Electromagnetic sounding in the Columbia Basin, Yakima, Washington

M. J. Wilt; H. F. Morrison; K. H. Lee; N. E. Goldstein

doi:10.1190/1.1442738

Electromagnetic sounding in the Columbia Basin, Yakima, Washington

Geophysics ◽

10.1190/1.1442738 ◽

1989 ◽

Vol 54 (8) ◽

pp. 952-961 ◽

Cited By ~ 6

Author(s):

M. J. Wilt ◽

H. F. Morrison ◽

K. H. Lee ◽

N. E. Goldstein

Keyword(s):

Geoelectric Section ◽

Electromagnetic Sounding ◽

Surface Inhomogeneity ◽

Varying Thickness ◽

Anticlinal Structure ◽

Scale Models ◽

Target Layer ◽

Induction Logs ◽

Lateral Structure ◽

Deep Target

A controlled‐source electromagnetic (CSEM) survey was conducted near Yakima, Washington, to map the thickness and resistivity of a thick volcanic sequence overlying a sedimentary section and to infer structure in the sediments. The survey was conducted with a frequency‐domain system employing loop transmitters 400–500 m on a side and three‐component SQUID magnetometer receivers separated from the loop by 1 to 5 km. Data collected along a 30 km profile orthogonal to regional strike were interpreted initially with 1-D layered models, which were then pieced together to make a geoelectric section. Induction logs in a 5000 m exploration hole at one end of the profile agree very well with the CSEM soundings made around the hole. The geoelectric section reveals a smoothly varying thickness of volcanics with a pronounced anticlinal structure approximately concordant with a surface topographic ridge. To assess the validity of inferring lateral structure from 1-D interpretations, we made scale models of an anticlinal structure and of a surface inhomogeneity and conducted CSEM measurements over the scale models. Layered‐model inversions of these data show that the anticlinal structure and its location are very well determined by 1-D inversion, but its height and width are not accurately determined. CSEM sounding over the surface inhomogeneity model shows that this feature does not significantly degrade the interpretation of a deep target layer. In this setting, a geoelectric section made up from 1-D interpretations provides good qualitative measures of subsurface structure and also provides excellent starting models for detailed 2-D or 3-D modeling.

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Transient electromagnetic modeling of shallow A‐type sections with 3-D inhomogeneities

Geophysics ◽

10.1190/1.1443291 ◽

1992 ◽

Vol 57 (6) ◽

pp. 774-780 ◽

Cited By ~ 3

Author(s):

M. Poddar ◽

Walter L. Anderson

Keyword(s):

Apparent Resistivity ◽

Middle Layer ◽

Late Time ◽

Geoelectric Section ◽

Resistivity Sounding ◽

Transient Electromagnetic ◽

Target Layer ◽

Weathered Layer ◽

Target Depth ◽

Transient Electric Field

A hard rock area underlain by granitic/gneissic or basaltic rocks often has an A‐type three‐layer geoelectric section in which resistivity increases with depth. The middle layer of moderate resistivity caused by fracturing/fissuring that lies between the surface‐weathered layer and the substratum of unfractured rock is not a good target for a direct current (DC) resistivity sounding since it is generally suppressed in the observations. Moreover, its definition requires expanding the electrode spacing to a length several times the depth of the target layer, and this may be a drawback if the target layer is either laterally variable or limited in its horizontal extent. We first studied the transient electric field of a horizontal electric dipole (HED) source excited by a step turn‐off current for a 1-D model of an A‐type geoelectric section. The results of this theoretical study are presented as graphs of normalized apparent resistivity versus a time‐related dimensionless parameter. Irrespective of the separation between the transmitter and receiver dipoles, these transient sounding curves become similar to the corresponding Schlumberger sounding curves at late time. Hence the transient electric field measurement offers the possibility of sounding at a fixed transmitter‐receiver spacing that may be shorter than the target depth. Also, at early times, for a certain ratio of the dipole separation to the target depth, there is a dramatic increase in the resolution of the response. Thus, it is possible to resolve suppressed layers of an A‐type section in this type of sounding. A study of the effects of transmitter ramp time and receiver bandwidth on the transient apparent resistivity curves shows that a very fast current shut‐off and wideband measurement are required to realize all the possibilities suggested by this modeling. Some 3-D transient electromagnetic (TEM) modeling was also done to simulate (1) a lateral variation in the resistivity of the middle layer of an A‐type section and (2) a weak zone of limited horizontal extent in the substratum of a two‐layer section. We observed that the 3-D inclusion has less effect at late time but is more pronounced at early time. In view of the above results, we conclude that the transient E‐field sounding with a grounded wire source can be used in place of a conventional DC resistivity sounding to overcome the problem of poor resolution due to the suppression of the intermediate layer in a geoelectric section where the resistivity increases with depth. As such, it has a potential application in groundwater as well as geotechnical surveys, because together with the overlying weathered layer, the fractured rock constitutes the aquifer in hard rocks.

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Focused resolution of thin conducting layers by various dipole EM systems

Geophysics ◽

10.1190/1.1443774 ◽

1995 ◽

Vol 60 (2) ◽

pp. 381-389 ◽

Cited By ~ 5

Author(s):

Saurabh Kumar Verma ◽

S. P. Sharma

Keyword(s):

Narrow Range ◽

Conducting Layer ◽

Good Resolution ◽

Fixed Frequency ◽

Electromagnetic Sounding ◽

Maximum Resolution ◽

Thin Conducting ◽

Wide Range ◽

Target Layer ◽

Separation Parameters

Electromagnetic sounding in the frequency domain can be performed in two ways—either by changing frequency at a location (frequency sounding) or by changing the transmitter‐receiver (T-R) separation using a fixed frequency (geometric sounding). These changes in frequency or separation parameters effect vertical scanning of conductivity distributions below the earth’s surface. In case of thin conducting layers, there could be an optimum range of frequencies or T-R separations that provide maximum resolution of the layer parameters. Thus, for a given buried target layer, it should be possible to find ranges of frequencies or separations that yield the best focusing. The present study deals with the focused resolution of a thin conducting layer in frequency sounding with variable T-R separation for four different dipole configurations. It is observed from the inversion of the data from various dipole electromagnetic (EM) systems that different T-R separations have different resolutions for the same target layer. It is also observed that for a particular loop system, the best resolution is observed at an optimum T-R separation. The resolution becomes poorer when the T-R separation is either increased or decreased from this particular separation. Thus it has been possible to propose a “zone of focusing” for various dipole EM configurations. The study reveals that this zone is broadest for the horizontal coplanar loops system, implying that this system yields good resolution over a wide range of T-R separations. Compared to this, the perpendicular loops system yields a very sharp peak implying that it resolves the target over a very narrow range of separations. However, the perpendicular loops system provides resolution most parsimoniously requiring the least T-R separation. This is followed by the vertical coplanar, vertical coaxial, and horizontal coplanar loops systems.

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Log-log scale roughness spectroscopy of surfaces

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100146965 ◽

1993 ◽

Vol 51 ◽

pp. 224-225

Author(s):

P. Fraundorf ◽

B. Armbruster

Keyword(s):

Power Spectrum ◽

Autocorrelation Function ◽

Power Spectra ◽

Scanning Force Microscopy ◽

Scanning Tunneling ◽

Tunneling Microscopy ◽

Force Microscopy ◽

Scanning Force ◽

Lateral Structure ◽

Scale Roughness

Optical interferometry, confocal light microscopy, stereopair scanning electron microscopy, scanning tunneling microscopy, and scanning force microscopy, can produce topographic images of surfaces on size scales reaching from centimeters to Angstroms. Second moment (height variance) statistics of surface topography can be very helpful in quantifying “visually suggested” differences from one surface to the next. The two most common methods for displaying this information are the Fourier power spectrum and its direct space transform, the autocorrelation function or interferogram. Unfortunately, for a surface exhibiting lateral structure over several orders of magnitude in size, both the power spectrum and the autocorrelation function will find most of the information they contain pressed into the plot’s origin. This suggests that we plot power in units of LOG(frequency)≡-LOG(period), but rather than add this logarithmic constraint as another element of abstraction to the analysis of power spectra, we further recommend a shift in paradigm.

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