Marine Heat Flow Research Articles

Predicting Marine In Situ Heat Flow Using a Geospatial Machine Learning Conformal Prediction

AbstractHeat flow measurements over the past decades have drastically increased in number. However, even with vast increases in measurements, large geospatial gaps exist, most notably across Earth's oceans. As marine heat flow data acquisition can be tedious, time consuming, and ultimately expensive, new methods have arisen in an attempt to quantify heat flow across the seafloor using empirical proxies. The goal herein is to obtain a high quality, fully data‐driven, global estimate of marine in situ heat flow with calculated uncertainty. We present a heat flow analysis for Earth's marine environment using a geospatial machine learning algorithm (MLA) utilizing 33,746 heat flow measurements from the International Heat Flow Commission's most recent Global Heat Flow Database. The mean heat flow for the observational data set is 95.83 mWm−2 with a spatially averaged heat loss of 40.7 TW. We use a random forest regression MLA combined with a conformal prediction to predict in situ heat flow and its associated uncertainty for marine environments at a 100 km2 global resolution. Ten‐fold cross‐validation yields an R2 value of 0.4, indicating predictive skill. Geospatial prediction results in a global marine surface weighted in situ heat flow average of 69.67 ± 20.01 mWm−2 with a corresponding heat loss of 29.59 ± 5.78 TW. Additionally, parametric isolation is calculated to highlight areas where additional heat flow measurements would be beneficial to directly improve the overall prediction. The resultant in situ heat flow and parametric isolation maps serve as a tool to further heat flow studies within the community.

The subsurface thermal state of Svalbard and implications for geothermal potential

Svalbard is a High Arctic Archipelago at 74–81°N and 15–35 °E under the sovereignty of Norway. All settlements in Svalbard, including the capital of Longyearbyen (population 2400), currently have isolated energy systems with coal or diesel as the main energy source. Geothermal energy is considered as a possible alternative for electricity production, as a heat source in district heating systems or harnessed for heating and cooling using geothermal heat pump installations. In this contribution we present the until now fragmented data sets relevant to characterize and assess the geothermal potential of Svalbard. Data sets include petroleum and deep research boreholes drilled onshore Svalbard, 14 of which have recorded subsurface temperature data at depths below 200 m. Geothermal gradients on Spitsbergen vary from 24 °C/km in the west to 55 °C/km in the south-east, with an average of 33 °C/km. Four deep research boreholes were fully cored and analyzed for thermal conductivity. These analyses were complemented by thermal conductivity calculated from wireline logs in selected boreholes and four measurements on outcrop samples. 1D heat flow modelling on five boreholes calibrated with the measured thermal conductivities offers insights into heat transfer through the heterogeneous sedimentary succession. Offshore petroleum boreholes in the south-western Barents Sea and marine heat flow stations around Svalbard provide a regional framework for discussing spatial variation in heat flow onshore Svalbard, with emphasis on the effects of erosion and deposition on the thermal regime. We conclude that Svalbard's geology is well suited for geothermal exploration and potential production, though challenges related to permafrost, the presence of natural gas, heterogeneous reservoir quality and strongly lateral varying heat flow need to be adequately addressed prior to geothermal energy production. Specifically for Longyearbyen, high geothermal gradients of 40–43 °C/km in the nearest borehole (DH4) suggest promising sub-surface thermal conditions for further exploration of deep geothermal potential near the settlement.

Thermal structure and sediment bulk density of the MV420 mud volcano on the continental slope of the Canadian Beaufort Sea and its implications for the plumbing system

The shallow migration path of mudflow of the mud volcano MV420 on the continental slope of the Canadian Beaufort Sea is investigated in terms of thermal and geotechnical characteristics. MV420 is a nearly flat topped active mud volcano that emits methane and fluidized mud. Its top is at a depth of water of 420 m, within the gas hydrate stability zone. During the summer 2017 IBRV Araon expedition, several measurements of marine heat flow were conducted in the area of juvenile mud mound/pond morphologies identified by multibeam bathymetry and backscatter intensity images obtained by the Monterey Bay Aquarium Research Institute’s autonomous underwater vehicle. The heat probe (a gravity corer with temperature, pressure, and tilt sensors) appeared to penetrate to a depth of >70 m below the seafloor. The sediment bulk density in the mudflow migration path of MV420, estimated by pressure change with controlled stretches of a winch cable, was 1.56 g/cm3. The subsurface temperature reaches up to 18°C and profiles show that the maximum local temperature occurs from 20 to 40 m below the seafloor, implying substantial vertical variability of geothermal gradients. Our finding of large positive and small negative geothermal gradients above and below the depth of the local temperature maximum may represent stagnant hot mud along the mudflow migration path, indicating a pulsative eruption of the mudflow. Gas hydrate is stable only within a few meter thick layer near the seafloor above the path because of the cold bottom water (0.4°C) covering the top of the mud volcano. Furthermore, the thermal conductivity of 0.939 W/m/K and the marine heat flow of 22.5 mW/m2 at a control site outside MV420 is estimated as the background heat flow in the slope, for which little data exists.

Prediction of marine heat flow based on the random forest method and geological and geophysical features

Prediction of marine heat flow based on the random forest method and geological and geophysical features

Towards Closing the Polar Gap: New Marine Heat Flow Observations in Antarctica and the Arctic Ocean

The thermal state of the lithosphere and related geothermal heat flow (GHF) is a crucial parameter to understand a variety of processes related to cryospheric, geospheric, and/or biospheric interactions. Indirect estimates of GHF in polar regions from magnetic, seismological, or petrological data often show large discrepancies when compared to thermal in situ observations. Here, the lack of in situ data represents a fundamental limitation for both investigating thermal processes of the lithosphere and validating indirect heat flow estimates. During RV Polarstern expeditions PS86 and PS118, we obtained in situ thermal measurements and present the derived GHF in key regions, such as the Antarctic Peninsula and the Gakkel Ridge in the Arctic. By comparison with indirect models, our results indicate (1) elevated geothermal heat flow (75 ± 5 mW m−2 to 139 ± 26 mW m−2) to the west of the Antarctic Peninsula, which should be considered for future investigations of ice-sheet dynamics and the visco-elastic behavior of the crust. (2) The thermal signature of the Powell Basin characteristic for oceanic crust of an age between 32 and 18 Ma. Further, we propose (3) that at different heat sources at the slow-spreading Gakkel Ridge in the Aurora Vent Field region might explain the geothermal heat flow distribution. We conclude that in situ observations are urgently required to ground-truth and fine-tune existing models and that a multidisciplinary approach is of high importance for the scientific community’s understanding of this parameter.

Heat-flow anomaly and residual topography in the Mascarene hotspot swell (Indian Ocean)

We review the sea-bottom heat-flow determinations and present a new heat-flow observation on the Mauritius island, which is part of the long-lived Reunion hotspot track. The marine heat flow is on average 66 ± 11 mW m−2 and is consistent with the on-land value of 61 ± 18 mW m−2 found in Mauritius. Since these values do not significantly deviate from the reference cooling-plate model, lithosphere erosion does not seem a likely mechanism for the swell formation. The lack of significant reheating due to a mantle plume impacting the lithosphere base is confirmed by thermal modelling. Moreover, the coherency between on-land and marine data is argument against advective redistribution of heat near the axis of the swell. We also analyse the large-scale features of the ocean lithosphere, which are not simply a function of the plate cooling and can reflect variations in mantle dynamic topography. The predicted topography variation along the swell shows amplitude and wavelength comparable to other hotspots. Both the topographic swell magnitude and the wavelength increase northwards with the increase of the age of volcanism. The estimated flux of material from the mantle follows the same trend, being larger in the northern part of the swell. The result that residual topography and the buoyancy flux are smaller at the active volcano of Reunion could be evidence that the activity of the plume has decreased with time.

The characteristics of heat flow in the Shenhu gas hydrate drilling area, northern South China Sea

Marine heat flow is of great significance for the formation and occurrence of seabed oil, gas and gas hydrate resources. Geothermal gradient is an important parameter in determining the thickness of the hydrate stability zone. The northern slope of the South China Sea is rich in gas hydrate resources. Several borehole drilling attempts were successful in finding hydrates in the Shenhu area, while others were not. The failures demand further study on the distribution regularities of heat flow and its controlling effects on hydrate occurrence. In this study, forty-eight heat flow measurements are analyzed in the Shenhu gas hydrate drilling area, located in the northern South China Sea, together with their relationship to topography, sedimentary environment and tectonic setting. Canyons are well developed in the study area, caused mainly by the development of faults, faster sediment supply and slumping of the Pearl River Estuary since the late Miocene in the northern South China Sea. The heat flow values in grooves, occurring always in fault zones, are higher than those of ridges. Additionally, the heat flow values gradually increase from the inner fan, to the middle fan, to the external fan subfacies. The locations with low heat flow such as ridges, locations away from faults and the middle fan subfacies, are more conducive to gas hydrate occurrence.

Richard P. Von Herzen (1930–2016)

Richard P. Von Herzen, a pioneer of marine heat flow studies who helped validate plate tectonics and discover oceanic hydrothermal vents, passed away on 28 January 2016. He was 85.

Conductive heat flow and nonlinear geothermal gradients in marine sediments—observations from Ocean Drilling Program boreholes

A basic premise in marine heat flow studies is that the temperature gradient varies with depth as a function of the bulk thermal conductivity of the sediments. As sediments become more deeply buried, compaction reduces the porosity and causes an increase in the bulk thermal conductivity. Therefore, while the heat flow may remain constant with depth, the thermal gradient is not necessarily linear. However, it has been argued that measurements showing increased sediment thermal conductivity with burial depth may be caused by a horizontal measurement bias generated by increasing anisotropy in sediments during consolidation. This study reanalyses a synthesis of Ocean Drilling Program data from 186 boreholes, and investigates the occurrence of nonlinear geothermal gradients in marine sediments. The aim is to identify whether observed downhole changes in thermal conductivity influence the measured temperature gradient, and to investigate potential errors in the prediction of in-situ temperatures derived from the extrapolation of near-surface thermal gradients. The results indicate that the measured thermal conductivity does influence the geothermal gradient. Furthermore, comparisons between shallow measurements (<10 m) from surface heat flow surveys and the deeply constrained temperature data from 98 ODP boreholes indicate that the shallow gradients are consistently higher by on average 19 °C km–1. This is consistent with higher porosity and generally lower thermal conductivity in near-seafloor sediments, and highlights the need to develop robust porosity–thermal conductivity models to accurately predict temperatures at depth from shallow heat flow surveys.

A synthesis of heat flow determinations and thermal modeling along the Nankai Trough, Japan

We review marine heat flow data along the Nankai Trough and show that observations &gt;30 km seaward of the deformation front are 20% below conductive predictions (129–94 mW m−2) but consistent with the global heat flow average for oceanic crust of the same age (16–28 Ma). Heat flow values ≤30 km seaward of the deformation front are generally 20% higher than conductive predictions. This heat flow pattern is consistent with the advection of heat by fluid flow in the subducting oceanic crust and explains both the high heat flux in the vicinity of the trench, &gt;200 and &gt;140 mW m−2, and steep landward declines to values of approximately 60 mW m−2 over distances of 65 and 50 km along the Muroto and Kumano transects, respectively. Along the Ashizuri transect, the lack of heat flow data precludes a definitive interpretation. We conclude that fluid flow in the subducting oceanic crust leads to temperatures that are generally 25°C higher near the toe of the margin wedge and 50–100°C lower near the downdip limit of the seismogenic zone than estimated by purely conductive models.

High resolution temperature measurement technique for measuring marine heat flow

High resolution temperature measurement technique is one of the key techniques for measuring marine heat flow. Basing on Pt1000 platinum resistance which has the characteristics of high accuracy and good stability, we designed a bridge reversal excitation circuit for high resolution temperature measurement. And the deep ocean floor in-situ test results show that: (1) temperature deviation and peak-to-peak resolution of the first version circuit board (V1) are 1.960-1.990 mK and 0.980-0.995 mK at 1.2-2.7A degrees C, respectively; and temperature deviation and peak-to-peak resolution of the second circuit board (V2) are 2.260 mK and 1.130 mK at 1.2-1.3A degrees C, respectively; (2) During the 2012NSFC-IndOcean cruise, seafloor geothermal gradient at Ind2012HF03, -07 and -12 stations (water depth ranges from 3841 to 4541 m) were successfully measured, the values are 59.1, 75.1 and 71.6A degrees C/km, respectively. And the measurement errors of geothermal gradient at these three stations are less than 3.0% in terms of the peak-to-peak resolution. These indicate that the high resolution temperature measurement technique based on Pt1000 platinum resistance in this paper can be applied to marine heat flow measurement to obtain high precision geothermal parameters.

New heat flow measurements in the Ulleung Basin, East Sea (Sea of Japan): relationship to local BSR depth, and implications for regional heat flow distribution

In July 2007, new marine heat flow data were collected at ten sites (HF01–10) in the central and southwestern sectors of the Ulleung Basin (East Sea or Sea of Japan) as part of regional gas hydrate research. In addition, cores were collected at five of these sites for laboratory analysis. The results show that the geothermal gradient ranged from 103–137 mK/m, and the in-situ thermal conductivity from 0.82–0.95 W/m·K. Laboratory measurements of thermal conductivity were found to deviate by as much as 40% from the in-situ measurements, despite the precautions taken to preserve the cores. Based on the in-situ conductivity, the heat flow was found to increase with water depth toward the center of the basin, ranging from 84–130 mW/m2. Using a simple model, we estimated the heat flow from the depths of the BSR, and compared this with the observed heat flow. In our study area, the two sets of values were quite consistent, the observed heat flows being slightly higher than the BSR-derived ones. The evaluation of regional pre-1994 data revealed that the heat flow varied widely from 51–157 mW/m2 in and around the basin. Due to a large scatter in these older data, a clear relationship between heat flow and water depth was not evident, in contrast to what would be expected for a rifted sedimentary basin. This raises the question as to whether the pre-1994 data represent the true background heat flow from the underlying basin crust since the basin opening, and/or whether they contain large measurement errors. In fact, evidence in support of the latter explanation exists. BSRs are generally found in the deep parts of the basin, and vary by only ±15 m in depth below the seafloor. From the average BSR depth, we inferred the background heat flow using a simple model, which in the case of the Ulleung Basin is approximately 120 and 80 mW/m2 for 2.5 and 1 km below sea level, respectively.

Modern Perspectives on Measuring and Interpreting Seafloor Heat Flux: The Future of Marine Heat Flow: Defining Scientific Goals and Experimental Needs for the 21st Century; Salt Lake City, Utah, 6–7 September 2007

There has been a resurgence of interest in marine heat flow in the past 10–15 years, coinciding with fundamental achievements in understanding the Earth's thermal state and quantifying the dynamics and impacts of material and energy fluxes within and between the lithosphere and hydrosphere. At the same time, technical capabilities have dwindled to the point that no U.S. academic institution currently operates a seagoing heat flow capacity.In September 2007, a workshop was convened in Salt Lake City with sponsorship from the U.S. National Science Foundation (NSF) and participation by scientists and engineers from North America, Europe, and Asia. The primary goals of the workshop were to (1) assess high‐priority scientific and technical needs and (2) to evaluate options for developing and maintaining essential capabilities in marine heat flow for the U.S. scientific community.

Marine heat flow measurements in hard ground offshore Sumatra

The hydrocarbon potential of the fore arc basins between the Siberut, Nias, and Simeulue islands and Sumatra was investigated in 2006 by the German Federal Geological Survey (BGR) with marinegeophysical and marine‐geological techniques (cruises SO‐189‐1 and SO‐189‐2). The basin sediments are relatively coarse grained (in part, of volcanic origin). In most locations, they are too hard for the use of conventional heat flow probes, which penetrate only muddy to soft seafloor sediments.To overcome this restriction and to measure in situ heat flow in hard‐ground terrain of these fore arc basins, a new ‘hard‐ground heat flow probe’ was developed and successfully employed at 10 stations during the SO‐189 cruises. A conventional heat flow probe was operated at six additional stations. The development of the new probe initially was motivated by the observation of widespread ‘hard ground’ along the outer‐arc high of the accretionary prism at the southern coast of Java in 1999 (marine cruise GINCO, leg SO‐139‐2 of BGR).

Determination of Seafloor Temperatures Using Data from High-Resolution Marine He at Probes

Marine heat flow surveys typically utilize Lister-type heat probes, which allow multi-penetrations to measure both temperature and thermal conductivity in situ. The temperature rise resulting from penetration friction must first be removed in order to obtain the equilibrium temperature and the thermal conductivity of the sediments. However, the time needed to return to the original sediment temperature may be very long. Using high- resolution heat probes, temperatures with resolution of 0.1 mK were obtained within the first few meters below the seafloor off southwestern Taiwan. This research introduces an improved data processing technique that can extrapolate friction-raised temperature to infinite time and obtain equilibrium ambient temperature accurately from a limited amount of high-resolution temperature recordings. The computing technique is to regress the temperature decay function numerically on parameters of ambient temperature, initial temperature rise, thermal conductivity, and heat capacity of the sediment. In order to obtain a better fit between the data and the model as well as a better prediction of the friction raised temperature, we allowed erroneous solutions to parameters for regression. This technique was tested on both synthetic and high-resolution field data to demonstrate accuracy and efficiency. From the field experiment, we estimated that the error between the prediction and the true equilibrium ambient temperatures to be within a few 0.1 mK which is much smaller than a typical resolution, of a few mK in marine heat flow measurement.

Determination of thermal conductivity and formation temperature from cooling history of friction-heated probes

SUMMARY The formation temperature below the sea or lake floor is frequently measured with a sensor probe that is inserted into the unconsolidated sediment, and the thermal conductivity of sediment is measured in situ with an independent heating experiment. Friction during the insertion raises the temperature of a probe. This paper presents the method and the test results of using the cooling history of friction-heated probes for the determination of equilibrium formation temperature and thermal conductivity. The inverse modelling (IM) for the desired parameters is formulated together with the finite-element simulation of the cooling history. The starting parameter values for IM are generated with a genetic algorithm (GA) that searches for them in the likely parameter ranges. The IM is constrained in an objective function by a commonly used asymptotic relation between the declining temperature and the inverse time. In addition to satisfying the root-mean-square misfit, the results are assessed by the equality between the model conductivity and the conductivity obtained through the asymptotic relation and by how well the models can predict the cooling behaviour at time beyond the record period for IM. The results of testing synthetic data with random noise up to ±0.005 K and twenty 8‐11 min long data sets from four sites in Lake Baikal indicate that the desired parameters can be determined from the first 2.5 min of the cooling records. By comparison with independent measurements or through repeated GA-IM, equilibrium temperatures can be consistently determined to within 0.002 K and thermal properties typically to ±5 per cent. The method has also been successfully applied to seven sets of 5 min long data sets at one marine heat flow station.

Thermal models of the Mexico subduction zone: Implications for the megathrust seismogenic zone

It has been proposed that the seismogenic zone of subduction thrust faults is primarily controlled by temperature or rock composition changes. We have developed numerical models of the thermal structure of the Mexico subduction zone to examine the factors that affect the temperature of the subduction thrust fault. Although the oceanic plates subducting beneath Mexico are young, the top of the oceanic plate at the trench is cool, because of the lack of a thick cover of insulating sediments. Marine heat flow observations suggest that hydrothermal circulation may further cool the oceanic plate. This results in a cool subduction thrust fault, where the brittle part of the fault extends to depths of over 40 km. At these depths, even slight frictional heating may have significant effects on temperature along the thrust fault, particularly for regions with a high convergence rate and shallow plate dip. With the addition of a small amount of frictional heating, the temperatures of the deep (30–40 km) thrust fault are increased by over 200°C. As the observed downdip limit of rupture in recent well‐constrained megathrust earthquakes is confined to depths above the intersection of the thrust fault and the continental Moho, a temperature of 350°C may control the downdip extent of the seismogenic zone. Thus, in order to be consistent with the observed shallow rupture areas, it is necessary to include a small amount of frictional heating, corresponding to an average shear stress of 15 MPa.

Inversion of marine heat flow measurements by expansion of the temperature decay function

Marine heat flow data, obtained with a Lister type probe, consists of two temperature decay curves, frictional and heat pulse decay. Both follow the same physical model of a cooling cylinder. The mathematical model describing the decays is non linear as to the thermal sediment parameters thus a direct inversion is not possible. To overcome this difficulty, the model equations are expanded using a first order Taylor series. The linearised model equations are used in an iterative scheme to invert the temperature decay for undisturbed temperature and thermal conductivity of the sediment. The inversion scheme is tested first for its theoretical limitations using synthetic data. Inversion of heat flow measurements obtained during a cruise of R/V SONNE in 1996 and needle probe measurements in material of known thermal conductivity show that the algorithm is robust and gives reliable results. The programme can be obtained from the authors.

Deep‐penetration heat flow probes raise questions about interpretations from shorter probes

More than 40% of the marine heat flow data collected since the early experiments of Sir Edward Bullard in 1949 were obtained using shallow penetration probes less than 5 m long [Louden and Wright, 1989]. The common belief that these data are reliable enough to model deep‐seated thermal processes is supported by a few experiments in which heat flow measurements made in the Deep Sea Drilling Program (DSDP) and the Ocean Drilling Program (ODP) were compared to nearby surface heat flow measurements [e.g.,Hyndman et al., 1984]. However, thermal measurements made with 18‐m penetrations recently collected on the northern flank of the South‐East Indian Ridge (SEIR) bring a new perspective to this belief. In the study area, measurements of heat flow taken at the surface ( 0–5 m) and measurements taken at greater depths (3–18 m) did not always concur. Investigating this lack of agreement will help address difficult questions about the interpretation of shallow penetration (&lt;5 m) marine heat flow measurements.

Electric Potential Arising from a Point-Source Near a Cylinder in Layered Earth Structures: Design Criteria for a Modified Marine Heat-Flow Probe

A modified marine heat-flow probe has been proposed to conduct simultaneous measurements of thermal and electric properties of seafloor sediments. A major concern in the electric aspect is the shielding effect caused by an extremely conductive solid steel strength-member of the probe.To appraise the feasibility of the proposed probe in electric measurements, we develop a novel numerical algorithm to calculate potential and hence apparent resistivities arising from a point-source of current near a cylinder situated in layered earth structures. The scheme is validated against an analytical solution with respect to a uniform whole-space in the presence of the cylinder. Image theory is employed to transform the layered earth structures into the whole-space models. Model parameters are taken from the design blueprint. Numerical computations for varieties of electrode configurations reveal that the dipole-dipole array suffers the smallest disturbance, and can be employed to directly measure the true electric properties of seafloor sediments as long as its electrode spacing is less than 10 cm. The electric properties are also measurable when the pole-dipole array or the Wenner array is used with the electrode spacing less than 5 cm. The pole-pole array is however not applicable.