This paper describes several geodetic studies that consolidate the reliability and precision of monitoring subsidence due to hydrocarbon production: the deployment of Integrated Geodetic Reference Stations (IGRS); the application of high resolution InSAR; the comparison of different GNSS processing methodologies; the implementation of an efficient InSAR stochastic model, and the framework of integrated geodetic processing (levelling, GNSS, InSAR). The advances that have been made are applicable for any other subsidence monitoring project.

Since the start of hydrocarbon production in the Netherlands, the
Nederlandse Aardolie Maatschappij (NAM) has performed subsidence monitoring
over its gas and oil fields, following Dutch legislation and the commitment
to produce in a safe and responsible manner. Innovative geodetic acquisition
techniques for subsidence monitoring (GPS/GNSS, InSAR) have been actively
investigated and deployed over the past decades, as well as state-of-the-art
processing and geodetic testing techniques. The solid foundation that has
been established for subsidence monitoring has been reinforced towards the
future by incorporating the latest (scientific) developments. This paper
addresses recent results from the geodetic studies that are part of the
production plan of the Groningen gas field:

The deployment of Integrated Geodetic Reference Stations (IGRS).

The application of high-resolution InSAR.

Improvement of the InSAR stochastic model.

Comparison of GNSS processing methodologies.

Integrated geodetic processing of levelling, GNSS and InSAR measurements (concept).

To further improve the subsidence monitoring network over the Groningen gas field, NAM has deployed 25 Integrated Geodetic Reference Stations (IGRS) since 2018 (Hanssen, 2017; Kamphuis, 2019). The IGRS consist of a GNSS receiver, two InSAR corner reflectors (for ascending and descending tracks) and several levelling benchmarks, all mounted on the same deeply founded monument (Fig. 1). The advantage of the IGRS is that the accuracy of levelling, GNSS and InSAR deformation estimates can be cross-validated directly, since the measurements are with respect to the same monument and hence reflect the same deformation cause. Also, spatially-dependent biases and noise can be assessed and mitigated.

IGRS; design by Delft University of Technology.

The IGRS support minimizing subsurface uncertainties and optimizing future subsidence predictions. The density and location of the IGRS have been chosen such that the areas with the largest uncertainty in subsurface behaviour are captured (e.g. areas with potential aquifer depletion that do not contain wells). These areas are primarily located at the edge of the Groningen gas field, where horizontal movements are expected to be the largest. The IGRS target density was chosen such that the expected horizontal deformation signal can be reconstructed, considering the GNSS noise structure.

From the three geodetic techniques, only GNSS delivers all 3D deformation
components (East, North, Height) with high precision. GNSS measurement
precision (

Figure 2 depicts the latest analysis results of the vertical and horizontal
movements of the Groningen IGRS. Based on geomechanical predictions,
horizontal deformation rates are expected 0 in the center of the gas field,
and

Horizontal and vertical deformation in mm yr

Three TerraSAR-X tracks (two descending and one ascending) have been
processed by SkyGeo B.V. for NAM covering the time period 2013–2019 (Qin et
al., 2019). The high spatial and temporal resolution enables near-realtime
building and infrastructure stability monitoring. Furthermore, a more
detailed insight into horizontal movements is possible. However, due to the
almost north–south oriented orbit, only the east–west component of the
horizontal movements can be estimated well from the TerraSAR-X results.
Figure 3 shows the horizontal movements over the
Groningen gas field in the direction of the ascending look direction
projected on the horizontal plane (almost east-west oriented,

Horizontal deformation (mm) in the direction of the ascending look direction projected on the horizontal plane (arrow) for the time periods 2013–2019 and 2018–2019, including the location of the IGRS that are operational minimal from April 2018. The outlines of the gas fields are depicted in grey.

The high-resolution InSAR processing results have also been used to
investigate whether it is possible to discriminate between shallow and deep
(at hydrocarbon reservoir level) deformation. Two methodologies can be used
for this: separation based on the deformation histogram of neighbouring
Persistent Scatterers (PS), and PS separation based on height above ground
level. The latter turned out to be the most effective for high-resolution
InSAR and has led for the first time to a localized clear shallow
deformation pattern (up to 2 mm yr

Shallow compaction rates (mm yr

Despite the successful application of InSAR in practice, one of the main concerns is that the quality description of InSAR deformation measurements in terms of precision is not adequate. Often, the error structure is simplified such as neglecting the spatio-temporal correlation between InSAR deformation measurements. The unrealistic quality description could negatively affect decision-making based on the InSAR results.

In order to describe the quality of the InSAR data in terms of precision, it
is needed to mathematically describe the spatio-temporal variability of
noise components in the data. It should be noted that, in the context of
deformation monitoring and modeling, the term

In order to evaluate the spatio-temporal variability of the measurement
noise, an InSAR dataset over an assumedly signal-free area (

From the obtained timeseries of all the points in the selected area, the
spatio-temporal empirical variograms are computed using robust algorithms
(Cressie and Hawkins, 1980; Genton, 1998) in order to reduce the sensitivity
to outliers. The results are visualized in Fig. 5. By visual/qualitative
analysis of this figure, we can recognize three different types of behavior:
(i) a nugget effect (lower bound of

Spatio-temporal variograms computed from RadarSAT-2 InSAR data over the assumed stable area.

To combine these three effects in a generic model, the following exponential
variogram model has been used:

Estimated variogram model parameters.

Using the estimated variogram model, it is possible to construct the full
covariance matrix of measurement noise for the spatio-temporal InSAR data
(Yaglom, 1962). Note that the large volume of InSAR data (usually consisting
of tens of thousands of points with tens to hundred epochs) result in a huge
covariance matrix that is not practically useful due to the computational
and numerical limitations. In this regard, a proper data reduction is
usually required for InSAR data. Therefore, the covariance matrix of the
full dataset should be propagated to the covariance matrix of the reduced
dataset. In the context of InSAR processing in Groningen, reduction
techniques have been used based on averaging in time and space (e.g. see
Samiei-Esfahany and Bähr, 2015). As the averaging-based data reduction
can be formulated as a linear operator (e.g., as

With the proposed approach, a reduced InSAR dataset is delivered to the subsurface community, including its full covariance matrix, incorporating both spatial and temporal correlation of data measurement noise. The covariance matrix can be further used as a quality descriptor of the InSAR data, as well as a proper weight matrix in geomechanical and subsurface modeling.

Reduced InSAR timeseries over the Groningen area, with spatial
averaging over grids of

In the NAM GNSS processing methodologies project (Van der Marel, 2020) different methodologies have been investigated with the aim to obtain transparent time series estimates to support conclusions on subsidence rates with realistic confidence levels. The three different processing methodologies that have been investigated are: state-space modeling (SSR), baseline network processing (BSW), and Precise Point Positioning (PPP). An overview of the main characteristics of each method is given in Table 2.

GNSS processing methodologies. SSR processing has been
carried out by 06-GPS using Geo

Besides the NAM monitoring and NAM reference stations (of which the coordinates are kept fixed – with incremental updates – in the SSR processing), IGS and EUREF stations have been included in the BSW and PPP processing, as well as the Dutch AGRS and NETPOS stations in the BSW processing.

The time series results have been decomposed into components: a long term trend using a spline function, annual and semi-annual components, temperature influence, atmospheric loading, time series steps (e.g. due to antenna changes), and residuals. In the estimation of the temperature influence and atmospheric loading, temperature and pressure data from the Royal Netherlands Meteorological Institute (KNMI) is used. During a first iteration also two common mode components are estimated, the common mode in the residuals (residual stack), and common mode of the periodic parameters (harmonics, temperature influence, and atmospheric loading). For the estimation of the common mode a subset of the stations is used. The common mode is removed in the second iteration.

All three processing chains estimate, after removal of the common mode, a similar annual, semi-annual, temperature influence and atmospheric loading for each station. The periodic common mode signals themselves are however very different for each solution; the common modes in the BSW and PPP solutions are significantly larger than the common mode in the SSR solutions.

The agreement between the estimated trend signals of the BSW and PPP is
good, which is what should be expected because both solutions use ITRF2008
as reference frame. For the final results, the known tectonic motion of the
Eurasian plate has been removed in the horizontal component (conversion to
ETRS89), but heights stay in ITRF2008. This is because the conversion to
ETRS89 introduces a small extra vertical velocity component in the results
(

When comparing the BSW and PPP solutions with respect to the SSR solutions, the overall patterns in the time series are similar, however – for some stations – small deviations are present. Figures 7, 8 and 9 show the East, North and Height/Up time series for a selection of Groningen and Wadden Sea GNSS continuous monitoring stations, for the SSR and PPP solution. The BSW solution is similar to the PPP solution, but with a lower noise level.

The reason for the differences lies in the reference stations. The SSR solution is computed in a local reference network. The reference station coordinates are checked periodically by relaxing the coordinate constraints. In case movement is detected in one or more reference stations, the coordinates of the reference stations are updated. For the BSW and PPP, ITRF2008 is used as reference frame. For ITRF2008, reference stations are used that lie well outside the area of interest.

The results indicate that there may be a possibility to further optimize the procedure for the reference stations in the SSR solution. Instead of incremental corrections of a local set of reference station coordinates, the results of the BSW and/or PPP processing could be utilized to strengthen the solution over longer periods. However, a local reference network stays key for high precision local deformation monitoring.

GNSS time series East component (relative, mm), PPP (light) and SSR (dark) solution.

GNSS time series North component (relative, mm), PPP (light) and SSR (dark) solution.

GNSS time series Height/Up component (relative, mm), PPP (light) and SSR (dark) solution.

The available observations acquired by the different techniques (levelling, GNSS, InSAR, but potentially also gravity, tilt) are complementary to each other due to their spatial density and coverage, temporal density and coverage, and sensitivity (1D versus 3D). Because of this complementary nature, an integration of the techniques to generate an optimal output product is desirable. However, the differences between the techniques make this integration non-trivial. Conventional geodetic processing methodologies require for instance measurements at common locations (e.g., benchmarks). Therefore, geodetic adjustment and testing procedures are typically applied for each technique/dataset separately, followed by a final integration step.

The Integrated Geodetic Processing (IGP) approach enables the adjustment and testing of the various observation types simultaneously. Hereby, the complementary nature of the techniques is better used. The overall concept of the IGP approach is shown in Fig. 10. The approach meets a number of pre-defined requirements. For instance, the user is able to select the area and period of interest, together with the signal of interest (e.g., surface motion due to deep causes, shallow causes, or the total). Furthermore, various output products can be generated. Before the integration, each dataset is pre-processed to account for certain technique-dependent error sources, such as benchmark identification errors in case of levelling data. Each dataset is accompanied with its covariance matrix. Within the integration step, possible differences between the geodetic datums is accounted for.

Overall concept of the Integrated Geodetic Processing approach.

Multiple advances have been made in the recent years on monitoring subsidence due to hydrocarbon production in the Netherlands. Integrated Geodetic Reference Stations (IGRS) enable to cross-validate levelling, GNSS and InSAR independent of the deformation cause, and will contribute to minimizing subsurface uncertainties. The application of high-resolution InSAR has quantified shallow deformation components in the Groningen area. The comparison of different GNSS processing methodologies has strengthened the confidence in the information that can be derived from the measurements. The InSAR stochastic model has been improved to incorporate correlated noise structures in an efficient way. All these “ingredients” have consolidated the foundation for integrated geodetic processing for future subsidence monitoring.

Subsidence monitoring data that is publicly available can be accessed via

All authors have participated in the collaboration projects between NAM and Delft University of Technology, and contributed to the paper. Particular contributions have been made by SSE on the InSAR stochastic model, by HvdM on GNSS processing methodologies, and by SL on high resolution InSAR in cooperation with SkyGeo B.V.

The authors declare that they have no conflict of interest.

This article is part of the special issue “TISOLS: the Tenth International Symposium On Land Subsidence – living with subsidence”. It is a result of the Tenth International Symposium on Land Subsidence, Delft, the Netherlands, 17–21 May 2021.

The studies described in this paper have been made possible by the contributions from the following parties: high-resolution InSAR processing by SkyGeo B.V.; placement and processing of GNSS receivers by 06-GPS B.V.; GNSS regional network processing by the Dutch Cadastre, and GNSS Precise Point Positioning processing by Nevada Geodetic Laboratory.