Inversions of surface and borehole data from large‐loop transient electromagnetic system over a 1‐D earth

Geophysics ◽  
2001 ◽  
Vol 66 (4) ◽  
pp. 1090-1096 ◽  
Author(s):  
Z. Zhang ◽  
J. Xiao
Keyword(s):  
Early Time ◽  
Late Time ◽  
Inversion Algorithm ◽  
Borehole Data ◽  
Electromagnetic System ◽  
Large Loop ◽  
Transient Electromagnetic ◽  
Surface Data ◽  
The Time Domain ◽  
The Magnetic Field

Large‐loop electromagnetic (EM) systems that measure transient EM (TEM) data on the surface or in boreholes have shown increased application in exploration geophysics. Accurate interpretation of borehole TEM data is necessary to discover deep hidden targets that cannot be detected with surface systems. However, the inversion of borehole TEM data has not been fully addressed. In this paper, we study the propagation of the TEM field from a large‐loop EM borehole system inside a layered earth and develop a new inversion algorithm to reconstruct layered conductivity structures from large‐loop TEM data measured with both surface and borehole configurations. The magnetic field and sensitivities are first computed in the frequency domain and are then transformed into the time domain where the inversion is performed. The surface data have a higher S/N ratio at early time channels, while the borehole data have a higher S/N ratio at late time channels. Consequently, the surface data can be inverted to better resolve shallow structures, and the borehole data can be used to better detect deep structures. The merits of joint inversions of borehole and surface data are explored. We test our inversion algorithm using numeric examples.

The transient EM step response of a dipping plate in a conductive half‐space

Geophysics ◽  
1992 ◽  
Vol 57 (9) ◽  
pp. 1116-1126 ◽  
Author(s):  
James E. Hanneson
Keyword(s):  
Half Space ◽  
Free Space ◽  
Early Time ◽  
Step Response ◽  
Current Loop ◽  
Late Time ◽  
Electromagnetic System ◽  
University Of Toronto ◽  
Transient Electromagnetic ◽  
Induction Process

An algorithm for computing the transient electromagnetic (TEM) response of a dipping plate in a conductive half‐space has been developed. For a stationary [Formula: see text] current loop source, calculated profiles simulate the response of the University of Toronto electromagnetic system (UTEM) over a plate in a 1000 Ω ⋅ m half‐space. The objective is to add to knowledge of the galvanic process (causing poloidal plate currents) and the local induction process (causing toroidal currents) by studying host and plate currents with respect to surface profiles. Both processes can occur during TEM surveys. Plates are all [Formula: see text] thick with various depths, dips, and conductances. Calculated host and plate currents provide quantitative examples of several effects. For sufficiently conductive plates, the late time currents are toroidal as for a free‐space host. At earlier times, or at all times for poorly conducting plates, the plate currents are poloidal, and the transitions to toroidal currents, if they occur, are gradual. At very late times, poloidal currents again dominate any toroidal currents but this effect is rarely observed. Stripped, point‐normalized profiles, which reflect secondary fields caused by the anomalous plate currents, illustrate effects such as early time blanking (caused by noninstantaneous diffusion of fields into the target), mid‐time anomaly enhancement (caused by galvanic currents), and late time plate‐in‐free‐space asymptotic behavior.

The Development and Applications of the Helicopter-borne Transient Electromagnetic System CAS-HTEM

2019 ◽  
Vol 24 (4) ◽  
pp. 653-663 ◽  
Author(s):  
Xin Wu ◽  
Guangyou Fang ◽  
Guoqiang Xue ◽  
Lihua Liu ◽  
Leisong Liu ◽  
...  
Keyword(s):  
Dynamic Range ◽  
Early Time ◽  
Time Signal ◽  
Late Time ◽  
Time Data ◽  
Electromagnetic System ◽  
Transient Electromagnetic ◽  
Chinese Academy Of Sciences ◽  
Band Limited ◽  
Academy Of Sciences

Over the past decade, helicopter-borne transient electromagnetic (HTEM) systems have been rapidly developed. A new HTEM prototype (referred to as a CAS-HTEM) has been developed by the Chinese Academy of Sciences. In terms of hardware, the CAS-HTEM system uses an inflatable structure to carry the transmitting loop, which significantly reduces the weight of the system and makes it easier to transport. A dual gain receiver was innovated to extend the dynamic range of the system. In addition, an observation circuit for transmitting voltage waveform is introduced, so that the derivative waveform of transmitting current with higher SNR could be calculated. In terms of data processing, more reliable early time data could be obtained by band-limited effect removal; a weighted stacking algorithm is introduced to reduce the narrow band noise more effectively and increase the sensitivity of data to the anomaly location; a method based on τ-domain transform is used for late time signal processing. The results of the field test which was carried out in Inner Mongolia were found to be consistent with the drill data, which effectively verified the performance of this HTEM prototype.

The use and misuse of apparent resistivity in electromagnetic methods

Geophysics ◽  
1986 ◽  
Vol 51 (7) ◽  
pp. 1462-1471 ◽  
Author(s):  
Brian R. Spies ◽  
Dwight E. Eggers
Keyword(s):  
Apparent Resistivity ◽  
Early Time ◽  
Asymptotic Formulas ◽  
Voltage Response ◽  
Late Time ◽  
Induced Voltage ◽  
Resistivity Curve ◽  
Transient Electromagnetic ◽  
Electromagnetic Methods ◽  
Tem Method

Problems and misunderstandings arise with the concept of apparent resistivity when the analogy between an apparent resistivity computed from geophysical observations and the true resistivity structure of the subsurface is drawn too tightly. Several definitions of apparent resistivity are available for use in electromagnetic methods; however, those most commonly used do not always exhibit the best behavior. Many of the features of the apparent resistivity curve which have been interpreted as physically significant with one definition disappear when alternative definitions are used. It is misleading to compare the detection or resolution capabilities of different field systems or configurations solely on the basis of the apparent resistivity curve. For the in‐loop transient electromagnetic (TEM) method, apparent resistivity computed from the magnetic field response displays much better behavior than that computed from the induced voltage response. A comparison of “exact” and “asymptotic” formulas for the TEM method reveals that automated schemes for distinguishing early‐time and late‐time branches are at best tenuous, and those schemes are doomed to failure for a certain class of resistivity structures (e.g., the loop size is large compared to the layer thickness). For the magnetotelluric (MT) method, apparent resistivity curves defined from the real part of the impedance exhibit much better behavior than curves based on the conventional definition that uses the magnitude of the impedance. Results of using this new definition have characteristics similar to apparent resistivity obtained from time‐domain processing.

A single apparent resistivity expression for long‐offset transient electromagnetics

Geophysics ◽  
1986 ◽  
Vol 51 (6) ◽  
pp. 1291-1297 ◽  
Author(s):  
Yang Sheng
Keyword(s):  
Apparent Resistivity ◽  
Early Time ◽  
Point Of View ◽  
Time Range ◽  
Late Time ◽  
Careful Study ◽  
Physical Point ◽  
Layered Earth ◽  
Transient Electromagnetic ◽  
Long Offset

Early‐time and late‐time apparent resistivity approximations have been widely used for interpretation of long‐offset transient electromagnetic (LOTEM) measurements because it is difficult to find a single apparent resistivity over the whole time range. From a physical point of view, Dr. C. H. Stoyer defined an apparent resistivity for the whole time range. However, there are two problems which hinder its use: one is that there is no explicit formula to calculate the apparent resistivity, and the other is that the apparent resistivity has no single solution. A careful study of the two problems shows that a numerical method can be used to calculate a single apparent resistivity. A formula for the maximum receiver voltage over a uniform earth, when compared with the receiver voltage for a layered earth, leads to the conclusion that, in some cases, a layered earth can produce a larger voltage than any uniform earth can produce. Therefore, our apparent resistivity definition cannot be applied to those cases. In some other cases, the two possible solutions from our definition do not merge, so that neither of them is meaningful for the whole time range.

Joint inversion of surface and three‐component borehole magnetic data

Geophysics ◽  
2000 ◽  
Vol 65 (2) ◽  
pp. 540-552 ◽  
Author(s):  
Yaoguo Li ◽  
Douglas W. Oldenburg
Keyword(s):  
Field Data ◽  
Joint Inversion ◽  
Magnetic Data ◽  
Data Sets ◽  
Source Depth ◽  
Inversion Algorithm ◽  
Borehole Data ◽  
Weighting Functions ◽  
Surface Data ◽  
Geometric Decay

The inversion of magnetic data is inherently nonunique with respect to the distance between the source and observation locations. This manifests itself as an ambiguity in the source depth when surface data are inverted and as an ambiguity in the distance between the source and boreholes if borehole data are inverted. Joint inversion of surface and borehole data can help to reduce this nonuniqueness. To achieve this, we develop an algorithm for inverting data sets that have arbitrary observation locations in boreholes and above the surface. The algorithm depends upon weighting functions that counteract the geometric decay of magnetic kernels with distance from the observer. We apply these weighting functions to the inversion of three‐component magnetic data collected in boreholes and then to the joint inversion of surface and borehole data. Both synthetic and field data sets are used to illustrate the new inversion algorithm. When borehole data are inverted directly, three‐component data are far more useful in constructing good susceptibility models than are single‐component data. However, either can be used effectively in a joint inversion with surface data to produce models that are superior to those obtained by inversion of surface data alone.

Analytic formulas for complete and incomplete first-order TEM moments below ground in a conductive half-space

Geophysics ◽  
2020 ◽  
Vol 86 (1) ◽  
pp. E1-E11
Author(s):  
Peter K. Fullagar ◽  
Ralf Schaa
Keyword(s):  
Half Space ◽  
Host Response ◽  
Early Time ◽  
Host Responses ◽  
Inversion Algorithm ◽  
Time Data ◽  
First Order ◽  
Transient Electromagnetic ◽  
Rectangular Mesh ◽  
Horizontal Electric Dipole

In the resistive limit, discrete conductors give rise to magnetic dipole fields. Magnetostatic modeling of time integrals, or moments, of transient electromagnetic (TEM) data therefore offers a means for fast approximate 3D modeling and inversion of TEM data sets. In our approximate inversion scheme, the net TEM moment response is represented as the combination of discrete conductor and uniform host responses. The inversion algorithm first estimates a homogeneous host conductivity, and then it subtracts the host response and fits the residual moment data by adjusting the conductivities of cells comprising a 3D rectangular mesh. To expedite calculation of the host response, we have derived analytic formulas for first-order TEM moments produced on and under the ground by a horizontal electric dipole on the surface of a homogeneous conductive half-space. We present analytic expressions for idealized all-time “complete” moments, or resistive limits, as well as for realizable finite-time “incomplete” moments. The moments produced by an arbitrary horizontal polygonal loop are determined by combining contributions from appropriately oriented electric dipoles. Downhole TEM moments computed with the new expressions reveal substantial differences between incomplete and complete moments when early time data are excluded and between step and impulse response incomplete moments. The role of the formulas in the first stage of our moment-based 3D inversion scheme is illustrated via analysis of downhole TEM data recorded at Santander, Peru. The host conductivity of best fit for early time B-field moments is 2.40 mS/m, consistent with apparent conductivities derived from ground TEM data recorded in the same area.

scholarly journals Numerical modelling analysis of multi-source semi-airborne TEM systems using a TFEM

2020 ◽  
Vol 17 (3) ◽  
pp. 399-410
Author(s):  
He Li ◽  
Zhipeng Qi ◽  
Xiu Li ◽  
Yingying Zhang
Keyword(s):  
Magnetic Field ◽  
Three Dimensional ◽  
Homogeneous Medium ◽  
Specific Area ◽  
Multiple Sources ◽  
Electromagnetic System ◽  
Transient Electromagnetic ◽  
Vector Finite Element ◽  
Data Collections ◽  
The Magnetic Field

Abstract Traditional transient electromagnetic methods use single sources for excitation and extract the characteristics of underground media by improving interpretation technology. This study focused the improvements of transient electromagnetic interpretation using complex source technology. A time-domain vector finite element method (TFEM) was applied on three-dimensional forward modelling of semi-airborne transient electromagnetic (TEM) with multiple electrical sources, and it analysed the characteristics of fields with multiple sources. The study used a model of an isolated anomalous body in a homogeneous medium as an example. The effects of different combinations of excitation sources on the distributions of the magnetic field characteristics were analysed. Numerical results showed that the magnetic field components in a specific area could be strengthened by changing the layout of the sources, which was significant for future field data collections. By comparing the transient electromagnetic fields of the vertical array dipole sources with that of the loop source, the anomaly transient electromagnetic field of multi-source was more obvious than the field with a single source. Taking a complex orebody model as an example, a cross-electric source was used to calculate the magnetic field components of the semi-airborne TEM method. The resistivity distribution characteristics of the underground medium were obtained using an apparent resistivity interpretation method of the vertical magnetic field, which fully demonstrated that a multi-source transient electromagnetic system had the ability to determine abundant resistivity information of a complex medium.

Effect of conductive host rock on borehole transient electromagnetic responses

Geophysics ◽  
1989 ◽  
Vol 54 (5) ◽  
pp. 598-608 ◽  
Author(s):  
Gregory A. Newman ◽  
Walter L. Anderson ◽  
Gerald W. Hohmann
Keyword(s):  
Magnetic Field ◽  
Half Space ◽  
Time Derivative ◽  
The Body ◽  
Galvanic Current ◽  
Large Loop ◽  
Transient Electromagnetic ◽  
Electromagnetic Responses ◽  
Pass Through ◽  
The Magnetic Field

Transient electromagnetic (TEM) borehole responses of 3-D vertical and horizontal tabular bodies in a half‐space are calculated to assess the effect of a conductive host. The transmitter is a large loop at the surface of the earth, and the receiver measures the time derivative of the vertical magnetic field. When the host is conductive (100 Ω ⋅ m), the borehole response is due mainly to current channeled through the body. The observed magnetic‐field response can be visualized as due to galvanic currents that pass through the conductor and return in the half‐space. When the host resistivity is increased, the magnetic field of the conductor is influenced more by vortex currents that flow in closed loops inside the conductor. For a moderately resistive host (1000 Ω ⋅ m), the magnetic field of the body is caused by both vortex and galvanic currents. The galvanic response is observed at early times, followed by the vortex response at later times if the body is well coupled to the transmitter. If the host is very resistive, the galvanic response vanishes; and the response of the conductor is caused only by vortex currents. The shapes of the borehole profiles change considerably with changes in the host resistivity because vortex and galvanic current distributions are very different. When only the vortex response is observed, it is easy to distinguish vertical and horizontal conductors. However, in a conductive host where the galvanic response is dominant, it is difficult to interpret the geometry of the body; only the approximate location of the body can be determined easily. For a horizontal conductor and a single transmitting loop, only the galvanic response enables one to determine whether the conductor is between the transmitter and the borehole or beyond the borehole. A field example shows behavior similar to that of our theoretical results.

A model study of a thin plate in free space for the EM37 transient electromagnetic system

Geophysics ◽  
1985 ◽  
Vol 50 (6) ◽  
pp. 1002-1019 ◽  
Author(s):  
Peggie R. Gallagher ◽  
Stanley H. Ward ◽  
G. W. Hohmann
Keyword(s):  
Thin Plate ◽  
Free Space ◽  
Time Derivative ◽  
Late Time ◽  
Loop Analysis ◽  
Response Curves ◽  
Electromagnetic System ◽  
Transient Electromagnetic ◽  
Small Depth ◽  
Depth Extent

The computer program PLATE, developed at the University of Toronto, models the electromagnetic (EM) response of an inductively thin plate in free space. We used PLATE to compute two components of the time derivative of the magnetic field for a range of models for the EM37 fixed‐source transient system ([Formula: see text] loop). Analysis of the response curves produced methods of interpretation for obtaining plate geometry and conductance. The overall width of an anomaly, the distance between peaks and the width of the updip lobes, can provide an estimate of depth. Dip has the dominant effect on the ratio of the peak amplitudes. A rough estimate of plate size and the position in time (early or late) of the currents is essential before proceeding with interpretation. Strike length is not obviously reflected in the shape of the curves, but depth extent is indicated by the rate at which the downdip tail returns to the baseline, except for vertical plates. For vertical plates, curve matching may be the only method of obtaining an estimate of depth extent. Varying conductance for a particular model in free space affects whether a channel represents an early, intermediate, or late time response. The shape of a profile varies with the time of measurement. The estimated time constant can be used to calculate the conductance, provided an estimate of the shortest dimension of the plate is available. Extinction angles appear frequently for plates of small depth extent but do not occur for plates which are of infinite strike and depth extent with respect to the size of the transmitting loop.

Identifying Deep Saturated Coal Bed Zones in China through the Use of Large Loop TEM

2018 ◽  
Vol 23 (1) ◽  
pp. 135-142
Author(s):  
Guo-qiang Xue ◽  
Wei-ying Chen ◽  
Zhen-jun Ma ◽  
Dong-yang Hou
Keyword(s):  
Geophysical Methods ◽  
Cost Effective ◽  
Shanxi Province ◽  
Coal Bed ◽  
Detection Accuracy ◽  
Coal Beds ◽  
Large Loop ◽  
Transient Electromagnetic ◽  
Coal Deposits ◽  
The Time Domain

Most of the shallow mineral deposits of China have already been mined because of the country's fast-growing social and economic development. Consequently, the exploration and mining of deep-seated coal deposits have become alternative ways to satisfy the energy needs for both domestic and industrial use. To ensure safe mining practices and avoid the intrusion of water from the bases of coal deposits, it has become essential to investigate the distribution of deep-seated coal deposits using appropriate and cost-effective geophysical methods. For example, a coal mine located in the southern part of China's Shanxi Province is characterized by a roof of dry rocks, whereas the coal beds in this area are saturated. A new technique which uses a large loop transient electromagnetic method (hereafter referred to as a modified central loop TEM) to detect geological targets located at deeper levels in the subsurface was successfully developed and applied in this study. The detection abilities of this technique, such as the time-domain responses and depth of investigation, as well as its sensitivity to deep targets, were analyzed. This new method was successfully employed to detect the floor of a 900 m deep coal seam at the location of the saturated coal beds in Shanxi Province. Wells were drilled to confirm the results, showing that the proposed method is both applicable and convenient for future exploration purposes. TEM could potentially be used to detect geological materials located at greater depths with higher detection accuracy. [Figure: see text]

Sign in / Sign up

Export Citation Format

Share Document