Passive Seismic Research Articles

Passive seismic horizontal to vertical spectral ratio (HVSR) surveying to help define bedrock depth, structure and layering in shallow coal basins

SummaryTrial passive seismic HVSR survey lines were carried out in the north-western part of the Wilga sedimentary basin in Western Australia, which is a small and shallow isolated sub-basin to the Perth Basin, sitting above rifted Archaean granite-gneiss basement rocks. The HVSR survey method was trialled to determine if it could resolve subsurface basin features quickly and cheaply. This trial HVSR survey was carried out as a series of transects along open walking tracks. It was very successful for mapping the general basin architecture by resolving bedrock depth to a maximum depth of close to 200 m along the survey transects, and defining a half-graben structure that forms the faulted eastern boundary of the basin, with the basin depth gently deepening toward the west. The HVSR data are also interpreted to have detected subtle layering within the basin sedimentary sequence to possibly map Permian black shale beds, coal measures and splits sitting below an unconformity of Cretaceous sedimentary cover. The passive seismic HVSR method has potential to be a robust survey method for defining basin architecture in shallow coal basins with poorly indurated basin fill deposits, especially in remote parts of the world with difficult access and limited exploration funds to carry out active seismic surveys.

A new empirical correlation between pressuremeter modules (EM) and shear wave velocity (Vs) for clay soils

The pressuremeter (PMT) and standard penetration (SPT) tests are the most common in situ tests used for determining the engineering properties of soils and rocks. PMT method can be used to determine the deformation and strength properties of soil or very blocky rock masses. PMT procedure is time-consuming and expensive, and it requires advanced testing equipment. Both SPT and PMT methods also require drilling to be performed in the area. The shear wave velocity (Vs) is a parameter obtained using active and passive seismic methods and provides insight into the strength properties of the soil and rock. Vs is easy to obtain with these methods and can be determined in all kinds of field conditions. Due to the difficulties experienced during many types of in-situ tests, numerous empirical equations for the soil or rock units have been proposed in the literature that are based on Vs.In this paper, correlations of Menard Deformation Modules (EM) with the corrected SPT blow counts (SPT-N60) and shear wave velocity (Vs) data were conducted. For this purpose, parameters of the pressuremeter were defined as a function of two variables. In order to determine the relationship between the results of these field tests and the results obtained from high-consolidated clayey soils with high and low plasticity properties, 10 boreholes with a depth of 15 m were drilled and in-situ tests were carried out at diverse depths. In addition, seismic measurements were performed at the same locations and depth-based Vs velocity data was obtained. It was concluded that EM could be predicted as a function of SPT-N60 and Vs values, and the predictions had relatively high R2 values of 0.77 and 0.75, respectively. This study thereby introduces to the literature empirical equations between EM and Vs for the first time. As soil properties are heterogeneous and anisotropic, it has been shown that it is more appropriate to use the equations produced from logarithmic and exponential relations in both single and multiple statistical analysis.

Ambient-noise tomography of the Greater Geneva Basin in a geothermal exploration context

SUMMARYThe Greater Geneva Basin is one of the key targets for geothermal exploration in Switzerland. Until recently, information about the subsurface structure of this region was mostly composed of well-logs, seismic reflection lines, and gravity measurements. As part of the current effort to further reduce subsurface uncertainty, and to test passive seismic methods for exploration purposes, we performed an ambient-noise tomography of the Greater Geneva Basin. We used ∼1.5 yr of continuous data collected on a temporary seismic network composed of 28 broad-band stations deployed within and around the basin. From the vertical component of the continuous noise recordings, we computed cross-correlation functions and retrieved Rayleigh-wave group-velocity dispersion curves. We then inverted the dispersion curves to obtain 2-D group-velocity maps and proceeded to a subsequent inversion step to retrieve a large-scale 3-D shear-wave velocity model of the basin. We discuss the retrieved features of the basin in the light of local geology, previously acquired geophysical data sets, and ongoing geothermal exploration. The Greater Geneva Basin is an ideal natural laboratory to test innovative geothermal exploration methods because of the substantial geophysical data sets available for comparison. While we point out the limits of ambient-noise exploration with sparse networks and current methodology, we also discuss possible ways to develop ambient-noise tomography as an affordable and efficient subsurface exploration method.

Passive processing of active nodal seismic data: estimation of <i>V</i><sub>P</sub>∕<i>V</i><sub>S</sub> ratios to characterize structure and hydrology of an alpine valley infill

Abstract. The advent of cable-free nodal arrays for conventional seismic reflection and refraction experiments is changing the acquisition style for active-source surveys. Instead of triggering short recording windows for each shot, the nodes are continuously recording over the entire acquisition period from the first to the last shot. The main benefit is a significant increase in geometrical and logistical flexibility. As a by-product, a significant amount of continuous data might also be collected. These data can be analyzed with passive seismic methods and therefore offer the possibility to complement subsurface characterization at marginal additional cost. We present data and results from a 2.4 km long active-source profile, which have recently been acquired in western Colorado (US) to characterize the structure and sedimentary infill of an over-deepened alpine valley. We show how the “leftover” passive data from the active-source acquisition can be processed towards a shear wave velocity model with seismic interferometry. The shear wave velocity model supports the structural interpretation of the active P-wave data, and the P-to-S-wave velocity ratio provides new insights into the nature and hydrological properties of the sedimentary infill. We discuss the benefits and limitations of our workflow and conclude with recommendations for the acquisition and processing of similar datasets.

Hydropower Dam State and Its Foundation Soil Survey Using Industrial Seismic Oscillations

In this article, we suggest a new type of seismic source for surveying both structure state and foundation soil conditions regardless of the level of seismic noise. In our opinion, powerful industrial equipment can be treated as seismic sources. We describe the results of a survey conducted on the Song Tranh-2 hydropower dam located in Central Vietnam. After a М = 4.7 earthquake, the dam visual inspection revealed zones of the excessive durability loss: cracks and areas with an elevated infiltration of water into the dam galleries. Powerful industrial equipment generates continuous quasi-harmonic mechanical oscillations (seismic waves) that travel through layers of rocks. These seismic oscillations are recorded by receivers in different measurement points such as the dam body, abutments, and the foundation soils. Anomalous amplitudes of these oscillations indicate the presence of weakened zones in the structure or in foundation soil. We coupled passive and active seismic methods to more precisely find such zones. In this case, active seismic methods allowed us to investigate dam abutment zones, and passive seismic methods were used to localize weakened zones in the dam-foundation soil system. We assumed that the joint contribution of two factors was the cause of the dam weakening. One of them was caused by increased water infiltration through the rock mass and its contact zones, and the other was reservoir-induced seismicity contributing to the deterioration of the foundation soils, which was possibly the reason for a shift in the dam in the contact zone with the rock mass foundation. It is necessary to perform computer modelling, which was not included in our research. The developed method can be used for the safety control of the hydropower station dams.

Sedimentary basins investigation using teleseismic P-wave time delays.

ABSTRACTPassive seismic methods have been proven successful in recent years at retrieving information about the large‐scale structure of a sedimentary basin. These methods are based on ambient noise recordings, and local and distant (teleseismic) earthquake data. In particular, it has been previously observed that the arrival time of teleseismic P‐waves recorded inside a sedimentary basin shows time delays and polarization that both strongly depend on the basin properties and structure. In this paper, we present a new methodology for determining seismic P‐wave velocity in a sedimentary basin, based on the time delay of a teleseismic P‐wave travelling through the low‐velocity basin infill, with respect to a teleseismic wave recorded outside the basin. The new methodology is developed in a Bayesian framework and, thus, it includes estimates of the uncertainties of the P‐wave velocities. For this study, we exploit synchronous recordings of teleseismic P‐wave arrivals at a dense linear array of broadband seismic stations, using data from two teleseismic events coming from two different incoming angles. The results obtained by the new proposed methodology are successfully compared to classical cross‐correlation measurements, and are used to infer properties of a sedimentary basin, such as the basin bounding fault's geometry and the average P‐wave velocity of the sedimentary basin fill.

Designing and testing a network of passive seismic surveying and monitoring in Dehdasht (South Western Iran)

ABSTRACTFrom August 2016 to July 2017, a passive seismic survey was conducted in South Western Iran as a part of a pilot project aimed to improve the imaging in geologically complex areas. Passive seismic methods have shown to be a useful tool to infer the physical properties of the underground geological structures where traditional hydrocarbon exploration methods are challenging. For this purpose, a dense passive seismic network consisting of 119 three‐component borehole seismic stations was deployed over an area of 400 km2 around the city of Dehdasht. This paper focuses on the details of the network design, which was devoted to high‐resolution seismological applications, including local earthquake tomography and seismic attenuation imaging. In this regard, we describe the instrument types and the station installation procedures used to obtain high‐quality data that were used to retrieve three‐dimensional models of P‐ and S‐wave velocity and P‐wave attenuation in the area using tomographic inversion techniques. We also assess the network performance in terms of the seismic ambient noise levels recorded at each station site, and we revise the horizontal orientation of the sensors using surface waves from teleseismic earthquakes.

Near-surface site investigation by seismic interferometry using urban traffic noise in Singapore

We have evaluated a field test in the city of Singapore to assess the feasibility of the passive seismic survey for bedrock depth determination and to further investigate the optimal acquisition parameters. The ambient noise field, dominated by urban traffic noise, is recorded passively for the application of seismic interferometry. Spectral analysis indicates that the traffic-induced noise by local roads is concentrated between 3 and 25 Hz. We use multiple signal classification beamforming for wavefield direction of propagation analysis. We apply seismic interferometry to retrieve the surface-wave part of the Green’s functions, based on which surface-wave dispersion relations are extracted and further inverted for 1D S-wave velocity profiles. Subsequently, we compare the inversion results from seismic interferometry with borehole logs at multiple sites in Singapore and establish that the bedrock depths are well-determined using passive seismic methods within a maximum error of 3 m. By investigating the convergence of the crosscorrelograms, we ascertain that the best compromise of cost, efficiency, and accuracy for a passive site investigation in Singapore can be achieved in 15 min in the morning of a working day using an array as short as 30 m with six vertical geophones, although these parameters should be reinvestigated at other sites and other times. The success of this case study demonstrates that accurate near-surface site investigation can be achieved with faster acquisition, fewer receivers, and a smaller acquisition footprint compared with conventional methods, all of which improve the efficiency particularly in a highly developed urban environment.

Tracking the Diprotodon - microtremor passive seismic profiling as a tool for location of megafauna bone beds

Bone beds containing Pleistocene megafauna fossils dated to between 35ka and 60ka occur near Lancefield (Victoria). The bones lie within clays above gravels and extensive Quaternary basalt flows. Evidence that the bones have been subject to alluvial transportation suggests that profiling the basalt basement for paleo-channels will assist with locating further bone beds. Passive seismic (microtremor) methods, as developed variously for earthquake hazard studies and regolith studies, have been applied to this problem, using both HVSR spectral ratio methods and two-station SPAC (spatially averaged coherency) methods. Clay layers have shear-wave velocities (Vs) in the range 120-180m/s and thicknesses 1.5 to 2.5m. Microtremor data in the frequency band 5-60Hz provides excellent resolution of the Vs and thickness of the clay layers, allowing the bedrock profile to be established to an accuracy of 0.5m or better. The results indicate existence of soft clays some 40m west of an existing excavation site and thus identify a future site for excavation.

Effective Active and Passive Seismics for the Characterization of Urban and Remote Areas: Four Channels for Seven Objective Functions

An efficient system for the joint acquisition and analysis of multi-component active and passive seismic data is presented. It is shown how, in spite of the limited field equipment (the system requires just a 4-channel seismograph, one 3-component and four vertical-component geophones), it is nevertheless possible to define up to seven different (but mutually related and complementary) objects used to constrain a multi-objective joint inversion capable of providing a robust subsurface shear-wave velocity (VS) profile for both geotechnical and seismic-hazard studies. The presented approach relies on acquisition techniques that require simple and straightforward field procedures useful in particular, but not solely, in the characterization of urban and remote areas where, due to logistical problems, standard acquisition procedures cannot be easily applied. Active data recorded by a single 3-component geophone are processed so to define up to five objective functions: the group-velocity spectra of the three components, the radial-to-vertical spectral ratio and the Rayleigh-wave particle motion frequency curve. Passive data are used to compute two further objects: the horizontal-to-vertical spectral ratio and the phase-velocity dispersion curve obtained via miniature array analysis of microtremors. These seven objects are jointly inverted by means of a multi-objective inversion procedure based on the Pareto criterion. Performances are assessed through a comprehensive field dataset acquired in an urban area of NW-Italy. The consistency of the overall procedure is assessed by comparing the results with the analyses accomplished by considering classical multi-channel active and passive data and methodologies (multi-component MASW, multichannel analysis of surface waves and ESAC, extended spatial auto-correlation).

A Quantitative Assessment of the Annual Contribution of Platform Downwearing to Beach Sediment Budget: Happisburgh, England, UK

Field and numerical investigations at Happisburgh, East coast of England, UK, sought to characterize beach thickness and determine geologic framework controls on coastal change. After a major failure of coastal protection infrastructure, removal of about 1 km of coastal defence along the otherwise protected cliffed coastline of Happisburgh triggered a period of rapid erosion over 20 years of ca. 140 m. Previous sensitivity studies suggest that beach thickness plays a major role in coastal recession. These studies were limited, however, by a lack of beach volume data. In this study, we have integrated the insights gained from our understanding of the Quaternary geology of the area, a novel non-intrusive passive seismic survey method, and a 3D novel representation of the subsurface source and transportable material into a coastal modelling environment to explore the role of beach thickness on the back wearing and downwearing of the cliffs and consolidated platform, respectively. Results show that beach thickness is non-homogeneous along the study site: we estimate that the contribution to near-shore sediment budget via platform downwearing is of a similar order of magnitude as sediment lost from the beach and therefore non-negligible. We have provided a range of evidence to support the idea that the Happisburgh beach is a relatively thin layer perched on a sediment rich platform of sand and gravel. This conceptualization differs from previous publications, which assume that the platform was mostly till and fine material. This has direct implication on regional sediment management along this coastline. The present study contributes to our understanding of a poorly known aspect of coastal sediment budgeting and outlines a quantitative approach that allows for simple integration of geological understanding for coastline evolution assessments worldwide.

A simple inversion algorithm to estimate a linearly increasing velocity model for microseismic monitoring

Microseismic monitoring is used to optimise shale gas production or enhanced geothermal stimulation. The technical tools for microseismic monitoring, which is a passive seismic method, are similar to those used in earthquake detection, but differ in that the target area is much smaller than areas affected by earthquakes. Therefore, it is important to use an accurate velocity model. However, such models require conducting an additional survey, which can be both expensive and time-consuming. Many microseismic monitoring studies have used an approximated velocity model constructed from well logging data to reduce these additional costs. In this study, we used a simple approximated model in which velocity increases linearly with depth and creates an accurate velocity model, eliminating the need for an additional survey. We analytically derived formulas for seismic ray traveltime and inverted the velocity gradient using the Gauss–Newton method. Using a numerical example, we verified that the proposed algorithm accurately describes the long-wavelength trend of the true velocity model in a negligibly short time. We performed a Monte Carlo simulation to evaluate the effects of traveltime picking errors. The simulation results indicated that the proposed algorithm provides a reasonable solution under the probable uncertainty of traveltime picking. Finally, we verified that our algorithm was not sensitive to the initial velocity gradient through inversion tests using various initial values. Thus, the numerical example and analysis confirm that the proposed algorithm is efficient and robust.

A Comprehensive Work Flow To Characterize Waterflood-Induced Fractures by Integrating Real-Time Monitoring, Formation Test, and Dynamic Production Analysis Applied to Changqing Oil Field, China

Summary It is well-known that water injection may induce formation fracturing in tight reservoirs. Especially when the field-geology condition is complex and the waterflood-induced fractures (WIFs) are not well-identified in time, the induced fractures can be of the same order as the well spacing, which has a significant, and generally undesired, impact on both areal sweep and vertical conformance. Therefore, the onset of WIFs must be identified in a timely manner, and the waterflooding performance must be evaluated comprehensively to formulate an appropriate strategy over time. A new work flow, containing analytical/semianalytical, statistical, and numerical techniques that are based on flow-rate/BHP and formation-testing data, is applied to identify the WIFs, diagnose waterflooding direction and front distribution, analyze interwell connectivity, and interpret abnormal bottomhole-pressure (BHP) behaviors in the Changqing Oil field. The work flow includes three modules: First, real-time monitoring and analysis, including modified Hall plot, evolving skin analysis, and injection/fracturing index methods, are used to identify the start of WIFs. Then, the formation-testing module, consisting of step-rate test (SRT), radioactive-tracer logging, and passive seismic method, is applied to investigate the formation-fracturing pressure, and uneven waterflooding performance in the areal and vertical directions. On the basis of the two former modules, we adapt the third module, which includes injector/producer relationships (IPRs) and the constrained multiple-linear-regression (MLR) method, to quantitatively investigate the waterflooding direction by injection/production rates. A new model—injection well with waterflood-induced fracture (IWWIF)—is proposed to characterize the abnormal BHP behaviors considering the properties variation (shrinking fracture length and decreasing fracture conductivity) of WIFs during the falloff period. Compared with an individual method, the ITD (which is the abbreviation of WIF identification, formation testing, and dynamic production analysis) work flow is developed to obtain a comprehensive and deep understanding of waterflooding performance. The main emphasis of this study is to integrate different approaches to address the key uncertainties rather than analyze each data source individually. On the basis of the results obtained by this work flow, the operators can make a more-proactive and -reasonable decision on waterflooding management. The work flow proposed in this paper gives a useful guidance in short- and long-term waterflooding management in tight reservoirs.

Application of the Spatial Auto-Correlation Method for Shear-Wave Velocity Studies Using Ambient Noise

Ambient seismic noise or microtremor observations used in spatial auto-correlation (SPAC) array methods consist of a wide frequency range of surface waves from the frequency of about 0.1 Hz to several tens of Hz. The wavelengths (and hence depth sensitivity of such surface waves) allow determination of the site S-wave velocity model from a depth of 1 or 2 m down to a maximum of several kilometres; it is a passive seismic method using only ambient noise as the energy source. Application usually uses a 2D seismic array with a small number of seismometers (generally between 2 and 15) to estimate the phase velocity dispersion curve and hence the S-wave velocity depth profile for the site. A large number of methods have been proposed and used to estimate the dispersion curve; SPAC is the one of the oldest and the most commonly used methods due to its versatility and minimal instrumentation requirements. We show that direct fitting of observed and model SPAC spectra generally gives a superior bandwidth of useable data than does the more common approach of inversion after the intermediate step of constructing an observed dispersion curve. Current case histories demonstrate the method with a range of array types including two-station arrays, L-shaped multi-station arrays, triangular and circular arrays. Array sizes from a few metres to several-km in diameter have been successfully deployed in sites ranging from downtown urban settings to rural and remote desert sites. A fundamental requirement of the method is the ability to average wave propagation over a range of azimuths; this can be achieved with either or both of the wave sources being widely distributed in azimuth, and the use of a 2D array sampling the wave field over a range of azimuths. Several variants of the method extend its applicability to under-sampled data from sparse arrays, the complexity of multiple-mode propagation of energy, and the problem of precise estimation where array geometry departs from an ideal regular array. We find that sparse nested triangular arrays are generally sufficient, and the use of high-density circular arrays is unlikely to be cost-effective in routine applications. We recommend that passive seismic arrays should be the method of first choice when characterizing average S-wave velocity to a depth of 30 m (Vs30) and deeper, with active seismic methods such as multichannel analysis of surface waves (MASW) being a complementary method for use if and when conditions so require. The use of computer inversion methodology allows estimation of not only the S-wave velocity profile but also parameter uncertainties in terms of layer thickness and velocity. The coupling of SPAC methods with horizontal/vertical particle motion spectral ratio analysis generally allows use of lower frequency data, with consequent resolution of deeper layers than is possible with SPAC alone. Considering its non-invasive methodology, logistical flexibility, simplicity, applicability, and stability, the SPAC method and its various modified extensions will play an increasingly important role in site effect evaluation. The paper summarizes the fundamental theory of the SPAC method, reviews recent developments, and offers recommendations for future blind studies.

Application of deep-penetrating geophysical methods to mineral exploration: Examples from Western Australia

The emergence of the concept of a “mineral system” has changed the way regional-scale mineral prospectivity is assessed. Geographically widespread data sets and deep-penetrating geophysical methods are required to map the various components of the mineral system, which may encompass areas of perhaps thousands of square kilometers and extend to mantle depths. Key mineral system components that can be detected in this fashion include deep-penetrating faults and the suture zones between major geologic blocks, which are important controls on the movement of metal-carrying magmas and brines in a variety of mineral systems. Two case studies from mineralized terrains in Western Australia illustrate the use of deep-penetrating geophysical methods in mineral exploration. Magnetotelluric (MT) data from a 300-km-long traverse in the Archean Yilgarn Craton map numerous steeply dipping conductive zones, which coincide with linear anomalies in potential field data and are interpreted as deep-penetrating faults. Also, lateral changes in crustal and upper mantle resistivity structure suggest the juxtaposition of two, or perhaps three, different major crustal blocks with intervening suture zones. Teleseismic data from a 250-km-long traverse in the Proterozoic Capricorn Orogen provide information on deep crustal structure and composition. Interpretation in association with deep seismic reflection data allows previously unrecognized suture zones to be recognized in the deep crust and under thick cover. Passive seismic and MT methods represent a comparatively cost effective way to identify key mineral system indicators of regional prospectivity, even in the geologically complex terrains of Western Australia.

Passive Seismic for Hydrocarbon Indicator : Between Expectation and Reality

In between 5 – 10 years, in our country, passive seismic method became more popular to finding hydrocarbon. Low price, nondestructive acquisition and easy to mobilization is the best reason for choose the method. But in the other part, some people are pessimistically to deal with the result. Instrument specification, data condition and processing methods is several points which influence characteristic and interpretation passive seismic result. In 2010 one prospect in East Java Basin has been measurement constist of 112 objective points and several calibration points. Data measurement results indicate a positive response. Furthermore, in 2013 exploration drliing conducted on the prospect. Drill steam test showes 22 MMCFD in objective zone, upper – late oligocene. In 2015, remeasurement taken in objective area and show consistent responses with previous measurement. Passive seismic is unique method, sometimes will have difference results on dry, gas and oil area, in field production and also temporary suspend area with hidrocarbon content.

ТЕХНОЛОГИЯ МСП - МАЛОКАНАЛЬНОЕ СЕЙСМИЧЕСКОЕ ПРОФИЛИРОВАНИЕ НА БАЗЕ МОВСР

ТЕХНОЛОГИЯ МСП - МАЛОКАНАЛЬНОЕ СЕЙСМИЧЕСКОЕ ПРОФИЛИРОВАНИЕ НА БАЗЕ МОВСР

Feasibility of using molecular-electronic seismometers in passive seismic prospecting: Deep structure of the Kaluga ring structure from microseismic sounding

Field experiments involving molecular-electronic seismometers, along with conventional pendulum geophones, were performed to study the deep structure of the upper crust (in the area of the Kaluga ring structure) by passive seismic methods. The microseismic sounding survey was carried out along a geophysical profile crossing the central part of the structure with simultaneous data acquisition by molecular-electronic and conventional seismometers at each measurement point. Experimental data on the propagation of Rayleigh waves along the curvilinear surface have been collected. The feasibility of using molecular-electronic seismometers for passive seismic studies has been confirmed by the results of comparative analysis of the vertical geophysical cross sections, which reveal upper crustal heterogeneities, and by the results of a series of laboratory tests.

Combining active and passive seismic methods for the characterization of urban sites in Cairo, Egypt

Combining active and passive seismic methods for the characterization of urban sites in Cairo, Egypt

Hydrocarbon Prospect Derived from Attributes Analysis on Low-Frequency Passive Seismic Survey: a Case Study from Kalimantan, Indonesia

Hydrocarbon Microtremor Analysis is a low-frequency passive seismic method which derives a quick look estimates new hydrocarbon reservoir prospect area. This method based on the empirical study which investigated an increasing of spectra anomaly between 2 – 4 Hz above the reservoir. We determined five attributes on low-frequency band of microtremors including Power Spectral Density integral of vertical component (PSD-IZ), Power Spectral Density (PSD) on 3 Hz frequency, frequency shifting, the spectral ratio of vertical and horizontal components (V/H) maximum and integral of spectral ratio of vertical and horizontal components (V/H). We deployed 105 points of measurement spreading in our suspect area. We used time series data that recorded from particle velocity of three components with 80 minutes duration and 100 Hz of the sampling frequency. The noise identification analysis in each station data set has been made from the measurement location, considering the suspect area had different local cultural noise. We proceed attributes for each data acquired from all station then used the interpolated map using a standard kriging algorithm spatially. As a result, each attribute analysis and spatial attribute map are combined to identify and estimate a good prospect of the hydrocarbon reservoir.