Geothermal Research
Among geophysical studies deployed for investigation of a geothermal system, EM technique, particularly magnetotellurics (MT) and controlled-source electromagnetics (CSEM), are known to be highly effective since the subsurface electrical conductivity is known to be a very important parameter characterizing a geothermal setting in a target area.
According to the Archie equation, the electric resistivity of rocks with relatively small amount of clay is directly proportional to the electrical resistivity of the pore fluid. The latter, in its turn, monotonically decreases with temperature (right figure) with the beta coefficient being close to 0.02 (e.g., for NaCl solutions beta = 0.026). Thus, for example, increasing the temperature to 200°С will decrease rock resistivity in about 6 times, which makes the EM geophysical methods especially useful for geothermal exploration.

A magmatic intrusion or any other high temperature zone (e.g. partially melted zone) as well as a recharge area (increased fracturing, high temperature and fluid mineralization) are characterized by reduced resistivity. Often hydrothermal processes result in forming clay caps. Clay mineral alterations resulting from the hydrothermal processes also have a low resistivity signature. This makes geothermal systems ideal targets for EM methods, which have become an industry standard for exploration of geothermal systems in many countries.

The reliably identified low resistivity zones produced by brines and clays that cap a geothermal system represent attractive targets for exploration. In particular, the magnetotelluric method become a favorite tool to detect and image such significantly anomalous conductive zones over a range of depths in a given region of geothermal significance [Muños, 2014; Patro, 2017].

Sketch of an ideal geothermal system (the picture taken from
Kamchatka peninsula, Russia
Kamchatka Peninsula is located in the Far East of Russia. The Pacific Ocean and the Sea of Okhotsk make up the peninsula's eastern and western coastlines, respectively. Kamchatka is famous for its volcanoes. The central valley of Kamchatka is flanked by large volcanic belts containing around 160 volcanoes, 29 of them are active. Thus, Kamchatka is a natural lab for studying volcanoes and associated volcanic phenomena. The power system of the region is isolated, about 20% of electric power in Kamchatka comes from geothermal sources. Kamchatka is the leader of geothermal power generation in Russia and has good opportunities for development, the first one linked to the possibility of the construction of new blocks in the Mutnovskoe geothermal field with the largest (50 MW) geothermal power plant in Russia.

EM studies in vicinity of the Mutnovkskaya geothermal power plant was carried out by Nord-West in several phases from, which took place between 2004 to 2014. The studies included both profile and array AMT and MT survey accompanied by a limited amount of CSEM exploration.

An MT survey at Ozernovsky block was carried out in 2018 and was aimed for estimating the geothermal prospects of the survey area and for proposing new exploration well sites location. The depth of investigation was about 1 km and low resistivity zones were its main targets.

In 2016-2018 an MT survey was also carried out in the vicinity of Avacha volcano. The research was a part of a large interdisciplinary survey aimed at estimation prospects of the Avacha hydrothermal field. The specific objectives of the survey were (1) understanding of the geological structure and hydrothermal systems; (2) studies of the proposed zone of interaction of a deep thermal source and a near surface aquifer; (3) constructing of a resistivity model at the depth up to 10 km; (4) a proposal for location of exploration wells and (5) preliminary assessment of the possibility and feasibility of developing geothermal resources in the survey area.

Location of our EM study areas in Kamchatka, Russia.
1 – Mutnovskaya geopthermal plant, 2 – Avacha volcano,
3 - Ozernovsky block.
Mutnovskaya geothermal field
The integrated interpretation of MT and logging data is summarized as a conceptual model for one of the profiles (right). In the central part of the model in the upper part of the subvertical discharge zone in depth interval from 300 to 1200 m a small highly resistive zone was distinguished. The existence of this zone was confirmed by drilling. The highly resistive zone corresponds to hydrothermally altered silicificated rocks, which acts as a cork that prevents thermal fluids from reaching the surface. The model is supported by the absence of surface manifestations of hydrothermal activity in the area. At the same time the productive intervals in the well located behind the highly resistive zone are at depth from 1600 to 2200 m, which is below “the cork”, while another well located nearby (650 m to southeast) entered productive depth interval at 700 m. Approximately at the same depth a conductive lens was revealed. Its thickness is up to 250 m and width of about 1200 m. This is a natural hot water reservoir according to the integrated interpretation of geological, geophysical and logging data. The uppermost layer is a sequence of terrigenous and volcanic sediments.

Several locations of new exploration wells were proposed and our model was supported by drilling. One borehole is now used by the Mutnovkskaya Geothermal power plant for commercial purposes.

Resistivity image (top panel) and conceptual model (bottom panel). 1 - MT sites location, 2 – productive and empty wells, 3 – Neogenic volcanic and terrigenous sediments; 4 – low resistive zones and hydrothermally altered rocks; 5 – granulite intrusion; 6 – hot fluid channel and possible partially melted deep zone; 7 – proposed fluid paths: blue is downward cold meteoritic fuid path and red is upward hot fluid path.
Ozernovsky block
The observed resistivity structure of the survey consists of a homogeneous uppermost layer corresponding to Quaternary terrigenous and volcanic deposits with resistivity of about 300 – 400 Ohm·m and thickness ranging from 60 to 200 m. The second layer is characterized by low resistivity of about 1 - 30 Ohm·m and thickness ranging from 250 to 330 m and corresponds to fractured and hydrothermally altered Neogene rocks. More resistive (> 500 Ohm·m) and heterogeneous Neogene deposits occurring deeper are associated with the main hydrothermal circulation systems. The fractured zone is evidently identified as conductive zones (< 20 Ohm·m) at depth interval from 400 to 800 m.

A big resistivity contrast between fractured hydrothermally altered rocks and host rocks makes interpretation of MT data efficient. Data analysis showed that elongated low resistive anomaly running from southwest to northwest divides survey area into resistive zones corresponding to intrusions. In the northern part of the survey area an isometric conductive anomaly was imaged. Both anomalies are interconnected by a relative narrow low resistive channel.

The suggested spatial structure of anomalous zones is supported by geological data. Deep subvertical low resistive zone represents a pathway of hot fluids, while two low resistive zones correspond to the above mentioned fractured and hydrothermally altered terrigenous and volcanic Neogene rocks. Several locations of new exploration wells were proposed at the peripheral parts of outlines anomalous zone, as the lowest resistivity usually corresponds to clay cap behind the reservoir.

2D resistivity cross section for typical proifle. 1 – MT sites location, 2 – fractured and geothermally altered Neogen rocks, 3 – resistive intrusions, 4 – consolidated Neogen rocks, 5 – possible heat flow, 6 – geothermal fluid paths.