Source Overprint Research Articles

A novel approach to address source overprint and shadow effects in controlled-source audio-frequency magnetotelluric exploration

Source overprint and shadow effects can significantly affect controlled-source audio-frequency magnetotelluric (CSAMT) data and mask subsurface geologic structures; however, few solutions have been presented. Based on the traditional observation approach, we develop a new approach to solve this problem. An additional transmitter and measurement zone are introduced in the novel approach to constrain the anomalous body causing source overprint or shadow effects in the inversion. To test this approach, six experimental models are designed. Three-dimensional CSAMT responses on the six synthetic models have been simulated, and the results indicate that source overprint and shadow effects have a strong influence on the data. Strong source overprint and shadow effects are found at low frequencies. The distribution of the source overprint or shadow distortions is affected by the shape of the anomalies below the transmitter or between the transmitter and receiver. We evaluate our approach by comparing the inversion results obtained with the new and traditional approaches based on the six synthetic data sets. The results indicate that the new approach not only effectively resolves the anomalies causing source overprint or shadow distortions but also yields more accurate target results in terms of resistivity value, structure geometry, and position than the conventional scheme. The inversion of field data from Yanqing, China, further verifies the effectiveness of our approach. Compared with the traditional inversion, the new scheme inversion obtains results that are more consistent with the rock type revealed by a drillhole.

Shadow effect and source overprint effect of short-offset transient electromagnetic method

AbstractIn artificial-source electromagnetic explorations, the shadow effects and source overprint effects are two of the key factors to affect the detection reliability. Compared with frequency-domain electromagnetic methods, the transient electromagnetic (TEM) shadow effect and source overprint effect are more complex, especially for the short-offset TEM (SOTEM) observed in the near-source region. However, there is less relevant research. Therefore, in this paper, we first realize 3D SOTEM simulations based on the vector time-domain finite element method, then further analyze the generation of SOTEM shadow effect and source overprint effect from view of the diffused electromagnetic fields, as well as their influence on ground horizontal electric field Ex and vertical induced voltage dBz/dt responses. Results show that the attraction or repulsion of the electrical inhomogeneity would affect the diffusion speed of the electromagnetic fields, which is the essential in the two source effects. Besides, from view of the ground Ex and dBz/dt responses, the relative anomaly of SOTEM shadow effect is almost twice those of the SOTEM source overprint effect, i.e. the SOTEM shadow effect is easier to record, especially for dBz/dt responses. In addition, owing to the shadow effect, SOTEM may fail to identify the real target anomaly, whereas the influence of the source overprint effect for recognizing the target anomaly mainly occurs on all-time dBz/dt responses and early-time Ex responses (before about 10 ms), that is, observing late-time SOTEM Ex responses can effectively avoid such influence. Hence, to fully ensure avoiding the influence of the source overprint effect, we suggest adopting high-power transmission to increase the late-time signal-to-noise ratio in field measurements.

Distortion effects caused by target abnormal bodies in CSAMT exploration

In CSAMT exploration, the using of the artificial sources not only improves the signal-to-noise ratio of the data, but also brings a series of distortion effects, such as shadow and source overprint effects. This paper attempts to introduce a distortion effect caused by the target in the survey area. Although it is often ignored, it always plagues the data interpretation. In CSAMT method, the primary current has determined direction due to the source. When the primary current encounters electrical interfaces, the induced charge will accumulate on it and generate local current, causing local distortion. The anomaly body stretches in the direction of the vertical primary current, and a false anomaly with opposite polarity appears on both sides of the target. If the direction of the primary current is different, the accumulation position of the induced charge is also different, which will result in different shapes of the anomalies in observed data. This paper confirms the existence of the distortion by taking four simple models as examples and explains it from the physical mechanism. On this basis, the paper summarizes the relationship between inversion and distortion. If our code can simulate the distortion effect in the forward, we do not need to remove it before the inversion. Otherwise, it must be removed.

The stable tungsten isotope composition of modern igneous reservoirs

Abstract Tungsten is a highly incompatible and fluid mobile element in the Earth’s crust and mantle, but its geochemical cycle in magmatic systems still remains poorly understood. Tungsten stable isotopes represent a novel tool with the potential to better constrain this cycle, because of possible W isotope fractionation during changes in redox-state (valence states +4 and +6) and coordination (tetrahedral and octahedral). By employing a 180W-183W double spike our analytical protocol for stable W isotope measurements yields an external reproducibility of ±0.018‰ (2 s.d.) in δ186/184W and enables the resolution of small scale stable W isotope variations between different igneous reservoirs. Here, we present the first stable W isotope data for representative volcanic rocks from different igneous settings including mid-ocean ridge basalts (MORBs), ocean island basalts (OIBs) and basalts to dacites from various subduction-related settings. The δ186/184W values of MORB samples from the Mid-Atlantic Ridge (+0.088 ± 0.017‰, n = 8) and OIB samples from the Canary Islands and Reunion Island (+0.084 ± 0.019; 2 s.d.; n = 17) show a narrow range and are analytically indistinguishable. This calls for a homogeneous stable W isotope composition in the mantle. Subduction-related rocks are more variable in their δ186/184W values (−0.009 to +0.195‰). Samples from the Troodos ophiolite complex in Cyprus show elevated W/Th and mainly high δ186/184W values (+0.101 to +0.195‰, n = 16) due to relative W enrichment from a subducted sediment component with heavy W isotope composition. In support of this model, Eastern Mediterranean sediments are isotopically heavier than mantle-derived rocks (+0.085 to +0.144‰ in δ186/184W, n = 5). Lavas from the Sunda arc show regional variability with a tendency towards lower δ186/184W values in the Central/East Java region (as low as −0.009‰). However, sediments from this region show uniformly high δ186/184W values of up to +0.301‰ and thus fail to explain the light compositions of the lavas. Hence, we rather propose that either the local subduction of an oceanic basement relief (Roo Rise) or source overprint by partial melts from deeply subducted crust accounts for low δ186/184W values in respective lavas. In arc settings, where the trace element budget is controlled by fluid-like components (New Britain) or melt-like components (Papua New Guinea) derived from shallowly subducted oceanic crust, we observe mantle-like W/Th and a narrow range in δ186/184W values (+0.109 ± 0.027‰, n = 19), which is only slightly elevated compared to MORBs and OIBs. This might hint towards the preferential release of isotopically heavy W during shallow fluid-induced melting events. Our results demonstrate that the stable W isotope composition of igneous rocks represents a novel tool to trace processes controlling the geochemical cycle of W in the silicate Earth.

Effects of Shadow and Source Overprint on Grounded-Wire Transient Electromagnetic Response

The detection accuracy of the grounded-wire transient electromagnetic method is affected by the incorrect geoelectric structure and burial depth due to shadow and source overprint effects. To better understand these effects, the three conditions under which these effects are present are analyzed separately. First, the analytical expressions are derived for computing the response of the formative wave and the surface wave in homogenous earth. The effects of the formative wave at the anomaly’s location and at the receiving site are analyzed. Then, the response variation due to the presence of an anomaly between the transmitter and the receiver is quantitatively analyzed using the finite-difference method. Finally, the effects of shadow and source overprint on the data inversion are investigated, which is verified by a case study. The wrong geoelectric information is caused by the shadow and source overprint effects. To avoid this, multisource observation can be used to weaken and utilize the shadow and source overprint effects.

The origin of highly radiogenic Hf isotope compositions in 3.33 Ga Commondale komatiite lavas (South Africa)

Drill core samples from the 3.33Ga high-alumina-type Commondale komatiites from the southern Kaapvaal craton (South Africa) were analysed for major, trace elements and Hf- isotope compositions in order to place possible constraints on their mantle sources and the process and timing of mantle depletion. The initial εHf isotope values range from +6.5 to +9.1 indicating a strongly depleted mantle source. This contrasts with previously published initial εNd values on the same geologic unit that reflect only minor depleted mantle sources of +2. Trace element modelling and modelling of the Hf-Nd isotope composition of the mantle source indicate that the strongly incompatible element-depleted trace element patterns may be explained by a mantle source that was depleted to high-degrees (ca. 40%) in the garnet stability field prior to high- degree melting of the restitic mantle in the spinel stability field. The depletion in the garnet-stability field may have been related to a plume event that produced the low-alumina Barberton-type komatiites. However, the decoupled Hf-Nd isotope compositions cannot be explained by such a mechanism. We discuss several possible scenarios that may explain the trace element distribution as well as the isotope composition of the Commondale komatiites. Based on trace and isotope modelling, we consider three different models (1) the decoupled Hf-Nd isotope compositions resulted from a depleted mantle source overprint by melt-depleted garnet-pyroxenite components that originated from older crustal differentiation events and were delaminated into the mantle; (2) the decoupling resulted from subduction fluids that overprinted a highly depleted mantle source and added Th, Nd and other light-rare-Earth elements (LREE) shortly before eruption of the komatiites and triggered the high degree of melting; (3) a possible origin from a mantle source that experienced fractionation processes in an early magma ocean, followed by at least three mantle depletion events. The trace element and Hf-Nd isotope composition of the Commondale komatiites reveal that parts of the Archean mantle must have been highly heterogeneous due to komatiite extraction and possible delamination processes of lower crustal restites.

Modeling and analysis of CSAMT field source effect and its characteristics

Controlled-source audio-frequency magnetotellurics (CSAMT) has been a highly successful geophysical tool used in a variety of geological exploration studies for many years. However, due to the artificial source used in the CSAMT technique, two important factors are considered during interpretation: non-plane-wave or geometric effects and source overprint effects. Hence, in this paper we simulate the source overprint effects and analyzed the rule and characteristics of its influence on CSAMT applications. Two-dimensional modeling was carried out using an adaptive unstructured finite element method to simulate several typical models. Also, we summarized the characteristics and rule of the source overprint effects and analyzed its influence on the data taken over several mining areas. The results obtained from the study shows that the occurrence and strength of the source overprint effect is dependent on the location of the source dipole, in relation to the receiver and the subsurface geology. In order to avoid source overprint effects, three principle were suggested to determine the best location for the grounded dipole source in the field.

The behaviour of the extended HFSE group (Nb, Ta, Zr, Hf, W, Mo) during the petrogenesis of mafic K-rich lavas: The Eastern Mediterranean case

In arc lavas, elements of the extended high field strength element group (HFSE; Nb, Ta, Zr, Hf, W, and Mo) are valuable tracers to unravel magma source processes. These elements can also help to identify residual mineral assemblages in subducting slabs and in the mantle. Most high-precision studies on HFSE behaviour to date only focused on intra-oceanic arc suites and data for mafic lavas of the K-rich series (medium-K, high-K and shoshonitic) are scarce. Arguably, K-rich series are the most incompatible element-rich end-members of subduction zone magmatism, and they often record sediment recycling into the mantle. Understanding HFSE fractionation in K-rich lavas can therefore provide important insight into the global HFSE budget. Here we present a comprehensive extended HFSE dataset obtained by isotope dilution on well-characterised K-rich lavas from the Eastern Mediterranean, also including subducting sediment samples drilled during DSDP Leg 13 and ODP Leg 160 South and West of Crete. The volcanic samples include mafic calc-alkaline lavas from the active Aegean Island arc (Santorini) and post-collisional Tertiary lavas from SE Bulgaria. The Santorini lavas record a hydrous sediment melt-mediated source overprint of a depleted mantle source by components from the subducting African plate. The Bulgarian lavas tap lithospheric mantle sources that were overprinted by fluid- and melt-like subduction components during Eocene subduction of the African Plate. The sediments in this study comprise silts/sands, marl oozes, limestones and clay-rich debris flows and approximate the bulk sediment subducted beneath the Hellenic arc.The marked enrichment of all HFSE in the lavas is controlled by the composition of the subducted sediments as shown by low 176Lu/177Hf (0.008630–0.02433) and Zr/Nb (11.3–29.4), combined with variable εHf (−3 to +11) and elevated W contents (up to 2.45ppm) in the lavas. Nevertheless, the lavas display unfractionated ratios of Nb/Ta and Zr/Hf of 12.3–16.5 and 34.4–38.6, respectively, with respect to MORB. This feature may be explained by the Nb/Ta in the sediments that are on average higher (15.2±2) compared to typical upper crustal estimates (12.5±1.8), indicating a strong regional control on the HFSE ratios in Eastern Mediterranean sediments. The lavas exhibit low Ta/W (<1.33), but do not show elevated W/Th, as expected for island-arcs entirely dominated by subduction zone fluids. Ratios of Mo/W in all lavas are low (<1.79), which, according to experimental evidence, can be explained by low fluid salinities at moderately oxidised conditions. Collectively, our data therefore confirm the mobility of W in subduction zones, but the selective mobility of W relative to the similar incompatible Th and Mo depends on the proportion of melt-like components, the composition of the material being subducted and ambient redox conditions in the subducted material.

The origin of decoupled Hf–Nd isotope compositions in Eoarchean rocks from southern West Greenland

Radiogenic isotope compositions of Hf and Nd are typically coupled in Phanerozoic and Proterozoic mafic rocks due to a similar behaviour of Lu–Hf and Sm–Nd during mantle melting. Eoarchean rocks, for instance those from southern West Greenland, exhibit an apparent decoupling of Hf and Nd isotope compositions. This apparent decoupling may either indicate metamorphic disturbance or, alternatively, mirror early differentiation processes in the silicate Earth. To evaluate the issue, we performed combined measurements of Hf–Nd isotope compositions together with major and trace element concentrations for well preserved >3720 to >3800Ma old tholeiitic metabasalts and gabbros from the ∼3700Ma and ∼3800Ma old terranes of the Isua Supracrustal Belt, southern West Greenland. In contrast to younger mafic rocks, calculated initial εHf–εNd values of the Isua tholeiites show similar spreads and are both near chondritic to strongly depleted (−0.7 to +6.3 and −0.8 to +4.4, respectively), also in contrast to previously reported more depleted signatures in nearby boninite-like metabasalts of the Garbenschiefer unit. An evaluation of alteration effects based on preserved major and trace element arrays reveals pristine magmatic trends and therefore the measured isotope compositions indeed in most cases characterize contrasting Eoarchean mantle sources. In accord with this view, compositions of the Isua metabasalts yield Eoarchean regression ages in Sm–Nd and Lu–Hf isochron spaces, overlapping with emplacement ages inferred from crosscutting relationships with tonalites. Lutetium–Hf systematics of the Isua metabasalts studied here, yield clear isochron relationships. For both terranes, there is some scatter in Sm–Nd space, indicating early disturbance of the Sm–Nd system close in time to the extrusion ages, possibly by seafloor alteration. Trace element compositions of the metabasalts indicate an arc setting and a strong source overprint by melt-like subduction components. It is likely, that the source overprint may have caused partial decoupling of the εHf–εNd values, due to selective addition of Nd as observed in modern subduction settings. In this case, the most radiogenic initial εNd and εHf isotope values characterize the most depleted mantle sources, and less radiogenic values would reflect increased contributions of isotopically more enriched subduction components. However, the most depleted samples still exhibit decoupled Hf–Nd compositions, making a case for the presence of even older mantle heterogeneities. A proposed superchondritic composition of the silicate Earth (SCHEM), however, cannot account for the most depleted sample compositions. Conversely, a depleted upper mantle formed by crystallization of perovskite-rich cumulates in the early Hadean may well explain these observed compositions. A literature survey reveals an overlap in initial Hf–Nd compositions between southern West Greenland TTGs and the metabasalts analyzed here. This overlap suggests a genetic relationship between these lithologies, where the TTGs may have inherited their unusual Hf–Nd compositions from mafic precursors isotopically similar in composition to the Isua tholeiites.

Petrogenesis of Lavas along the Solomon Island Arc, SW Pacific: Coupling of Compositional Variations and Subduction Zone Geometry

The Solomon island arc, SW Pacific, is of particular interest for understanding subduction zone volcanism, as magmatism in the active part of the arc is dominated by mafic melts, thus permitting direct insights into mantle processes. Along the Solomon island arc, the Indian^Australian plate is subducting at present beneath the Pacific plate. However, until at least c. 12 Myr ago, the Pacific plate was subducting beneath the Indian^Australian plate until the Cretaceous Ontong Java Plateau collided with the northern Solomon island arc.To evaluate the effects of the changes in tectonic regime on lava compositions, we present a comprehensive Sr^Nd^ Hf^Pb isotope, major element and trace element dataset, covering lavas erupted along the entire island arc (c. 1000 km). Basalts and andesites represent the most abundant rock types. Picrites and ankaramites occur in the New Georgia Group of the Solomon Islands, where they erupted above the subductingWoodlark spreading center, and also in the Santa Cruz archipelago, north of Vanuatu, where the Rennell Fracture Zone is subducting. Recent work has also identified the presence of adakites (Sr/Yup to c. 200), and high-Mg andesites (MgO45 wt %, Sr/Y c. 11^46). Most of the high-Mg andesites are genetically linked to the adakites, but some of the high-Mg andesites show affinities to boninitic compositions. Large ion lithophile element abundances in most Solomon island arc magmas indicate a strong source overprint by subduction components. Sr/Sr and eNd values along the arc range from 0 7029 to 0 7052 and from þ5 8 to þ8 3, respectively.The Sr^Nd values partially overlap the compositions of oceanic basalts from the Indian^Australian plate. Measured eHf values range from þ10 5 to þ14 6. If corrected for contributions from subduction components, combined eHf^eNd systematics also indicate that most of the studied Solomon arc lavas were generated within the Indian-type mantle domain. However, a few samples display eHf^eNd compositions resembling those of the Pacific-type mantle domain. These samples either originate from older Pacific basement (basalts) or represent melts derived from subducted Pacific crust (adakites). Lead isotope compositions, controlled by subduction components, can be used to identify the presence of two distinct types of subduction components that originate (1) from the Pacific plate including Ontong Java Plateau material (46 Myr old) and (2) from the more recently subducted Indian^ Australian plate. Combined Hf^Nd^Pb isotope data also reveal that lower parts of the Ontong Java Plateau entered the mantle wedge, as previously postulated by geophysical models.

Analytical Study on Field Zones, Record Rules, Shadow and Source Overprint Effects in CSAMT Exploration

With vector potential of magnetic type this paper derives the closed form expressions of subsurface electromagnetic fields generated by a grounded horizontal harmonic electric dipole on a homogeneous half-space, which distinguish formation waves and surface waves explicitly. According to this, we define surface- and subsurface- wave zone corresponding to far- and near-field zone. The record rules, shadow and source overprint effects, that have disturbed CSAMT explorers for a long time, are explained with physics. The methods of quantitatively dividing zones, the conditions of optimum record rule for each component and the prerequisites for causing shadow and source overprint effects are provided by the ratio of formation wave to surface wave. The deduction of covering points of CSAMT prospecting in Qinshui basin, Shanxi Province shows that the shadow and source overprint effects are real geological information carried by formation waves between transmitting source and observation point or beneath the source, which can be exploited.

An analytical method to estimate shadow and source overprint effects in CSAMT sounding

The controlled-source audio frequency magnetotelluric (CSAMT) method is well established in geophysical prospecting. By using a transmitter (usually a grounded electric dipole) to generate the source fields, the CSAMT method overcomes problems associated with weak natural EM fields used in conventional audio MT surveys. But the CSAMT method creates a new problem associated with active EM surveys: when anomalies are located between the transmitter and receiver, the measured data may be distorted so that the geological structure will be inferred incorrectly. Kuznetzov (1982) and Zonge and Hughes (1988) observe this phenomenon in 2D numerical model studies and term such behavior the shadow effect. This phenomenon has been simulated by numerical modeling and discussed in many papers (Newman et al., 1986; Zonge et al., 1986; Mitsuhata, 2000). Boschetto and Hohmann (1991) use numerical methods to study shadow and source overprint effects in CSAMT using a horizontal electric dipole source. For this problem, we separate the ground-wave and surface-wave terms from the closed-form expressions of the electric and magnetic fields in a uniform earth and use the ground-wave/surface-wave ratio to estimate the potential magnitude for producing shadow and source overprint effects. This is a new and effective analytical method.

2-D electromagnetic modeling by finite‐element method with a dipole source and topography

I present a method for calculating frequency‐domain electromagnetic responses caused by a dipole source over a 2-D structure. In modeling controlled‐source electromagnetic data, it is usual to separate the electromagnetic field into a primary (background) and a secondary (scattered) field to avoid a source singularity, and only the secondary field caused by anomalous bodies is computed numerically. However, this conventional scheme is not effective for complex structures lacking a simple background structure. The present modeling method uses a pseudo‐delta function to distribute the dipole source current, and does not need the separation of the primary and the secondary field. In addition, the method employs an isoparametric finite‐element technique to represent realistic topography. Numerical experiments are used to validate the code. Finally, a simulation of a source overprint effect and the response of topography for the long‐offset transient electromagnetic and the controlled‐source magnetotelluric measurements is presented.

Controlled‐source audiofrequency magnetotelluric responses of three‐dimensional bodies

Modeling the controlled‐source audiofrequency magnetotelluric (CSAMT) responses of simple three‐dimensional (3-D) structures due to a grounded electric bipole confirms that the CSAMT technique accurately simulates plane‐wave results in the far‐field zone of the transmitter. However, at receiver sites located above large conductive or resistive bodies, the presence of the inhomogeneity extends or reduces, respectively, the frequency range of the far‐field zone. Measurements made on the surface beyond a large 3-D body display a small but spatially extensive effect due to decay of the artificial primary field. Situating a 3-D inhomogeneity beneath the source permits an evaluation of “source overprint” effects. When such a body is resistive, a slight shift in the near‐field response to higher frequencies occurs. When a body below the transmitter is conductive, it is possible to make far‐field measurements closer to the transmitter or lower in frequency. However, as the size of the conductor and its secondary‐field response increases, large transition‐zone responses distort the data. For both a plane‐wave source and a finite source, current channeling into a 3-D conductor from conductive overburden enhances the response of a target. The modeled response of a dike‐like conductor shows no better results for either the broadside or collinear configuration. The location and extent of such a body are better defined when measuring the electric field perpendicular to the strike of the prism, but resistivity estimates are better when using the electric field parallel to the strike of the prism, irrespective of transmitter orientation. Models designed from data collected at Marionoak, Tasmania, yield results which indicate that the thin, vertical graphitic unit intersected by drilling is detectable by the CSAMT method, but probably is not the sole source of the large anomaly seen in the CSAMT data.