A comparative study of in-river geophysical techniques to define variations in riverbed salt load and aid managing river salinization

Geophysics ◽  
2010 ◽  
Vol 75 (4) ◽  
pp. WA135-WA147 ◽  
Author(s):  
Michael Hatch ◽  
Tim Munday ◽  
Graham Heinson
Keyword(s):  
River Flow ◽  
Relative Effectiveness ◽  
Sampling Interval ◽  
Southeastern Australia ◽  
Transient Electromagnetic ◽  
Conductivity Structure ◽  
Murray River ◽  
Conductive Substrate ◽  
Geophysical Techniques ◽  
Depth Of Investigation

Increased interest in the character of sediments at the base of waterways, for the purpose of managing river salinization, has led to the application of several geophysical techniques for collecting information from this zone. These instream methods are based on established ground and airborne electrical and electromagnetic technologies, including towed transient electromagnetic systems, towed direct current resistivity array systems, and frequency-domain helicopter electromagnetic systems. Although these systems are individually successful, a systematic examination of their relative effectiveness for identifying variations in substrate conductivity for a common stretch of a river remains lacking. We have compared results obtained from data collected using three instream geophysical techniques for a common stretch of the Murray River in southeastern Australia. The Murray River is an important water resource for drinking and agricultural purposes. Data from these surveys were acquired tolocate areas of significant saltwater accession to the Murray from a saline regional groundwater system that discharges into it. The three methods indirectly inform on those reaches that most likely contribute to higher salt loads in the river, and they do this through the identification of a conductive substrate (a gaining reach). For a [Formula: see text] stretch of the river, the methods identified similar variations in the conductivity structure of sediment substrate, although differences were observed in the modeled response relating to intrinsic differences between each system, including the sampling interval and resolution. The helicopter electromagnetic (EM) system is capable of acquiring hundreds of kil-ometers of data in a day, under any river flow condition, from near the river surface to depths in excess of [Formula: see text]. The other two techniques require safe river flow conditions for acquisition, with as much as 50 river km of data per day possible. The ground-based methods had enhanced lateral and vertical resolving capabilities relative to the helicopter EM system, but their depth of investigation was less (usually only [Formula: see text]).

Hydroelectric development and translocation of Galaxias brevipinnis: a cloud at the end of the tunnel?

2002 ◽  
Vol 59 (1) ◽  
pp. 49-56 ◽  
Author(s):  
Jonathan M Waters ◽  
Michael Shirley ◽  
Gerard P Closs
Keyword(s):  
Microsatellite Loci ◽  
River Flow ◽  
River System ◽  
River Diversion ◽  
Hydroelectric Project ◽  
Southeastern Australia ◽  
Haplotypic Diversity ◽  
Murray River ◽  
Alternative Hypotheses ◽  
Mtdna Diversity

Two major drainages of southeastern Australia, the Snowy River and the Murray River, were artificially linked by a major hydroelectric project during the early 20th century. This development diverts Snowy River flow into tributaries of the Murray River via a series of extensive tunnels. In 1990, fish surveys of the upper Murray River system recorded Galaxias brevipinnis, an aggressive migratory species previously unrecorded from the drainage. We used genetic analysis to discriminate between alternative hypotheses for Murray River G. brevipinnis: (i) anthropogenic translocation via the Snowy River diversion or (ii) a previously undiscovered natural population. Landlocked G. brevipinnis from the Murray River (43 fish, eight control region haplotypes) and Snowy River (39 fish, 11 haplotypes) exhibit similar levels of mtDNA diversity, share six haplotypes, and are not significantly differentiated for microsatellite loci (p = 0.0884). Coastal samples exhibit higher haplotypic diversity (40 fish, 20 haplotypes) but share only three haplotypes with Murray River and are significantly differentiated from Murray River samples for microsatellite loci (p = 0.0008). Our data are consistent with the translocation hypothesis but are generally inconsistent with a natural origin for Murray River G. brevipinnis. The suggested human-mediated translocation represents a risk to native fauna.

Finite-Element Modeling of Time-Domain Controlled-Source Electromagnetic Survey with Weighted Laguerre Polynomials

Geophysics ◽  
2021 ◽  
pp. 1-43
Author(s):  
Qingtao Sun ◽  
Runren Zhang ◽  
Yunyun Hu
Keyword(s):  
Finite Element ◽  
Time Domain ◽  
Laguerre Polynomials ◽  
Time Integration ◽  
Sampling Interval ◽  
Controlled Source ◽  
Time Integration Method ◽  
Transient Electromagnetic ◽  
Backward Euler ◽  
Land Survey

To facilitate the modeling of time-domain controlled-source electromagnetic survey, we propose an efficient finite-element method with weighted Laguerre polynomials, which shows a much lower computational complexity than conventional time integration methods. The proposed method allows sampling the field at arbitrary time steps and also its accuracy is determined by the number of polynomials, instead of the time sampling interval. Analysis is given regarding the optimization of the polynomial number to be used and the criterion of selecting the time scale factor. Two numerical examples in marine and land survey environments are included to demonstrate the superiority of the proposed method over the existing backward Euler time integration method. The proposed method is expected to facilitate the modeling of transient electromagnetic surveys in the geophysical regime.

Understanding LOTEM data from mountainous terrain

Geophysics ◽  
2000 ◽  
Vol 65 (4) ◽  
pp. 1113-1123 ◽  
Author(s):  
Andreas Hördt ◽  
Martin Müller
Keyword(s):  
Field Data ◽  
Electromagnetic Coupling ◽  
Theoretical Data ◽  
Correction Method ◽  
Topographic Effects ◽  
Mountainous Terrain ◽  
Large Loop ◽  
Transient Electromagnetic ◽  
Conductivity Structure ◽  
Long Offset

Long‐offset transient electromagnetic (LOTEM) data from the Vesuvius volcano, in Italy, show that the EM response of the topography is a potential cause of data distortions. A modeling study was carried out to simulate the effect of mountainous terrain on vertical magnetic‐field time derivatives using a 3-D finite‐difference code. The objectives were to assess the importance of topographic effects and to help identify them in existing field data. The total effect of topography on the LOTEM response can be considered as a combination of four distortions of the corresponding responses for a flat terrain. First, the receiver is at some height above the flat surface. Second, the mountain acts as a conductive body displacing air. Third, large loop receivers are nonhorizontal and sense a combination of horizontal and vertical magnetic fields. Finally, the electromagnetic coupling between the mountain and deeper‐lying structure modifies the structure response. Each of the effects can be identified in field data recorded at Mount Vesuvius. The topographic induced distortions for the model used in this study are moderate in the sense that 1-D inversions of the theoretical data still recover the gross conductivity structure, albeit with small deviations from the true parameters. Although this result might imply that topography might be ignored during the first stage of an interpretation, no simple correction method is evident, so topography will have to be included in any 2-D or 3-D inversion attempt.

Stratigraphic mapping of sedimentary formations in southern Ontario by ground electromagnetic methods

Geophysics ◽  
1990 ◽  
Vol 55 (9) ◽  
pp. 1148-1157 ◽  
Author(s):  
Ajit K. Sinha
Keyword(s):  
Oil And Gas ◽  
Structural Features ◽  
Southern Ontario ◽  
Transient Electromagnetic ◽  
Electromagnetic Methods ◽  
Geological Information ◽  
Survey Results ◽  
Multifrequency System ◽  
The Village ◽  
Depth Of Investigation

Multifrequency and transient electromagnetic (EM) soundings were done at several locations in southern Ontario to evaluate the possibility of stratigraphic mapping of gently dipping sedimentary formations by ground EM techniques. The possibility of detecting structural features such as folds, faults, and grabens in the formations was also explored. The two EM techniques were used to map a buried river valley near the village of Copetown, about 75 km southwest of Toronto. The valley, buried under fairly resistive glacial till and dolomite formations was mapped at depths ranging from 100 to 200 m. The interpreted shape and depth of the valley agreed well with information from a high resolution seismic reflection survey and data from wells penetrating the bedrock. Transient EM (TEM) data, using square transmitter loops with side dimensions comparable to the transmitter‐receiver separation for the multifrequency system, was found to be less affected by the presence of shallow conductors and lateral inhomogeneities than the multifrequency data, and had greater depth of investigation. TEM soundings were made on five additional profiles in the area. Lithologic logs from old oil and gas wells located near the profiles provided information for comparison with EM sounding interpretations. The depths to various dolomite, shale, and limestone formations interpreted from EM data agreed well with drillhole information. The EM soundings also detected structures in the Paleozoic formations such as faults and folds that were not previously known. The soundings confirmed the existence of a graben at one site at a depth of 100 m, which was postulated from logs from a cluster of closely spaced drillholes by geologists. The survey results indicated that EM sounding methods can be used for stratigraphic mapping in areas where detailed geological information is unavailable either because the bedrock is concealed by overburden, or when drillholes are sparsely distributed.

Noise reduction in TEM: Presenting a bandwidth- and sensitivity-optimized parallel recording setup and methods for adaptive synchronous detection

Geophysics ◽  
2012 ◽  
Vol 77 (3) ◽  
pp. E203-E212 ◽  
Author(s):  
Nicklas Skovgaard Nyboe ◽  
Kurt Sørensen
Keyword(s):  
Noise Reduction ◽  
Transient Signal ◽  
Depth Interval ◽  
Time Signal ◽  
Late Time ◽  
Synchronous Detection ◽  
Transient Electromagnetic ◽  
Receiver System ◽  
Depth Of Investigation ◽  
Measuring Scheme

The transient electromagnetic method (TEM) is a recognized tool for determining the subsurface resistivity structure over a wide depth interval. A key requirement for an accurate characterization of the shallow part of this interval is a sufficiently wideband receiver system. The maximum depth of investigation, on the other hand, is determined by the late-time signal-to-noise ratio (S/N). It has been demonstrated that the use of compact wideband receiver coils tends to deteriorate the late-time S/N due to their inherent low sensitivity to time-varying magnetic fields. To overcome this problem, we used a bandwidth- and sensitivity-optimized parallel recording setup using two separated receiver coils optimized for measuring the early- and late-time part of the transient signal, respectively. Using this setup, we experienced substantial improvements in the achievable depth of investigation while retaining shallow resolution. Further noise reduction may be obtained by adapting the applied synchronous detection measuring scheme to the local noise conditions. This is especially important when TEM are carried out in culturally developed areas, which is often the case for environmental investigations. Such soundings frequently suffer from inferior signal quality due to EM noise from local cultural sources. Comparing the frequency-domain filtering effect of the applied synchronous detection measuring scheme with the actual noise frequency content allowed us to evaluate the relative importance of the many sources of noise encountered in the field. Further improvements could be obtained by tailoring the applied synchronous detection to better reject specific noise frequencies through modification of the properties of time-gate integration and gate stacking.

Depth of investigation in electromagnetic sounding methods

Geophysics ◽  
1989 ◽  
Vol 54 (7) ◽  
pp. 872-888 ◽  
Author(s):  
Brian R. Spies
Keyword(s):  
Magnetic Field ◽  
Field Measurements ◽  
Sedimentary Basins ◽  
Step Response ◽  
High Signal ◽  
Depth Of Penetration ◽  
Transient Electromagnetic ◽  
Magnetic Field Measurements ◽  
Depth Of Investigation ◽  
Power Of Source

The time or frequency at which the electromagnetic (EM) response of a buried inhomogeneity can first be measured is determined by its depth of burial and the average conductivity of the overlying section; it is relatively independent of the type of source or receiver and their separation. The ability to make measurements at this time or frequency, however, depends on the sensitivity and accuracy of the instrumentation, the signal strength, and the ambient noise level. These factors affect different EM sounding systems in surprisingly different ways. For the magnetotelluric (MT) method, it is possible to detect a buried half‐space under about 1.5 skin depths of overburden. The maximum depth of investigation is virtually unbounded because of high signal strengths at low frequencies. Transient electromagnetic (TEM) soundings, on the other hand, have a limited depth of penetration, but are less affected by static shift errors. For TEM, a buried inhomogeneity can be detected under about one diffusion depth of overburden. For conventional near‐zone sounding in which induced voltage is measured (impulse response), the depth of investigation is proportional to the [Formula: see text] power of the source moment and ground resistivity. By contrast, if the receiver is a magnetometer (step response system), the depth of investigation is proportional to the [Formula: see text] power of source moment and is no longer a function of resistivity. Magnetic‐field measurements may, therefore, be superior for exploration in conductive areas such as sedimentary basins. Far‐zone, or long‐offset, TEM soundings are traditionally used for deep exploration. The depth of investigation for a voltage receiver is proportional to the [Formula: see text] power of source moment and resistivity and is inversely proportional to the source‐receiver separation. Magnetic‐field measurements are difficult to make at long offsets because instrumental accuracy limits the measurement of the very slow decay of the magnetic field. Frequency‐domain controlled‐source systems are ideally suited for sounding at the very shallow depths needed for engineering, archaeological, and groundwater applications because of the relative ease of extending the measurements to arbitrarily high frequencies, and also because geometric soundings can be made at low induction numbers.

Tooth Ablation along the Murray River in Southeastern Australia

2017 ◽  
Author(s):  
Colin Pardoe ◽  
Arthur C. Durband
Keyword(s):  
Central Incisor ◽  
Australian Aborigines ◽  
Southeastern Australia ◽  
Murray River ◽  
Tooth Type

Tooth ablation has a long history among Australian Aborigines. Here we present a study of four groups along a 370km stretch of the Murray River in southeastern Australia. The frequency and patterning are examined with respect to the individual’s sex, population, and tooth type. Within the study area, ablation is nine times more common among men than women (27 percent versus 5 percent). Although the samples are from cemeteries, there is no chronological control and from other evidence the remains probably date to the later Holocene. Larger regional linguistic and cultural groupings appear to be important indicators for ablation patterning, particularly differences between women upstream (7 percent) and downstream (absent). The tooth or teeth removed varies between groups and most combinations were seen, although ablation of a central incisor was the most common.

scholarly journals A geophysical analysis of Aboriginal earth mounds in the Murray River Valley, South Australia

2021 ◽  
Author(s):  
Ian Moffat ◽  
Dave Ross ◽  
Michael Morrison ◽  
Kleanthis Simyrdanis ◽  
Amy Roberts ◽  
...  
Keyword(s):  
Aboriginal People ◽  
Relative Effectiveness ◽  
Geophysical Methods ◽  
South Australia ◽  
Comparative Trial ◽  
Resistivity Tomography ◽  
Murray Darling Basin ◽  
Murray River ◽  
Repeated Use ◽  
Earth Oven

Earth mounds are common archaeological features in some regions of Australia, particularly within the Murray-Darling Basin. These features are generally considered to have formed via the repeated use of earth oven cookery methods employed by Aboriginal people during the mid- to late-Holocene. This study assesses the relative effectiveness of key geophysical methods including magnetometry, groundpenetrating radar (GPR) and electrical resistivity tomography (ERT) in mapping, and determining the stratigraphy of earth mound sites. Three earth mounds adjacent to Hunchee Creek, on Calperum Station in South Australia's Riverland region, were chosen to conduct a comparative trial of these methods. This research demonstrated that geophysics can be used to both locate mounds and provide information as to deposit thickness and size. Individual ovens within mounds can also be located. This suggests a greater potential role for geophysics in understanding the Holocene archaeological record in Australia.

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