The problem of current channelling: A critical review

Jones, Alan G.

doi:10.1007/bf01453996

Cited by 147 publications

(72 citation statements)

References 79 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In this case, such an interpretation yields a cross section which may be somewhat incorrect in its surficial features but represents accurately the regional resistivity structure of interest provided the regional structure itself is not arbitrarily three-dimensional [Jones, 1983;Bostick, 1986;Kariya, 1986;Wannarnaker, 1989, in press]. For two-dimensional modeling, the MT functions should be defined using a fixed coordinate system approximating the strike direction to overcome unwanted effects of near-surface structure, specifically on the impedance phase [Wannamaker et al, 1984].…”

Section: Discussionmentioning

confidence: 99%

Resistivity cross section through the Juan de Fuca Subduction System and its tectonic implications

Wannamaker¹,

Booker²,

Jones³

et al. 1989

J. Geophys. Res.

Self Cite

177

124

View full text Add to dashboard Cite

A resistivity cross section to depths exceeding 200 km has been derived from magnetotelluric observations along a profile near latitude 45øN from the Juan de Fuca spreading center, across the coastal subduction complex, the High Cascades volcanic arc, and into the back-arc Deschutes Basin region. In this two-dimensional interpretation, emphasis was placed on data approximating the transverse magnetic mode since these data are relatively robust to common departures from the two-dimensional assumption. The vertical magnetic field, however, has been very valuable in defining structure of the offshore sediments, of the oceanic asthenosphere and below the arc volcanics of the Westem and High Cascades. The transverse electric data on land suffer a variety of three-dimensional effects, making their interpretation very difficult. In contrast, the greater uniformity of upper crustal conditions on the seafloor allowed a good fit to both modes of the impedance plus the vertical magnetic field at least down to 104 s' Important components of . While a number of aspects of the subsurface resistivity can be deduced straight from inspection of the data, computer simulation of the observations helps to quantify more rigorously the permissible or required structures. In this paper, we derive a two-dimensional resistivity model, with a presumed northsouth strike, by trial-and-error fitting of our MT measurements with a finite element forward modeling algorithm. Constraints on model geometry from independent geological or geophysical investigations have been incorporated where justified. Our paper is divided into three major sections. First, we explain the approach toward two-dimensional modeling. On land, the transverse magnetic (TM) impedance functions are emphasized because theory and experience show that they are more robust to plausible three-dimensional effects in the region than are the vertical magnetic field or, especially, the transverse electric (TE) impedance. Upper crustal structure on the seafloor appears to be rhuch simpler than on land, however, and both modes of impedance plus the vertical field are fit fairly well. Second, the model cross section is described and its goodness of fit to the data demonstrated. Model uniqueness is investigated by perturbing certain features of the model and assessing the increase in misfit.

show abstract

Section: Discussionmentioning

confidence: 99%

Resistivity cross section through the Juan de Fuca Subduction System and its tectonic implications

Wannamaker¹,

Booker²,

Jones³

et al. 1989

J. Geophys. Res.

Self Cite

177

124

View full text Add to dashboard Cite

show abstract

“…These fluctuations arise from electric charge distributions originating wherever the field crosses resistivity boundaries [Price, 1973;Jones, 1983;Wannamaker et al, 1984]. The charge in turn reflects the requirement that electric current across boundaries be continuous.…”

Section: Transverse Magnetic Impedance Functionsmentioning

confidence: 99%

Magnetotelluric observations across the Juan de Fuca Subduction System in the EMSLAB Project

Wannamaker¹,

Booker²,

Filloux³

et al. 1989

J. Geophys. Res.

Self Cite

View full text Add to dashboard Cite

A magnetotelluric (MT) transect has been obtained near latitude 45"N from the active Juan de Fuca spreading center, across the subduction ,..one and Cascades volcanic arc, and into the back arc Deschutes Basin region. This paper presents the MT dat.a set and describes ~ts major characteristics as they pertain to the resistivity of the subduction system. In addition, we discu~s the measurement and processing procedures employed as well as important concerns in data interpretation. Broadband audiomagnetotelluric (AM1)/MT soundings (approx. 0.01-500 s period) were collectei on land with considerable redundancy in site location, and from which 39 sites were selected which con~train upper crustal heterogeneity but sense also into the upper mantle. Fifteen long-period MT recordings (about 50-10,000 s) on land confinn the broadband responses in their common period range and extend the d,:pths of exploration to hundreds of kilometers. On the Juan de Fuca plate offshore, 33 out of 39 sea floor instruments at 19 locations gave good results. Of these locations, five magnetotelluric soundings plus two additional geomagnetic variation sites, covering the period range 200-1OS s approximately, constitute the ocean bottom segment of our profile. The feature of the land observations which probably relates most closely to the subduction process is a peak in the impedance phase of the transverse magnetic mode around 30-50 s period. This phase anomaly, with a corresponding inflection in the apparent resistivity, is continuous eastward from the seacoast and ends abruptly at the High Cascades. It signifies an electrically conductive layer in otherwise resistive lower crust or upper mantle, with the layer conductance decreasing eastward from the coast to a minimum under the Coast Range but increasing suddenly to the east of the central WillanIette Basin. The higher conductance to the east is corroborated by the vertical magnetic field transfer function whose real component shows negative values in the period range 100-1000 s over the same distance. The transverse electric mode apparent resistivity and phase on the land display a variety of three-dimensional effects which make their interpretation difficult. Conversely, both modes of the ocean floor soundings exhibit a smooth progression laterally from the coastal area to the spreading ridge, indicating that the measurements here are reflecting primarily the large-scale tectonic structures of interest and are little disturbed by small near-surfac.e inhomogeneities. The impedance data near the ridge are strongly suggestive of a low-resistivity asthenosphere beneath resistive Juan de Fuca plate lithosphere. Approaching the coastline to the east, both impedance and vertical magnetic field responses appear increasingly affected by a thick wedge of deposited and accreted sediments and by the thinning of the seawater.

show abstract

“…Jones (1983), Wanamaker et al (1984), Berdichevsky et al (1998), Park & Mackie (2000), Ledo et al (2002) and Ledo (2005) studied the limitations of 2-D interpretation of 3-D MT data and made various recommendations, showing some situations where 2-D inversion of 3-D data can properly reproduce the geoelectrical subsurface when the content in the 2-D modes is appropriately chosen for inversion. Siripunvaraporn et al (2005b) suggest that if the data are 3-D in nature but collected along a profile rather than on a grid, the results from the 3-D inversion of the MT profile are superior to the results obtained by performing 2-D inversion of the same data, even with the far coarser mesh employed in 3-D as compared to 2-D inversion.…”

mentioning

confidence: 99%

The advantages of complementing MT profiles in 3-D environments with geomagnetic transfer function and interstation horizontal magnetic transfer function data: results from a synthetic case study

Campanyà¹,

Ogaya²,

Jones³

et al. 2016

Geophys. J. Int.

Self Cite

View full text Add to dashboard Cite

: The advantages of complementing MT profiles in 3-D environments with geomagnetic transfer function and interstation horizontal magnetic transfer function data: results from a synthetic case study. -Geophysical Journal International, 207, 3, pp. 1818-1836 S U M M A R YAs a consequence of measuring time variations of the electric and the magnetic field, which are related to current flow and charge distribution, magnetotelluric (MT) data in 2-D and 3-D environments are not only sensitive to the geoelectrical structures below the measuring points but also to any lateral anomalies surrounding the acquisition site. This behaviour complicates the characterization of the electrical resistivity distribution of the subsurface, particularly in complex areas. In this manuscript we assess the main advantages of complementing the standard MT impedance tensor (Z) data with interstation horizontal magnetic tensor (H) and geomagnetic transfer function (T) data in constraining the subsurface in a 3-D environment beneath a MT profile. Our analysis was performed using synthetic responses with added normally distributed and scattered random noise. The sensitivity of each type of data to different resistivity anomalies was evaluated, showing that the degree to which each site and each period is affected by the same anomaly depends on the type of data. A dimensionality analysis, using Z, H and T data, identified the presence of the 3-D anomalies close to the profile, suggesting a 3-D approach for recovering the electrical resistivity values of the subsurface. Finally, the capacity for recovering the geoelectrical structures of the subsurface was evaluated by performing joint inversion using different data combinations, quantifying the differences between the true synthetic model and the models from inversion process. Four main improvements were observed when performing joint inversion of Z, H and T data: (1) superior precision and accuracy at characterizing the electrical resistivity values of the anomalies below and outside the profile; (2) the potential to recover high electrical resistivity anomalies that are poorly recovered using Z data alone; (3) improvement in the characterization of the bottom and lateral boundaries of the anomalies with low electrical resistivity; and (4) superior imaging of the horizontal continuity of structures with low electrical resistivity. These advantages offer new opportunities for the MT method by making the results from a MT profile in a 3-D environment more convincing, supporting the possibility of high-resolution studies in 3-D areas without expending a large amount of economical and computational resources, and also offering better resolution of targets with high electrical resistivity.

show abstract

The problem of current channelling: A critical review

Cited by 147 publications

References 79 publications

Resistivity cross section through the Juan de Fuca Subduction System and its tectonic implications

Resistivity cross section through the Juan de Fuca Subduction System and its tectonic implications

Magnetotelluric observations across the Juan de Fuca Subduction System in the EMSLAB Project

The advantages of complementing MT profiles in 3-D environments with geomagnetic transfer function and interstation horizontal magnetic transfer function data: results from a synthetic case study

Contact Info

Product

Resources

About