摘要:

AbstractThis study utilizes ultrasonic water tank modelling to examine the complexities which can result when seismic energy is diffracted by even a simple 3D interface structure. Ultrasonic laboratory modelling is carried out in our in‐house water tank, using an aluminum block to simulate very simple ocean‐bottom topography. The topography consists of parallel depressions which simulate valleys running parallel to a mid‐ocean spreading ridge. This study shows that the phase and amplitude of primary ocean‐bottom reflections can vary strongly with the azimuth of propagation. The trailing energy coda also varies dramatically as a function of propagation direction, showing the importance of developing a seismic model which includes the full 3D azimuthal distribution of interface st

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1995.tb00268.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe aim of this work is to consider the phenomenon of negative response and its detectability in a polarizable half‐space formed of Clay‐water mixture which commonly exists in water aquifers and which can be used as a simple indicator of the presence of Underground water.We make use of an analytical treatment for the transient voltage induced in a coincident loop lying on a half‐space as a basis for our computations. A Cole‐Cole model is used to represent the Clay with appropriate parameters.As saturated Clay is characterized by high conductivity and moderate chargeability, it is hardly detectable relative to the practical noise level. However, there are optimum Clay parameters that offer a maximum negative response. It is also observed that for a very short time constant τ in the Cole‐Cole model, the amplitude of the negative response decreases with the decrease of the time constant due to the fast decay of the polarization current.Finally, it is found that there is an optimum value for the loop radius which gives the largest value of the negative response and this loop radius depends on the model

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1995.tb00269.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe prospecting of densely urbanized areas by the measurement of magnetic and electric natural fields is severely hampered by electromagnetic (EM) noise. Active man‐made EM noise sources can generally be considered fixed in space, thus affecting the magnetotelluric (MT) signals of a measuring site mainly along their polarization directions.Taking advantage of the impulsive nature of polarized EM noise, a time‐domain directional noise cancelling (DNC) technique is proposed. The comparison of noisy data with data predicted, using a low noise reference signal or with data interpolated whenever no reference is available, allows the detection of the most likely noise sources with prevailing directional patterns using a Bayes's criterion. The DNC approach is general and can be adapted, depending on the reference signal used (single‐site or remote‐reference).In field data, hodograms of the prediction residuals basically confirm the directional noise model assumed in DNC. An example is presented in which the DNC technique has been applied to a single‐site MT survey carried out in northern Italy, where the signal was heavily corrupted by noise with prevailing directional properties due to the densely urbanized area. MT apparent resistivities and phases obtained at the site of the survey before and after DNC are presented and

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1995.tb00270.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractAn integrated electromagnetic and nuclear magnetic resonance (NMR) method is proposed for investigating highly conductive soil areas. Using a simple model of a homogeneous half‐space and the same antenna for both methods, a significant improvement in the NMR data interpretation is obtained. A case study has shown fair agreement between the results from computer modelling, field tests, and data from a nearby observation well. The electromagnetic method and the half‐space model were selected for easy integration into an existing instrument used for the NMR method. A more accurate knowledge of the conductivity distribution with depth will further improve the final res

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1995.tb00271.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA geophysical test was carried out over a well‐located and fairly embedded karstic conduit. The MT‐VLF method was selected because of its high resolution and its ability to provide a resistivity parameter sensitive to water and Clay. This method was used together with DC resistivity methods which allow vertical adjustment of the VLF data and show consistency between the investigation and target depths. After correcting the deformations due to the polarization of the primary field, the MT‐VLF data show clearly, in the Central part of the site, that the conduit does not coincide with an anomaly axis but coincides with the boundary between a conductive area and a resistive area. 2D modelling confirms that direct detection of the conduit is not feasible and that the conduit is located close to a conductive zone corresponding to a completely Clay‐filled fractured zone. This situation was observed on the whole site and the conduit seemed systematically to thread its way between the conductive zones to join the outlet. The distribution of the conductive fractured zones and the direction of the hydraulic gradient were two important elements in predicting the location of the conduits. A 3D approach would increase the probability of finding the conduits in such

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1995.tb00272.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractLaboratory measurements of soil samples are necessary to assess the effect of mineralogy, grain size distribution, moisture content, and electrolyte composition on the resistivity spectrum of soil material. Laboratory results are also required for the interpretation of field data. Induced polarization phenomena in glacial soils are poorly understood and so far no convenient laboratory techniques are available for its measurement. Coarse grain size and the need to measure unsaturated samples and to monitor the homogeneity of the sample require a sample holder–electrode construction that differs from those presented in Clay mineral Studies.This study presents a spectral induced polarization laboratory system that is suitable for measuring fine‐ and coarse‐grained and both saturated and unsaturated soil samples. The noise caused by the electrode–electrolyte interface is studied in detail. It is shown that easy‐to‐use platinum or acid‐free steel potential electrodes are convenient over a broad frequency band ranging from 0.016Hz up to more than 1000 Hz. The laboratory experiments and comparisons between laboratory and field results also indicate that sampling and sample packing procedures have only a minor influence on the phase spectrum of

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1995.tb00273.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA new method is proposed for the quantitative interpretation of SP field data produced by a polarized ore body and simulated by an inclined sheet. The theoretical concept is based on the study of the amplitude Spectrum. It is shown that the SP amplitude Spectrum is not continuous at zero frequency; this leads to the dip angle determination of the inclined sheet. It is also shown that the SP amplitude spectrum is practically nullified at a characteristic amortization frequency that depends on the depth of the polarized body. The maximum amplitude Spectrum value of the SP gradient is used to estimate the depth to the bottom of the polarized body. Thus, the geometrical parametershandH, the depths to the top and bottom, respectively, as well as the dip angle of the inclined sheet, can be satisfactorily determined. Some problems may arise in the determination of these parameters, affecting their accuracy, whenever unwanted frequency noise is present.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1995.tb00274.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractTwelve ground penetrating radar (GPR) experiments were conducted on the modern, wave‐influenced William River delta, on the Southern shore of Lake Athabasca in northern Saskatchewan, Canada. The delta is a well‐sorted, quartzoserich, clean, sand‐dominated, water‐saturated geomorphic feature which provided an ideal site to test GPR. Penetration depths, resolution and continuity of reflections were compared for different antennae frequencies (25, 50, 100, 200 MHz) and transmitter powers (pulser voltage: 400 V, 1000 V). The data show significant variations in vertical resolution from 0.15 m to 0.76 m (200‐25 MHz), depth of penetration from 14 m‐28 m (200‐25 MHz), and continuity of reflections. Increasing the transmitter power from 400 V to 1000 V increases the depth of penetration by 5 to 14% and improves the continuity of reflections with little effect on t

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1995.tb00275.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY