Anomalous Low Resistivity Research Articles

A correction factor for the resistivity log in gas reservoirs with low resistivity pay zone in the Poti Formation, Parnaiba Basin, Brazil

The Parnaiba Basin, even being a new frontier basin, it’s already one of the most profitable for natural gas in Brazil. The shallow marine sandstones of the Poti Formation are the main gas reservoirs at Park of the Hawks. These reservoirs hold anomalously low resistivities which directly impacts water saturation calculations and leading to underestimations of their true potential as gas reservoirs. We propose a method for the characterization and mitigation of the problem and for the correction of the resistivity log based on the case study of well PGN-5, seeking to obtain more adjusted results for the reservoir evaluation. The methodology used is based on the integration data, where the main inputs were drill cuttings samples, well logs, gamma-spectral data and geochemical analyzes through X-rays fluorescence. We observed that the presence of pyrite and clay minerals are the causes of the low resistivities, still in the presence of gas-charged intervals, attested by crossover between density and neutron logs and through pressure gradients data. We propose a correction factor for the resistivity log in the anomalous low resistivity zone, allowing a new evaluation of the well, leading to a significant reduction in water saturation and a remarkable net pay gain.

Geoelectrical Investigation of Groundwater at Northern Gaza Strip, Palestine

A vertical electrical sounding survey was carried out in northern Gaza Strip (Gaza City, Jibalya, Bayt Lahya and Bayt Hanun), Palestine to study the groundwater conditions and evaluate its quality and distribution. The survey included twenty-six (26) traverses with variable lengths in the range 200-500 m. The geoelectrical survey revealed that the resistivity of the study area varies in a wide range mostly 9 to 850 Ωm, subdivided into three subsurface zones. The first zone represents the dry sandstone and alluvial that attains the highest resistivities (100-850 Ωm) with variable thicknesses in the range 26-80 m beneath the whole study area. This zone is characterized by the presence of thin layers of clay and silt in some localities that reduced the resistivity to 15-25 Ωm. The second zone represents the fresh water saturated calcareous sandstone (locally named Kurkur) with resistivities in the range 22 -39 Ωm). The thicknesses of this zone is about 55-70 m in the western part of the study area and increased to 110 m in the east. The surface of this zone shows a gentle westward gradient, which indicates a general flow of water towards the Mediterranean Sea. The third zone represents saline water saturation with low resistivities, mostly 9 Ωm -10 Ωm close to the Mediterranean shore, located at 60 m below sea level and 100 m beneath the eastern side of the study area. Beneath the eastern part of the study area, an anomalous low resistivity of 2-3 Ωm is observed within the fresh water zone, which indicates groundwater pollution near the buffer zone caused by agriculture activities and untreated pumped wastewater.

Dissecting a Zombie: Joint Analysis of Density and Resistivity Models Reveals Shallow Structure and Possible Sulfide Deposition at Uturuncu Volcano, Bolivia

The recent identification of unrest at multiple volcanoes that have not erupted in over 10 kyr presents an intriguing scientific problem. How can we distinguish between unrest signaling impending eruption after kyr of repose and non-magmatic unrest at a waning volcanic system? After ca. 250 kyr without a known eruption, in recent decades Uturuncu volcano in Bolivia has exhibited multiple signs of unrest, making the classification of this system as “active”, “dormant”, or “extinct” a complex question. Previous work identified anomalous low resistivity zones at <10 km depth with ambiguous interpretations. We investigate subsurface structure at Uturuncu with new gravity data and analysis, and compare these data with existing geophysical data sets. We collected new gravity data on the edifice in November 2018 with 1.5 km spacing, ±15 μGal precision, and ±5 cm positioning precision, improving the resolution of existing gravity data at Uturuncu. This high quality data set permitted both gradient analysis and full 3-D geophysical inversion, revealing a 5 km diameter, positive density anomaly beneath the summit of Uturuncu (1.5–3.5 km depth) and a 20 km diameter arc-shaped negative density anomaly around the volcano (0.5–7.5 depth). These structures often align with resistivity anomalies previously detected beneath Uturuncu, although the relationship is complex, with the two models highlighting different components of a common structure. Based on a joint analysis of the density and resistivity models, we interpret the positive density anomaly as a zone of sulfide deposition with connected brines, and the negative density arc as a surrounding zone of hydrothermal alteration. Based on this analysis we suggest that the unrest at Uturuncu is unlikely to be pre-eruptive. This study shows the value of joint analysis of multiple types of geophysical data in evaluating volcanic subsurface structure at a waning volcanic center.

Integrated seismic ambient noise, magnetotellurics and gravity data for the 2D interpretation of the Vallès basin structure in the geothermal system of La Garriga-Samalús (NE Spain)

The integration of geophysical methods, together with the previous information of the Vallès basin area, has resulted in the creation of a new conceptual model that explains La Garriga-Samalús geothermal system. The integration of complementary geophysical methods seems to be a good option for the preliminary stages of a geothermal system exploration, especially in urban areas.An integrated seismic ambient noise, magnetotellurics, and gravity methods were used to determine the geological units and structures which control the La Garriga-Samalús geothermal system. The 2D resistivity and density models have allowed the identification of the four main units which regulate the geothermal system: the Miocene basin, the Prelitoral Range unit, the Vallès Faut Zone, and the Paleozoic basement. The interpretation of our models set the Vallès Fault Zone, which is characterized by an anomalous low resistivity and low density, as the main path for the hot fluids. Moreover, the geophysical characterization established a new geometry for the Miocene basin. The Miocene basin presents a stepwise morphology, with the minor thickness towards the fault and an increasing thickness towards the center of the basin. This geometry seems to be related to synthetic normal faults.These results have evidenced that, although, in some geothermal systems, the warm water may create an insufficient physical contrast; the appropriate use of some techniques can still be useful for the exploration of medium and low-temperature geothermal systems.

Geo-electrical modeling of leachate contamination at a major waste disposal site in south-eastern Nigeria

The contamination of aquifers due to the leaching of organic and inorganic waste materials deposited in landfills and open dumpsites leads to serious degradation of the environment. To determine the extent of leaching and to assess the migration of the leachate into the surrounding aquifer, vertical electrical sounding (VES) and electrical resistivity tomography (ERT) surveys were carried out along a heavily contaminated waste disposal site in South-eastern Nigeria. The results obtained from the vertical electrical soundings show that the area comprises three subsurface layers, and that the aquifer lies between the second and third layer. Computation of longitudinal conductance values shows that the overall protective capacity of the aquifer is weak; hence, the capacity for leachate contamination within the aquifer is higher. Models obtained from the electrical resistivity tomography surveys indicate the direction of the leachate plumes and also show the extent of leachate percolation into the aquifer. On the electrical resistivity tomography models, the locations of anomalous low resistivity zones indicate the presence of leachate. The results show that the leachate is moving rapidly towards the aquifer and this will create serious environmental degradation to valuable water resources in the area, thereby affecting health of humans and livestock within this heavily populated zone. Further, the results from the resistivity surveys confirm that the resistivity technique is a powerful tool for delineating the presence of leachate plumes and for indicating to what extent the leachate has infiltrated into the surrounding aquifer.

Geophysical constraints on the Luhoi (Tanzania) geothermal conceptual model

We present the results of the gravimetric, magnetic and magnetotelluric geophysical campaign aimed to constrain the geothermal conceptual model of the Luhoi geothermal field, a poorly known system located in the Tanzanian southeastern coastal sedimentary basin. Regional assessment by existing potential field data reveals that Luhoi lies at the margin of a wide regional NE trending uplifted block that involves the Precambrian metamorphic basement complex. In the geothermal field area, 3D geophysical modelling has imaged a local asymmetric horst of Cretaceous age, 1 km wide and 5 km long, with the same regional NE trend. The horst is formed by the denser Kipatimu Lower Cretaceous sandstones surrounded by the Upper Cretaceous Ruaruke claystones that are down thrown up to 1 km by normal faulting. Far from the horst, lithology mainly controls the modelled electrical resistivity distribution: the Ruaruke claystones show low resistivity (1–6 Ωm) primarily due to its clay content, while the Kipatimu sandstones have generally higher resistivity (10–30 Ωm) because of their prevalent electrolytic conduction. However, anomalous low resistivity values (1–5 Ωm) in the shallower portion of the Kipatimu sandstones forming the horst indicate enhanced electrical conduction due to the cation exchange capacity (CEC) of clay. The CEC estimated values from electrical resistivity are in the range 2−30 meq/100 g, suggesting the presence of kaolinite and illite and excluding the occurrence of smectite, which is the typical clay mineral in the cap-rock of high-temperature geothermal systems. This shallow clay-rich layer together with the Ruaruke claystones act as a regionally continuous clay cap confining the reservoir formed by moderately hot water (close to 100 °C from water geothermometers) in the permeable Kipatimu sandstones layer that reach a maximum depth of ca. 2 km, likely heated by a slightly enhanced geothermal flux attributable to crustal extension. At Luhoi, the local horst structure imaged by the geophysical models pushes up the sandstone reservoir at very shallow depth (about 500 m from the surface), strongly enhancing the local geothermal gradient and originating the surface thermal manifestations that are controlled by the system of normal faults.

Electrical transport mechanism of the amorphous phase in Cr2Ge2Te6 phase change material

A Cr2Ge2Te6 (CrGT) phase change material (PCM) was studied. Different from conventional PCMs, it shows an inverse resistance change between a low-resistance amorphous phase and a high-resistance crystalline phase. Moreover, the anomalous low resistivity in the amorphous CrGT is considered to be due to a large carrier density, but the mechanism of electrical transport is still not clear. In this study, the electrical transport mechanism of the amorphous CrGT was discussed based on the temperature dependence of the resistivity, carrier density, mobility, and current–voltage characteristics. Above 300 K, the conduction mechanism of the amorphous CrGT was thermally activated band conduction, which is different from the conventional Ge–Sb–Te PCMs that show Poole–Frenkel conduction in the amorphous phase. Below 300 K, the amorphous CrGT shows hopping conduction, changing from variable range hopping (Mott VRH) to Efros–Shklovskii variable range hopping (ES-VRH) with decreasing temperature. The crossover from Mott VRH to ES-VRH was observed at around 200 K. Furthermore, the Fermi level was not pinned at the center of bandgap; instead, it was located near the valence band.

Anomalous Low Resistivity at Room and Elevated Temperatures of Manganin Alloy Implanted with High Dose Niobium Ions

The anomalous low resistivity metallic material at room temperature has been discovered. Known high resistive material called manganin in form of 10 μm thick foil was one side implanted with high dose of niobium ions of 2.5 × 10 Nb ions/cm and then with 2.5 × 10 Kr ions/cm. Before and after implantation procedure investigated manganin foils were temperature treated in 130 ◦C during 100 h. Basic resistance measurements were performed just after implantation observing large decrease of its resistivity. In repeated measurements after long time later (about twelve months), at temperature of 22 ◦C, practically no resistivity was measured. Four-terminal method for resistance measurements was used.

Two-dimensional electrical resistivity survey around the periphery of an artificial lake in the precambrian basement complex of northern Nigeria

A two-dimensional electrical resistivity survey was carried out at the periphery of the impounding reservoir of the Ahmadu Bello University farm dam in order to investigate the subsurface seepage conditions and identify possible weak zone that could serve as seepage paths within the subsurface in the vicinity of the dam. Eight profiles were acquired, using Abem Lund imaging system, along each of the northern and southern abutment (flanks) of the reservoir. The RES2DINV interpretation software was used to analyze the data and the resulting tomograms, generally, it shows a resistivity variation from 23 to 926 Ωm. Anomalous low resistivity zones within the bedrock are interpreted as zones of weakness representing preferential flow pathways for water from the impounding reservoir. Key words: Two-dimensional, electrical resistivity, subsurface seepage condition, artificial lake, Nigeria.

Evaluation of two-dimensional resistivity methods in a fractured crystalline-rock terrane

A series of two-dimensional resistivity profiles collected in the Blue Ridge Province of southwest Virginia and results from numerical modeling of synthetic data reveal substantial differences in depth of investigation, resolution, and sensitivity using Wenner, Wenner–Schlumberger, dipole–dipole, and pole–pole data collection techniques. Resistivity profiles were collected using short (2-m electrode spacing, 48-m profile length), intermediate (6-m electrode spacing, 144-m profile length), and long (10-m electrode spacing, 240-m profile length) arrays over shallow unconsolidated soils and regolith overlying crystalline bedrock. Pole–pole data were only collected with the short array. Numerical modeling was used to simulate both vertical and horizontal structures similar to subsurface conditions in the study area. All of the apparent resistivity data were inverted into earth models using a computer program that uses an l 1 norm smoothness constrained inversion technique. Earth models generated from both field data and numerical modeling acquired by the dipole–dipole technique consistently indicated more detail and greater depth of investigation than the other techniques. The dipole–dipole method uniquely imaged thin saturated sands and isolated high resistivity bodies beneath the 48-m-length array, significant horizontal and vertical resistivity variations including a thick transitional resistivity zone in the 144-m-length array, and anomalous low resistivity zones in crystalline bedrock in the 240-m-length array. Earth models created from the Wenner and Wenner–Schlumberger apparent resistivity data had shallower depths of investigation and revealed significantly less geologic detail than the profiles generated from the dipole–dipole surveys. The earth model from the pole–pole data had the greatest depth of investigation but low resolution and limited geologic detail when compared to the dipole–dipole survey.

都市における応用地質学の課題 新型空中電磁法による軟岩地域での地すべり調査

The new type Helicopter-borne Electro-Magnetic (HEM) system, which has been improved the frequencyrange of measurement to meet the needs in civil engineering fields, has been applied to the landslides of Sarukuyoji in Niigata prefecture and Yachi in Akita prefecture.The results of HEM survey show that both landslides correspond to the anomalous low resistivity zones, which shows that HEM method is effective to detect and delineate the landslides accompanied with cohesive soil, collapse soil or weathered zone.

Electrical conduction of bis(1,2-diondioximato)metals(II) and their partially oxidized complexes at high pressures

The resistivity of one-dimensional d 8-metal complexes with various kinds of 1,2-diondioxime(dimethylglyoxime, nioxime, and diphenylglyoxime) ligands has been measured at high pressures. The resistivity of M(dmg) 2 and M(niox) 2 decreased with increasing pressure. The resistivity of M(dmg) 2 is much lower than that of M(niox) 2 at high pressure, though M- M distances and optical energy gaps in both salts are almost the same at atmospheric pressure. An anomalous low resistivity has been observed in Pt(dmg) 2 at about 40 kbar. The electrical resistivity of M(niox) 2 and M(dmg) 2, oxidized by oxidizing agents, decreased with increasing pressure. The resistivity of partially oxidized Pt complexes is significantly higher than that of the parent compounds at high pressure. On the contrary, partially oxidized Ni and Pd complexes are more conductive than unoxidized compounds at high pressures. The electrocal resistivity of unoxidized and partially oxidized Pt complexes increased with increasing pressure at higher pressures. The resistivity minimum at high pressure has been observed only in Pt complexes. The infrared spectra of the d 8-metal complexes and their partially oxidized complexes have been investigated. The frequency of the CN stretching vibration in M(dmg) 2 and M(niox) 2 salts increases in the order Ni > Pd > Pt; on the other hand, the frequency of M(niox) 2 X n increases in the order Ni < Pd < Pt. The frequency of the CN stretching vibration is closely related to the effect of metal-ligand π∗ back bonding in d 8-metal complexes. Some anomalies in the resistance-pressure curve have been found in Ni(dpg) 2, Ni(dpg) 2I, Ni(dpg) 2I 0.5, and Ni(dpg) 2I 0.14. The anomalies are explained by new pressure-induced phase transitions.