Fix wrong citations of Cordell (1992) (#518)

Replace wrong citations to Cordell (1992) in equivalent sources docs and
examples.

doc/references.rst

@@ -4,7 +4,7 @@ References
 .. [Balmino1973] Balmino, G., Lambeck, K., & Kaula, W. M. (1973). A spherical harmonic analysis of the Earth's topography. Journal of Geophysical Research, 78(2), 478-481.
 .. [BarthelmesKohler2016] Barthelmes, F. and Kohler, W. (2016), International Centre for Global Earth Models (ICGEM), in: Drewes, H., Kuglitsch, F., Adam, J. et al., The Geodesists Handbook 2016, Journal of Geodesy (2016), 90(10), pp 907-1205, doi:`10.1007/s00190-016-0948-z <https://doi.org/10.1007/s00190-016-0948-z>`__
 .. [Blakely1995] Blakely, R. (1995). Potential Theory in Gravity and Magnetic Applications. Cambridge: Cambridge University Press. doi:`10.1017/CBO9780511549816 <https://doi.org/10.1017/CBO9780511549816>`__
-.. [Cooper2000] Cooper, G.R.J. (2000), Gridding gravity data using an equivalent layer, Computers & Geosciences, Computers & Geosciences, doi:`10.1016/S0098-3004(99)00089-8 <https://doi.org/10.1016/S0098-3004(99)00089-8>`__
+.. [Cordell1992] Lindrith Cordell (1992). A scattered equivalent‐source method for interpolation and gridding of potential‐field data in three dimensions. GEOPHYSICS 57: 629-636. doi:`10.1190/1.1443275 <https://doi.org/10.1190/1.1443275>`__
 .. [Dampney1969] Dampney, C. N. G. (1969). The equivalent source technique. Geophysics, 34(1), 39–53. doi:`10.1190/1.1439996 <https://doi.org/10.1190/1.1439996>`__
 .. [Fukushima2020] Fukushima, T. (2020). Speed and accuracy improvements in standard algorithm for prismatic gravitational field. Geophysical Journal International. doi:`10.1093/gji/ggaa240 <https://doi.org/10.1093/gji/ggaa240>`__
 .. [Geosoft1999] Geosoft Incorporated. (1999). Montaj MAGMAP filtering; 2–D frequency domain of potential field data extension for Oasis Montaj v. 6.1.

doc/user_guide/equivalent_sources/block-averaged-eqs.rst

@@ -6,7 +6,7 @@ Block-averaged equivalent sources
 When we introduced the :ref:`equivalent_sources` we saw that
 by default the :class:`harmonica.EquivalentSources` class locates one point
 source beneath each data point during the fitting process, following
-[Cooper2000]_.
+[Cordell1992]_.
 
 Alternatively, we can use another strategy: the *block-averaged sources*,
 introduced in [Soler2021]_.

doc/user_guide/equivalent_sources/index.rst

@@ -76,7 +76,7 @@ Now we can initialize the :class:`harmonica.EquivalentSources` class.
  equivalent_sources
 
 By default, it places the sources one beneath each data point at a relative
-depth from the elevation of the data point following [Cooper2000]_.
+depth from the elevation of the data point following [Cordell1992]_.
 This *relative depth* can be set through the ``depth`` argument.
 Deepest sources generate smoother predictions (*underfitting*), while shallow
 ones tend to overfit the data.

examples/equivalent_sources/cartesian.py

@@ -21,7 +21,7 @@
 to predict data values wherever we want, like on a grid at a certain altitude.
 By default, the sources for :class:`~harmonica.EquivalentSources` are placed
 one beneath each data point at a relative depth from the elevation of the data
-point following [Cooper2000]_.
+point following [Cordell1992]_.
 
 The advantage of using equivalent sources is that it takes into account the 3D
 nature of the observations, not just their horizontal positions. It also allows

harmonica/_equivalent_sources/cartesian.py

@@ -51,7 +51,7 @@ class EquivalentSources(vdb.BaseGridder):
  * Analytical derivative calculations
 
  By default, the point sources are located beneath the observed
- potential-field measurement points [Cooper2000]_ that are passed as
+ potential-field measurement points [Cordell1992]_ that are passed as
  arguments to the :meth:`EquivalentSources.fit` method, producing the same
  number of sources as data points.
  Alternatively, we can reduce the number of sources by using block-averaged
@@ -105,7 +105,7 @@ class EquivalentSources(vdb.BaseGridder):
  Coordinates are assumed to be in the following order:
  (``easting``, ``northing``, ``upward``).
  If None, will place one point source below each observation point at
- a fixed relative depth below the observation point [Cooper2000]_.
+ a fixed relative depth below the observation point [Cordell1992]_.
  Defaults to None.
  depth : float or "default"
  Parameter used to control the depth at which the point sources will be

harmonica/_equivalent_sources/gradient_boosted.py

@@ -47,7 +47,7 @@ class EquivalentSourcesGB(EquivalentSources):
  Coordinates are assumed to be in the following order:
  (``easting``, ``northing``, ``upward``).
  If None, will place one point source below each observation point at
- a fixed relative depth below the observation point [Cooper2000]_.
+ a fixed relative depth below the observation point [Cordell1992]_.
  Defaults to None.
  depth : float or "default"
  Parameter used to control the depth at which the point sources will be

harmonica/_equivalent_sources/spherical.py

@@ -48,7 +48,7 @@ class EquivalentSourcesSph(vdb.BaseGridder):
  * Analytical derivative calculations
 
  Point sources are located beneath the observed potential-field measurement
- points by default [Cooper2000]_. Custom source locations can be used by
+ points by default [Cordell1992]_. Custom source locations can be used by
  specifying the *points* argument. Coefficients associated with each point
  source are estimated through linear least-squares with damping (Tikhonov
  0th order) regularization.
@@ -75,7 +75,7 @@ class EquivalentSourcesSph(vdb.BaseGridder):
  (``longitude``, ``latitude``, ``radius``). Both ``longitude`` and
  ``latitude`` must be in degrees and ``radius`` in meters.
  If None, will place one point source below each observation point at
- a fixed relative depth below the observation point [Cooper2000]_.
+ a fixed relative depth below the observation point [Cordell1992]_.
  Defaults to None.
  relative_depth : float
  Relative depth at which the point sources are placed beneath the