Broadband frequency electromagnetic soundings with absolute phase reference

Electrical conductivity mapping is a prerequisite tool for hydrogeological or environmental studies. Its interpretation still remains qualitative but advantages can be expected from a quantitative approach. However a full 3D interpretation is too laborious a task in comparison with the limited cost and time which are involved in the majority of such field studies. It is then of value to define the situations where lateral variations are sufficiently smooth for a lD model to describe correctly the underlying features.For slingram conductivity measurements, criteria allowing an approximate 1D inversion are defined: these mainly consist of a limited rate of variation over three times the intercoil spacing.In geological contexts where the weathering has generated a conductive intermediate layer between the underlying sound rock and the soil, this processing can be applied to determine the thickness of the conductive layer from the apparent resistivity map when the other geoelectrical parameters are known. The examples presented illustrate this application. lntroduction Conductivity or resistivity mapping plays a major part in near-surface geophysics. These physical properties exhibit the widest range of variations and are sensitive to the presence of water in a soil or in a rock, to the salinity of this water or to the content in clay particles. These parameters are dominant in environmental or geotechnical studies. As the cost of such surveys is necessarily limited, electrical or electromagnetic methods used for resistivity mapping must involve fast measurements with light and robust instruments and must be qualitatively interpretable without difficulty. The speed of measurement determines both the extent of the surveyed area and the size of the sampling mesh, which are the main influences on interpretation. I

Section: Amplitudes Of the Anomaliesmentioning

confidence: 99%

Interpretation of slingram conductivity mapping in near‐surface geophysics: using a single parameter fitting with 1D model¹

Guérin¹,

Meheni

Rakotondrasoa

et al. 1996

“…Two variations of the method under the codename MAXI-PROBE (Sinha 1979a, also Ghosh, Nyitrai andSzabadvary, paper at EAEG meeting in Cannes 1982) and MELOS or SYSCAL EM (Valla and Gasnier 1984) are also relevant.…”

Section: Introductionmentioning

confidence: 99%

“…The principle of frequency electromagnetic depth sounding techniques over a layered earth is well described in literature (Keller and Frischknecht 1966, Vanyan 1967and Kaufman and Keller 1983. Two variations of the method under the codename MAXI-PROBE (Sinha 1979a, also Ghosh, Nyitrai andSzabadvary, paper at EAEG meeting in Cannes 1982) and MELOS or SYSCAL EM (Valla and Gasnier 1984) are also relevant.…”

Section: Introductionmentioning

confidence: 99%

Frequency Electromagnetic Sounding Using a Vertical Magnetic Dipole*

Mundry

Blohm

1987

A horizontal transmitter loop (vertical magnetic dipole) is used for frequency electromagnetic (FEM) soundings. The frequency ranges from approximately 6 Hz to about 4000 Hz. The vertical and radial magnetic field components are measured for 20 frequencies per decade several hundred meters from the transmitter loop. A very small bandwidth is selected for amplification using a reference signal. An Apple computer is used for data acquisition. A computer program for Marquardt inversion optimizes the parameters for the n‐layer case: the resistivities and thicknesses of individual beds and a correction factor for the primary magnetic field. Interpretation of each component individually yields practically the same parameters. Examples from the field are given with interpretation; comparison with dc resistivity measurements is provided. The ratio of vertical/radial magnetic field components vs. frequency can be transformed simply into apparent resistivity vs. apparent depth. This can be done in the field to obtain an estimation of the depth of the layer boundaries. FEM results are compared with Schlumberger d.c. sounding obtained at the same site.

FIXED LOOP SOURCE EM MODELLING RESULTS USING 2D FINITE ELEMENTS¹

Valla

1992

Self Cite

Among electromagnetic sounding techniques, the Mélos method possesses the specific feature of including an apparent resistivity computation. This acts as a normalizing scheme so that 2D modelling results can be obtained without accounting for a true 3D source. However, in order to get reliable numerical modelling results for a 2D magnetic dipole source, improved algorithms are required in order to apply the standard finite‐element technique: quadratic basis functions must be used in place of linear basis functions, and a more sophisticated method than conventional ones is necessary for properly solving the resulting system of linear equations. Such modelling results have been used to study theoretical responses for the Mélos method in the search for conductive bodies in mineral exploration. Two sets of models are presented and discussed. They show that the typical Mélos response to a conductive target is a bipolar anomaly on the apparent resistivity pseudo‐section, with a conductive pole at low frequency which is centred above the target.