Journal of critical reviews 905 Journal of Critical Reviews ISSN- 2394-5125 Vol 7, Issue 5, 2020 Review Article GEOTHERMAL GRADIENT AND SUBSURFACE TEMPERATURE FOR ESTIMATION OF SOURCES, PATTERNS AND HEAT FLOW DIRECTIONS IN THE HYDROTHERMAL AREA OF MINAHASA INDONESIA Donny R. Wenas a , Djeli A. Tulandi b , Cyrke A. N. Bujung c Universitas Negeri Manado a,bc donny_wenas@unima.ac.id a , djelitulandi02 b @gmail.com , cyrke@unima.ac.id c Received: 13.01.2020 Revised: 12.02.2020 Accepted: 04.03.2020 Abstract Geothermal gradient anomaly studies and subsurface temperature mapping play an important role in the estimation of heat flow sources and patterns, and also make a fundamental contribution in the field of geothermal physics, especially methods for geothermal energy exploration. The purpose of this study is to measure and mapping the temperature distribution in several subsurface layers in the manifestation of geothermal warm ground and steaming ground, and analyze the geothermal subsurface gradient, to determine the heat source zone, and the pattern and direction of heat flow from subsurface to surface in Hydrothermal area of Minahasa Indonesia. The method used is direct measurement in the field. To determine the coordinates of geothermal manifestations and location mapping, using remote sensing techniques. The results showed that the temperature gradient under the manifestation of geothermal energy increased with increasing depth. The pattern of heat flow in the warm ground manifestation is perpendicular from shallow depths to the surface, while the heat flow pattern under the steaming ground manifestation tends to spread towards the North-East. Keywords: Geothermal Gradient; subsurface temperature; heat flow © 2019 by Advance Scientific Research. This is an open-access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) DOI: http://dx.doi.org/10.31838/jcr.07.05.186 INTRODUCTION Geothermal gradients are caused by subsurface heat dissipation, which are not the same everywhere, and gradients vary from place to place due to differences in rocks and regional and local heat sources. In two places where the gradients in steady state are identical, the temperature at the given moderate depth will be different from the amount for which the average surface temperature is different (Lovering T.S. and H. D. Goode). Direct method of estimating deep subsurface temperatures is by extrapolating measured gradients. However, if the depths of interest lie significantly below the depth for which temperature measurements are available, this extrapolation becomes uncertain, and variation in conductivity must be accounted for. When the thermal conductivity has not been measured or cannot be estimated with confidence, the temperature data should be from drill holes sufficiently deep that any changes in thermal conductivity between the bottom of the hole and the target depth will not be significant. Figure 1. The Study Area LITERATURE REVIEW The amount of heat (Q) flowing from the heat source to the surface depends on thermal conductivity (k), geothermal gradient (dT/dx) and the area: Q=k dT dx and if the gradient is constant through the interval x, Q = kA( T 2 −T 1 x 2 −x 1 ) where x is the depth parameter, T is temperature, T2>T1 and A is area. If there is no other heat source between a given heat source and the surface, and no heat sink, the amount of heat transmitted when steady-state temperature conditions exist must be the same regardless of the differences in conductivity of the rocks: k( ∂T ∂x )=Q=k ′ ( ∂T ∂x )′ k k ′ = ( dT dx )′ ( dT dx ) and a geothermal gradient above a heat source will change in passing from one rock to another of different conductivity. The main modes of heat transfer in the crust to the surface are convection and conduction. In mapping regional heat flow, an important goal is to separate out near surface processes, such as groundwater flow and hydrothermal circulation, from the deeper heat flow from the Earth's interior. Knowledge on the spatial variation in geothermal gradient and heat flow is of direct importance for the growing geothermal investigation and harnessing worldwide (Hjartarson, A., 2015). The temperature of rock or soil at and near the surface of the earth results almost entirely from heating by the sun and cooling through radiation, evaporation, and various heat-absorbing processes. At any particular surface location the heat supplied from below the surface is relatively constant; it represents heat from the interior locally supplemented by heat from subsurface oxidation or other local heat sources and is responsible for rock temperatures below the zone where the effect of surface temperatures is perceptible. The temperatures at a given depth in any locality, however, depend not only on the heat flow through the rocks but on the thermal properties of the rock, and on the surface temperature with which the subsurface temperatures are in equilibrium or to which they are adjusting (Lovering, T.S. and H. D. Goode, 1963). Joseph Forrest, J., et. al (2007) measurement Geothermal Gradients and Subsurface Temperatures in the Northern Gulf of Mexico. The result is a map that illustrates below-mudline (BML) depths to the 300-degree (BMLD300)