amplitude anomalies simultaneously, we used the vertical derivatives of potential field to normalize the new defined filters. For the complex geological situation with positive and negative gravity or magnetic anomalies, we introduced a new strategy to reduce the false edges. The presented filters were tested on synthetic and real potential field data to verify its feasibility. All of the results have shown the new edge detection methods can not only display the sources edges precisely and clearly, but also bring out more geological subtle details.

The Granites - Tanami Orogen Subsurface Geometry as Revealed by and Integrated Potential Field Geophysical and Geological Study

The Granites Tanami Orogen (GTO) in central Australia is a significant gold producing province. Future exploration will be facilitated by determining the structural controls on mineralisation and crustal evolution of the orogen. A whole of crust model has been generated through the multi-scale integration and interpretation of geophysical, geological and remote sensing data. The architecture of the orogen is that of a basin that has been inverted, deformed and intruded during: a) the collision between the Kimberley and Tanami basins along the Halls Creek Orogen to form the North Australian Craton; and b) during the amalgamation of the North Australian Craton with the Central Australia Craton. These continent-continent collisions have resulted in a complex structural framework, which is further complicated by deep weathering and extensive regolith across the region. Reconnaissance style outcrop mapping coupled with potential field interpretation has identified two main phases of deformation. The first regional deformation event resulted in north- to northeast-trending isoclinal fold trains of wavelengths ~10 km or greater. These folds are recognised through the interpretation of joint gravity and magnetic anomalies and are confirmed in outcrop. Gold mineralisation within the GTO is coincident with the second regional deformation event, which is recognised in regional aeromagnetic data as poly-phase deformational interference patterns caused by the refolding of earlier folds around axes trending E to ESE. This defining of upper crustal architecture to structural features observable in sparse outcrop coverage could not be sufficiently identified without this combined geology and geophysical approach.

Three-dimensional geology from potential field geophysics: Applications to Bathurst Mining Camp, Eastern Canada

SUMMARY Interpretation methods and tools for geophysics datasets continue to evolve. Advances in clustering algorithms, the use of implicit functions to create 3D surfaces, new algorithms to estimate source depths and dips, and the availability of clever computational geometry libraries, contribute to the discipline of potential field interpretation techniques, allowing for a much more explicit statement of implied 3D description. While traditional scalar measures of potential fields have benefited from applying new ideas, perhaps more exciting is the reduction in ambiguity imposed from gradient measurement when used as the basis for field interpretation. Full tensor gravity gradiometry in particular, allows for 2D fault dip and throw calculations. Direct detection of high density bodies and faults via state-of-the-art gravity gradiometry is now a reality. Bodies greater than 200m in lateral extent are detectable. Implicit 3D structural geology modelling techniques derived from gravity curvature attributes of the observed gravity field present a leading edge technique for defining structurally controlled near surface geology geometry. A demonstration from the Bathurst camp dataset is given.

Tectonodynamic model of the Earth’s crust of the Urals and neighboring areas

On the foundation of developed technologies of analysis and generalization of seismic data along profiles of Deep Seismic Sounding (DSS), application of the interpretation of potential fields with 3D geological–geophysical models were compiled and a scheme of tectonic subdivision of the Urals region lithosphere on a 1: 2 500 000 scale has been constructed.

Early tectonic evolution of the Thomson Orogen in Queensland inferred from constrained magnetic and gravity data

The crustal architecture as well as the kinematic evolution of the Thomson Orogen in Queensland is poorly resolved because the region is concealed under thick Phanerozoic sedimentary basins and the basement geology is known from limited drill holes. Combined potential field and seismic interpretation indicates that the Thomson Orogen is characterized by prominent regional NE- and NW-trending structural grain defined by long wavelength and low amplitude geophysical anomalies. The ‘smooth’ magnetic signature is interpreted to reflect deeply buried source bodies in the mid- to lower crust. Short wavelength positive magnetic features that correlate with negative gravity anomalies are interpreted to represent shallower granitic intrusions. They appear to be focused along major fault zones that might have controlled the locus for magmatism. The eastern Thomson Orogen is characterized by a prominent NE structural grain and orthogonal faults and fold interference patterns resulting in a series of troughs and highs. The western Thomson Orogen consists of a series of NW-trending structures interpreted to reflect reverse faults. Sedimentation and basin development are interpreted to have initiated in the Neoproterozoic to Early Cambrian during E–W- to ENE–WSW extension, possibly related to the Rodinia break-up. This extensional event was followed by Late Cambrian shortening recorded in the Maneroo Platform and the Diamantina River Domain which possibly correlates with the Delamerian Orogeny. Renewed deposition and volcanism occurred during the Ordovician and may have continued until Late Silurian, resulting in thinned Proterozoic basement crust and extensive basin systems that formed in a distal continental back-arc environment. Our interpretation places the Thomson Orogen to the west of the Neoproterozoic passive margin preserved in the Anakie Inlier. The region is likely to represent the internal extensional architecture during the Rodinia break-up that has been subsequently extensively modified by multiple extensional basin forming events and transient episodes of crustal shortening and basin inversion.

A geophysically constrained multi-scale litho-structural analysis of the Trans-Tanami Fault, Granites-Tanami Orogen, Western Australia

The Trans-Tanami Fault in the poorly exposed Paleoproterozoic Granites-Tanami Orogen of Western Australia is an ∼100 km long curvilinear structure with ∼6 km right lateral displacement. Multi-scale integration and analysis of aeromagnetic, gravimetric, reflection seismic and remote sensing data have constrained the relative timing and architectural relationship of this structure. Interpretation of regional scale long-wavelength potential field (gravity and magnetic) anomalies, which are commonly used to define first-order structures, show that the fault is not a terrane boundary. Structural interpretation of short-wavelength potential field data illustrates that the structural domains on either side of the fault represent the products of a non-homogeneous stress regime developed between rigid granitic plutons. Additionally, 2D joint forward modelling of gravity and magnetic data and interpretation of reflection seismic data confirms the vertical displacement across this fault to be negligible indicating a predominant lateral displacement. The lateral displacement along a portion of this structure has exploited a pre-existing plane of a north-dipping thrust fault. Where this early thrust fault terminates, the Trans-Tanami Fault displaces previously unfaulted rock as a wrench fault step-over. These observations differ from previous findings in the area by constraining the absolute displacement of this structure and through the recognition of a wrench fault system that includes lateral step-overs between re-activated early thrust fault planes.

The Potentials Recorded Around the Spinal Cord: Different Sides of the Same Dice

Introduction. Clinical Neurophysiology is strongly based on the interpretation of electric potential fields. Such interpretations may sometimes create different conceptual objects that over time end up as different sides of the same phenomenon. An instructive example is represented by the potential fields recorded around the spinal cord viewed from a historical perspective. Method. A brief historical account is given of the potentials recorded around the spinal cord. Results. Dorsal root reflex, dorsal root potential, cord dorsum potential and primary afferent depolarization are described. Conclusion. all these potentials are mainly different aspects of the same generator the segmental spinal cord activities secondary to the recordings by different leads.

The Potentials Recorded Around the Spinal Cord

Introduction. Clinical Neurophysiology is strongly based on the interpretation of electric potential fields. Such interpretations may sometimes create different conceptual objects that over time end up as different sides of the same phenomenon. An instructive example is represented by the potential fields recorded around the spinal cord viewed from a historical perspective. Method. A brief historical ac­count is given of the potentials recorded around the spinal cord. Re­sults. Dorsal root reflex, dorsal root potential, cord dorsum potential and primary afferent depolarization are described. Conclusion. all these potentials are mainly different aspects of the same generator - the segmental spinal cord activities - secondary to the recordings by different leads.

V. N. Strakhov's mounting approach to solving inverse problems of gravity prospecting: the current state and real possibilities

Strakhov's mounting algorithms for solving nonlinear inverse gravity problems are considered from the point of view of the concept of problem-oriented methods of conditional minimization, which make up the core of any technology for quantitative interpretation of gravity data. The comparative analysis of editing algorithms has revealed their main advantage, which lies in the ability to bypass the richly acute angles in the theory and practice of interpretation of potential fields, have become a stumbling block for most known approaches to solving inverse problems, which rely on classical methods to solve the conditions of no-extremal problems . At the same time, the question of the actual resolution of the editing method remains unresolved, the answer to which can only be given by a large-scale computational experiment. In this article, this gap is partially filled.

On the choice of the zeroth approximation in the determination of a complex contact

Two analytical models of gravity field and geological medium have been suggested for the horizontally layered medium with several density interfaces present in the lower semi-space. The first model is acquired from the equation of gravity derived from 2D consideration of normal gravity potential in a local neighborhood of the measurement points. In addition to well-known in the theory of potential fields interpretation the main classes of contact surfaces a new medium model is introduced named a Chorny contact surfaces class. The examples of field separation theorems for a case of several one-coherent volumes and several non-crossed layers (two most popular initial approximations of a medium) are given in the model studied. A new numerical approach for definition of the start approximation of density interface and its asymptotes in the multilayered geological medium by means of several "contact" algorithms is given.

Skellefte mining district in 3D; results from integrated interpretation of potential field, resistivity/IP and reflection-seismic data

Located in northern Sweden, the Skellefte mining district has been subject to several geological and geophysical investigations, as it is hosting abundant volcanic-hosted massive sulfide deposits. The importance of mineral exploration at greater depths in the Skellefte district has been increased since most of mineralization at shallow depths are already discovered and exploited. Therefore, geophysical methods become particularly important as they can improve our knowledge about spatial relationship between geological features at the depth. In the first part (local-scale), we used resistivity/IP data to map the subsurface geometry down to 430m depth. Furthermore, the results of the resistivity/IP studies were constrained with potential field data down to 1.5 km depth. In the second part (Regional-scale), potential field data were used to constrain the interpretation of the reflection-seismic data down to 5 km depth. The result from the first part indicated a good correlation between the initial resistivity model and the magnetic and gravity field calculated from that model. The volcano sedimentary contact between the Skellefte and Vargfors group rocks and three proposed locations for sulphide mineralization were identified along the resistivity/IP profile. In second part, the gravity and magnetic data were investigated to better understand the contact between the Skellefte group, volcanic rocks and the Bothnian Basin sedimentary rocks. Furthermore the data was used to constrain the geometry of late-orogenic gabbro-diorite and granite intrusions, which occur along inferred shear zones that are only poorly indicated, or not visible at all in the reflection-seismic data. As the main outcome, the proposed integrated 3D model of the central Skellefte district (CSD) revealed crucial information about the spatial relationship between key lithologies, which will be further used to understand the evolution of CSD in the 4th dimension, time.

Potential-field interpretation of the Kars Belt, western NSW

The Cambrian Ponto Group of the Koonenberry Belt is a suite of fore-arc rocks of the Mt Wright Arc, and includes basalts that are associated with copper mineralisation. Magnetic derivative maps allow the Ponto Group to be followed southwest under shallow cover, where they define the magnetic texture of the Kars Zone that flanks the southeastern margin of the Curnamona Craton. Shallow-sourced positive magnetic anomalies in the Kars one are superimposed on a longer-wavelength magnetic low; this contrasts with their equivalents along the Koonenberry Belt, which sit on broad magnetic highs. Gravity images of the Koonenberry Belt suggest that shallow dense sources associated with the Ponto Group overlie deeper mass excesses, while equivalent shallow and deep mass excesses appear to be uncoupled in the Kars Zone. Joint magnetic and gravity modelling of a profile across the Kars Zone suggests that high susceptibility, dense bodies representing the Ponto Group have been displaced to the northwest, over-riding in their path deep magnetic sources below the Menindee Trough. Similar deep magnetic sources below the Bancannia Trough have been interpreted as very large intrusive features forming the base of the Mt Wright Arc. Together, the potential field imagery and modelling suggests very large scale, low-angle thrusting of the Mt Wright fore-arc over a rigid basement of the Mt Wright Arc.

Superimposed extension and shortening in the southern Salinas Basin and La Panza Range, California: A guide to Neogene deformation in the Salinian block of the central California Coast Ranges

We synthesized data from geologic maps, wells, seismic-reflection profiles, potential-field interpretations, and low-temperature thermochronology to refine our understanding of late Cenozoic extension and shortening in the Salinian block of the central California Coast Ranges. Data from the La Panza Range and southern Salinas Basin document early to middle Miocene extension, followed by Pliocene and younger shortening after a period of little deformation in the late Miocene. Extension took place on high-angle normal faults that accommodated ∼2% strain at the scale of the ∼50-km-wide Salinian block (oriented perpendicular to the San Andreas fault). Shortening was accommodated by new reverse faults, reactivation of older normal faults, and strike-slip faulting that resulted in a map-view change in the width of the Salinian block. The overall magnitude of shortening was ∼10% strain, roughly 4–5 times greater than the amount of extension. The timing and magnitude of deformation in our study area are comparable to that documented in other Salinian block basins, and we suggest that the entire block deformed in a similar manner over a similar time span. The timing and relative magnitude of extension and shortening may be understood in the context of central Coast Range tectonic boundary conditions linked to rotation of the western Transverse Ranges at the south end of the Salinian block. Older models for Coast Range shortening based on balanced fault-bend fold-style cross sections are a poor approximation of Salinian block deformation, and may lead to mechanically improbable fault geometries that overestimate the amount of shortening.

Locating the sources of potential fields in areal data using the singularity method

In an effort to develop and improve potential geophysical field interpretation we suggest that the singularity method should be extended toward areal data. This procedure enables mass and fast calculation of singularities in area and stable solutions. Studying the behavior of model fields and localization of singularities in the phase planes of vector fields permits the use of findings in interpreting geopotential fields. Model and real-life cases of this procedure as applied to tectonic dislocations and local geologic features are exemplified. We show that the proposed procedure significantly enhances the application of the singularity method to interprete geopotential fields.