Audiomagnetotelluric Soundings Research Articles

Groundwater electromagnetic imaging in complex geological and topographical regions: A case study of a tectonic boundary in the French Alps

Very‐low‐frequency (VLF), audiomagnetotelluric (AMT), and water geochemistry surveys were performed on the Sur‐Frêtes Ridge in the French Alps to evaluate the groundwater circulation system within the ridge. At this site, temporal variations of the electric field have been observed in association with water‐level variations of neighboring artificial lakes. The Sur‐Frêtes Ridge is 1 km wide and trends east–west. Water samples were collected at 52 points distributed throughout the area. VLF soundings were carried out along three parallel east–west profiles, and 7 AMT soundings were carried out along an east–west profile on the top of the ridge. This site is characterized by a rugged topography of the ridge where geological and topographic trends are almost perpendicular, making the structure fully three dimensional. We constructed a 2‐D resistivity model of the ridge from 2‐D and 3‐D analyses of the VLF and AMT data, associating geology and topography models. When combined with the water chemistry data, a model is proposed for groundwater percolation below the ridge across the geological contacts. This study demonstrates that electromagnetic imaging in a highly heterogeneous context can be combined with water chemistry to map groundwater circulation at the kilometer scale. The approach is relevant for hydrogeological and environmental applications.

Hydrogeological sections of the Pergamino basin (Buenos Aires province, Argentina): audio magnetotelluric and geochemical results

Aquifers in the northeastern zone of the Buenos Aires Province, mainly used for agricultural purposes, were studied regionally by several authors, but local studies are necessary because great variations may occur in different basins, and well data are limited. In this work, geophysical exploration by means of Audio-magnetotelluric soundings and geochemical analyses of some well data were carried out at the Pergamino rivulet basin. Depth and thicknesses of the upper hydrogeological section, the Epiparaniana, including the Pampeano and Puelches aquifers, were determined. Also, the hydro chemical evolution along the direction of regional groundwater flow was studied. The depth of the very conductive layer corresponding to the salty Puelches and the "Green Miocene", limiting the agricultural water usage, came out to be very variable depending on the zone. Geochemical analyses showed that the Pampeano aquifer has bicarbonate sodium water, with an increase of electrical conductivity, sodium and bicarbonates contents at a local recharge zone with impermeable and bad drainage soils and a topographical small slope.

New insights into the hydrogeology of a basaltic shield volcano from a comparison between self-potential and electromagnetic data: Piton de la Fournaise, Indian ocean

In order to investigate aquifers, several geophysical surveys have been carried out in the Baril area of the southern flank of Piton de la Fournaise volcano on Reunion in the Indian Ocean using audiomagnetotelluric (AMT), very-low-frequency (VLF) and self-potential (SP) methods. We present the results with emphasis on a comparison between SP data and the findings of geoelectric surveys. AMT soundings have indicated, from the surface downward, three layers: (i) resistive volcanic rocks, (ii) an intermediate resistivity layer, and (iii) a conductive basement attributed to a seawater-bearing aquifer. VLF measurements allow the mapping of the first layer apparent resistivity, and therefore its bottom, when the true resistivity is supposed to be isotropic and homogenous. When this assumption does not hold, only the SP method permits the mapping of this bottom. Because of the good agreement between the SP and electromagnetic results, we propose the SP method as the first tool that should be used in studying shallow hydrogeological structures in volcanic areas.

A resistivity cross-section of Usu volcano, Hokkaido, Japan, by audiomagnetotelluric soundings

We collected audio-magnetotelluric (AMT) data across Usu volcano, Hokkaido, Japan, which erupted in 1977 and is still active. We had a profile of 17 sites perpendicular to the regional tectonic strike, which crossed the 1977 cryptodome, Usu-Shinzan. Tensor-decomposed data were interpreted by a two-dimensional inversion. Outside the crater rim, the resistivity structure is simple. The resistive somma lava is underlain by a conductive substratum, implying altered Tertiary or Quaternary rocks. In the crater, there are two resistive bodies bisected by a vertical conductor, which corresponds to Usu-Shinzan fault, located at the foot of the uplift. The vertical conductor was not detected in the AMT sounding in 1985. One of the possible causes of the development of the vertical conductor is a cold water supply from the surface into the vapor dominant fracture zone. One of the resistive bodies is located beneath Usu-Shinzan and implies an intrusive magma body which caused the 1977 uplift.

Electromagnetic Investigations of Aquifers in the Grand Brulé Coastal Area of Piton de la Fournaise Volcano, Reunion Island

AbstractA geophysical survey was carried out on the eastern flank of Piton de la Fournaise volcano in the Grand Brulé area using combined audiomagnetotelluric (AMT) and time domain electromagnetic (TDEM) techniques. The aims of the present study were firstly to define the shallow geoelectrical structure, and subsequently to locate large bodies of ground water with anomalous electrical properties. Most of the AMT sites are isotropic across the entire observational bandwidth (7500‐10 Hz), except for a few sites where static‐shift effects are present. Joint one‐dimensional interpretation of the TDEM and AMT data indicates basically three‐layered structures with the following resistivity zones, progressing from the surface downward: (1) a resistive unsaturated surface layer of subaerial basaltic flows with a 2000 to 10,000 ohin‐m resistivity, its base being approximately at mean sea level, (2) an intermediate‐resistivity layer (100‐250 ohm‐m) 50–300 m thick, interpreted as being fresh‐water‐saturated basaltic formations, and (3) a low‐resistivity layer of less than 15 ohm‐m probably due to more saline water at depth and/or higher clay content in crushed materials. In the center of the study area, the results have also shown the presence of a prominent boundary dividing the area into a southern zone where the aquifer sequence is thin (60‐100 m) and a northern zone corresponding to a relatively thick aquifer (>150 m). Geoelectrical models are compared with the available geological information given by two geothermiic exploration drillings. The coastal drill hole gives a crucial information, i.e. the thick intermediate‐resistivity layer in the northern part of Grand Brule can be related to water‐saturated alluvium deposits down to 200 m below sea level. AMT and TDEM soundings have revealed a major paleoalluvial drainage system buried under recent lava flows. A delimitation of sectors with different hydrogeological characteristics can be proposed.

High-frequency magnetotelluric investigation of crustal structure in north-central Abitibi, Quebec, Canada

SUMMARY Thirty-one audiomagnetotelluric (AMT) soundings have been collected along a N-S profile across the northern Abitibi greenstone belt. The recording frequency ranges of 10000-1 Hz for 25 stations and 10000-0.001 Hz for the other six stations were appropriate for defining the near-surface geoelectric structures and for imaging major faults and shear zones down to approximately 10 km along the profile. Six time-domain electromagnetic (TEM) soundings were also collected near the southern end of the profile, to provide a correction for static shift. Extensive static distortion analysis reveals that most of the AMT data are not distorted by localized near-surface structures; however, they are affected by static shift. In order to remove this static shift, a spatial filter was designed and applied to the data, effectively reducing static shift as verified by TEM soundings at the same sites. Extensive 2-D inversions of the filtered data were carried out. The results suggest the presence of a thin overburden with both conductive and resistive blocks overlying a very resistive basement. Among the four geological deformation zones crossed by the profile, only two zones are imaged as subvertical conductive crustal structures extending to the limit of resolution of AMT. These correspond to major breaks (Porcupine-Destor Fault and Casa Berardi Tectonic Zone) as evidenced by the seismic reflection data collected over the same area.

Description of the shallow layers at Antinaco-Los Colorados Valley, NW Argentina

We performed a geophysical study in Antinaco-Los Colorados Valley to determine the thickness of the sedimentary cover and the sequence of the more superficial layers. Eight magnetotelturic and four audiomagnetotelluric soundings were performed along an east-west profile. Analysis of the results indicates that a deep basin and a very conductive layer can be detected at a depth of around 400 m, coincident with the depth of drillings. This layer may be associated with the presence of aquifers. These studies also indicate that the basement is shallower to the east (1 km) and deeper to the west (3 km).

Electromagnetic studies in the Fennoscandian Shield—electrical conductivity of Precambrian crust

Electromagnetic (EM) investigations of the 1980s in the Fennoscandian (Baltic) Shield produced an unique and unified EM data set. Studies include regional investigations by the magnetovariational (MV) method with large lateral sampling distance, investigations of anomalous conductivity structures by magnetotelluric (MT) soundings and other (EM) and electrical methods (audio MT soundings, d.c. dipole-dipole and VLF resistivity profilings) with shorter sampling distance, and studies of the near-surface conductivity by airborne EM surveys. The variety of methods provide an ability to map efficiently crustal conductivity structures from a regional scale of hundreds of kilometres down to local details of some metres in the anomalous structures. The Precambrian of the Fennoscandian Shield is characterized by roughly NW-SE-directed elongated belts of conductors which separate more resistive crustal blocks. The latter serve as transparent windows through which to probe deep electrical structure and belts of conductors as tectonic markers of ancient orogenic zones including (1) the Kittilä-Vetrenny Poyas conductor, (2) the Lapland Granulite Belt and Inari-Pechenga-Imandra-Varzuga conductors, (3) the Archaean-Proterozoic boundary conductor and (4) the Southern Finland Conductor. The conductive belts—orogenic conductors—indicate places where crustal masses collided and were finally sealed together. Enhanced conductivity in the orogenic conductors is caused primarily by an electronic conducting mechanism in graphite- and sulphide-bearing metasedimentary rocks. Estimations of the lower-crustal conductivity indicate a laterally heterogeneous lower crust in the Fennoscandian Shield. Archaean lower crust seems to be in general more resistive than the Early Proterozoic lower crust of the Karelian and Svecofennian Domains. The lower crust in the southwestern part of the Svecofennian Domain and in the Sveconorwegian Domain seems to be more resistive than in the central part of the Svecofennian Domain.

Audiomagnetotelluric soundings on the basement-sedimentary transition zone around the eastern margin of the Douala Basin in Cameroon

Audiomagnetotelluric (AMT) soundings were made on two profiles traversing a metamorphic basement-sedimentary transition zone on the eastern margin of the Douala basin in southern Cameroon. The sounding curves and pseudosections of both profiles suggest the existence of two blocks of different lithologies. Also the interpretation of the AMT data reveals a distinct electrical conductivity discontinuity on both profiles for each of the two orientations of the telluric line. This discontinuity marks the metamorphic basement-sedimentary contact. On one profile, some stations correlate well with each other, but not with one of them which presents singular characteristics. There is evidence of a conducting layer in the entire sedimentary section (5 Ohm.m resistivity) of thickness about 100 metres and the variability of its depth can help us to evaluate the dip of the sedimentary deposits. A close examination of the transition zone suggests vertical tectonic movements of several hundred metres. Using the base of the supposed basal sandstone layer as a reference horizon, we have estimated: the dip of the sedimentary syncline to be 5%; the top of sandstone layer to have a dip of 3%; the thickness of the basal sandstone to vary between 500 to 850 m and calculated the total thickness of the sedimentary deposits to be about 1100 m. The high electrical conductivity in the sedimentary section demands further soundings in the lower frequency range (10 −2 to 10 Hz) required in deep MT exploration work.

Etude structurale par la méthode audiomagnétotellurique des complexes du Protérozoïque inférieur au Sénégal oriental

Audiomagnetotelluric (AMT) soundings have been made at 14 sites along a 7 km profile in the Tonkouto-Léoba area across Early Proterozoic formations of the Kedougou Inlier in Eastern Senegal. The purpose of the study was to gain information about the electrical properties of the Mako Supergroup. The results, shown in the form of apparent resistivity pseudosections, indicate marked anisotropies for orthogonal components recorded at all sites. Consequently, two-dimensional (2-D) modelling is required to establish the electrical characteristics of the major geological units. The modelling results indicate resistivities in the range 20 000– 100 000 Ωm for the Early Proterozoic units which extend to depths of 18 km. A major discontinuity characterized by a resistivity of 1000 Ωm may arise from a structural change associated with the southeastern boundary of the Kakadian batholith. Beneath the highly resistive formations, a sharp fall of resistivity has been detected, which may be due in part to the presence of trapped fluids.

Audio-magnetotelluric investigation of allochthonous iron formations in the Archaean Reguibat shield (Mauritania): structural and mining implications

The M'Haoudat range, considered as an allochthonous unit amid the strongly metamorphosed Archaean basement (Tiris Group), belongs to the Lower Proterozoic Ijil Group, weakly metamorphosed, constituted mainly by iron quartzites including red jaspers and high grade iron ore. Audio-magnetotelluric (AMT) soundings (frequency range 1–7500 HZ) were performed together with the systematic survey of the range (SNIM mining company). The non-linear least squares method was used to perform a smoothness-constrained data model. The obvious AMT resistivity contrasts between the M'Haoudat Unit (150–3500 ohm. m) and the Archaean basement (20 000 ohm. m) allow to state precisely that the two thrust surfaces, on both sides of the range, join together at a depth which increases from North-West to South-East, as the ore bodies. Inside the steeply dipping M'Haoudat Unit, the main beds of iron quartzites (1500–3500 ohm. m), schists (1000–1500 ohm. m) and hematite ores (150–300 ohm. m) were distinguished when their thickness exceeded 30 to 50 m. The existence of an hydrostatic level (1–50 ohm. m) and the steeply dipping architecture, very likely responsible for the lack of resistivity contrast on the upper part of some profiles, complicate the interpretation at high frequencies the thin layers being poorly defined.

Audiomagnetotelluric soundings on White Island volcano

Abstract Audiomagnetotelluric soundings in the frequency range 100-1 Hz have been made at three locations within the crater region of White Island volcano, New Zealand. Exceptionally low apparent resistivities are prevalent. Despite the possibility of terrain effects and static shift, and the three-dimensional nature of the island, the measured values appear to be indicative of the true electrical structure. One-dimensional modelling of the E polarisation responses has been used to obtain approximations to the electrical structure beneath each site. The results suggest a variable depth to the acid hydrothermal system which is believed to underly the crater region, with low resistivities coming closest to the surface near the active fumaroles of Donald Mound.

Audiomagnetotelluric study in northeast region of Paraná Basin, South America

We conducted natural‐source audiomagnetotelluric (AMT) soundings at 31 stations along a 215 km profile in the Paraná Basin (South America) across the previously postulated Araxá‐Rio Grande alignment. Our objective was to study the resistive structures of the upper layers of the sedimentary sequence; our results made it possible to map the surficial layer of sediments of the Bauru group, a layer that reaches thicknesses of up to 250 m. We concluded that AMT soundings have reasonably good potential for groundwater exploration in the region. The varying thicknesses of Bauru group sediments and the irregularities of their contact surface with resistive volcanic rocks of the Serra Geral formation along the profile were not known previously. Our AMT profile crossed an important positive gravity anomaly located near the Rio Grande (a natural boundary between the states of São Paulo and Minas Gerais, in Brazil) which has not been fully interpreted. Five sites for deeper MT soundings, which may throw light on possible causes of the gravity anomaly, were selected based on the conclusions from the AMT survey.

The geoelectric model of the POLAR Profile, Northern Finland

Electromagnetic soundings have been made in order to construct a geoelectrical (conductivity) model of the crust along the European Geotraverse (EGT) POLAR Profile. Forty magnetotelluric (MT) soundings, eighteen audiomagnetotelluric (AMT) soundings and ten magnetohydrodynamic (MHD) soundings were made on the main POLAR Profile (POLAR I) and ten more MT soundings on a parallel profile (POLAR II), 40 km to the southeast of the main profile.Analysis of simultaneous recordings by the EISCAT magnetometer chain, and thin-sheet modelling of the effect of the Barents Sea, indicate that neither the source field effects nor the presence of the ocean are significant at periods below 200 s in the measurement area.The magnetotelluric data have been modelled with two-dimensional models representing the regional structure along the profiles. In addition to the regional structure, a thin inhomogeneous surface layer is included in the models in order to explain some local features of the measured response functions. Although details of the surface electrical structures are poorly resolved, the gross features of the geoelectrical cross section are considered to be reliable. The results divide the POLAR Profile into three different blocks. The better conducting Karasjok-Kittilä Greenstone Belt in the south has an average resistivity of less than 10 Ωm. The more resistant Lapland Granulite Belt, with a resistivity between 100 and 200 Ωm, is underlain by conductive (< 5 Ωm), N-dipping layers. The depth for the uppermost conductive layer varies from a few kilometres in the southwestern part of the granulite belt to 13 km in the northeastern part, from where it rises steeply towards the surface close to the boundary between the Lapland Granulite Belt and the Inari Terrain. These features appear to be continuous between the two parallel MT profiles. Within the Inari Terrain a conductive zone at an approximate depth of 10 km and with a resistivity of about 20 Ωm was identified in a resistive upper crust.The geoelectric cross section agrees, in gross detail, with the corresponding gravity, refraction seismic and reflection seismic cross sections of the POLAR Profile. All methods indicated a similar shape for the southwestern part of the Lapland Granulite Belt i.e., granulites have a gently NE-dipping boundary against the underlying Karelian Province. In the northeastern part of the granulite belt the geoelectric model and the gravimetric model show a rather steep S-dipping boundary against the Inari Terrain northeast of the granulite belt.

Fracturing associated with the 1977–1978 eruption of Usu volcano, North Japan, as revealed by geophysical measurements

None of the ground temperature measurements inside the somma caldera of Usu volcano in southwestern Hokkaido, Japan, show patterns that correlate with the development of surface faults and the hypocenter distribution of earthquake swarms observed during the 1977–1978 eruptions. Audiomagnetotelluric (A.M.T.) soundings on the surface of the somma caldera display well-defined high-resistivity zones (H.R.Z.) within the low-resistivity edifice. H.R.Z. are explained as being due to rising dry steam and low-resistivity zones are attributed to ionized water in the volcanic edifice. Inversion of data shows that elongated H.R.Z. have a shallow roof and are some hundred meters wide. Steam rising along narrow vertical fractures generates widely distributed vapor-dominated geothermal systems, because rising steam condenses near the surface. The condensed steam creates a water-filled zone which inhibits the free escape of succeeding steam. Distribution of elongated H.R.Z. correlates well with the high ground-temperature zones. A self-potential survey reveals a positive anomaly developed over the Usu edifice, about 350 mV in magnitude. Electrokinetic coupling effect due to intensive hydrothermal convection is a probable interpretation of the anomaly because the high-potential zone is closely related to a high ground-temperature zone. Hydraulic fracturing of the upper crust at the time of eruption allowed release of elastic strain energy due to magma pressure at depths with the spectacular consequence that one side of the major fracture was uplifted about 180 m. The geometry of such a process may be drawn from seismological results; the roof of the edifice is pushed up by expanding magma accompanying intensive seismic activities. We suggest that earthquake-free zones are proof of magma-filled fractures and that eruption took place when one of these fractures opened to the surface.

Minimizing Interpretation Ambiguities Through Joint Inversion of Surface Electrical Data

AbstractJoint inversion is an extension of standard least‐squares inversion techniques. With this approach, it is possible to construct a model that is simultaneously consistent with surface observations made using more than one geophysical method. Specifically, discussion focuses on applications to the Schlumberger and audiomagnetotelluric sounding methods, although the technique is not limited to these.The authors have found that the non‐uniqueness problems that often plague the interpretation of electrical sounding data can be substantially reduced if the model is required to be consistent with more than one set of surface measurements. If geological ground truth is available to further constrain the model, the chances of arriving at the true physical property distribution of the subsurface are far better than if a single data set is interpreted.To demonstrate the power of joint inversion, the authors present a study using synthetic data obtained from a three‐layer resistivity model. Finally, two case histories are presented to demonstrate that a much better representation of the true earth can be obtained with joint inversion than with standard interpretation methods.

Regional audiomagnetotelluric study of the Questa Caldera, New Mexico

Audiomagnetotelluric (AMT) soundings at Questa caldera were made in the course of a mineral resource study of the Columbine‐Hondo and Latir Peak Wilderness areas, New Mexico, in the fall of 1980 and 1981. This regional electrical survey covered an area of approximately 518 km2 and consisted of 92 AMT stations at an average spacing of approximately 3 km. An AMT apparent resistivity map and one‐dimensional models delineate several major lithologic and structural features. A large north‐south resistivity high is interpreted to outline a largely concealed batholith 20 km long, 2–6 km wide, and &gt;4 km thick. Two high‐resistivity east‐west trends coincide with the mapped margins of the Questa caldera, indicating the electrically contrasting rocks at its boundary. Apparent resistivity lows coincide with areas of hydrothermal alteration known to contain stockwork molybdenum deposits.

Geophysical expression of the batholith beneath Questa Caldera, New Mexico

Gravity gradients delineate uncharacteristically straight, north‐south trending graben faults in the Rio Grande rift west of Questa caldera, and gravity and audiomagnetotelluric (AMT) data show north‐south trending low‐density and high‐resistivity zones from the caldera southward. A gravity inversion technique was used to isolate the gravity anomaly of the caldera and related intrusive rocks from the complicated Bouguer gravity field. The residual gravity anomaly together with AMT data reported in a companion paper by C. L. Long seem to delineate the subcaldera batholith. Geophysical models indicate this to be a north‐south trending dikelike or beamlike body about 25 km long, 5 km wide, and &gt;4 km thick, thickness estimate being limited by the penetration depth of the AMT soundings. The north‐south trend is not consistent with the regional pattern of early Miocene southwest directed extension and instead may reflect passive control by preexisting north‐south strike‐slip faults.

Intermediate tectonic pattern and hydrodynamic process deduced from audiomagnetotelluric investigations on the volcanic island of Mayotte (Comores Archipelago)

Audiomagnetotelluric soundings were carried out on the volcanic island of Mayotte (Comores Archipelago). The field method and data are described and discussed. All the sounding cross sections show coherent curves with a high conductive layer between 100 and 200 m. depth. This layer is considered to be a fluctuating water table zone correlated to yearly climatic changes under tropical weathering processes. Dipping and sudden level variations are linked to tectonic volcanic features and are the visualization of a block faulting system of the primitive basaltic shield, a consequence of a distensive phase possibly connected with the collapse of the western flank of the volcano. Piano-key panels are the typical sub-surface style and the guide-line for any water prospection on the volcano and the activity of the island water transfer system.

An automatic audio-magnetotelluric equipment, controlled by microprocessor, for the telesurvellance of the volcano Momotombo (Nicaragua)

A campaign of audio-magnetotelluric soundings in the crater of Momotombo has shown that the structure of this volcano is suitable to a surveillance by this method; it has an only central activity, and the resistivity versus the depth decreases suddenly at least 100 times at about 270 m.