Horizon

An imaginary surface in the subsurface of the earth, usually thought of as representing a stratigraphic surface (either lithostratigraphic or chronostratigraphic).

Geoscientists often think of seismic horizons as geological horizons, or even as stratal surfaces, but this is an over-simplification. Seismic is prone to multiple reflections, interference effects, and distortion due to the velocity field.

Data-centric view


A horizon is a matrix of discrete samples in x,y,z that can be stored in a 3-column ASCII file. As such, a horizon is something that can be unambiguously drawn on a map, and treated like a raster image. Some software tools even call attribute maps horizons, blurring the definition further. The data-centric horizon is devoid of geology, and of geophysics; it is an artifact of software.

Geophysics-centric view
A horizon is an event, a reflection, in the seismic data; something you could pick with an automatic tracking tool. The quality is subject to the data itself. Change the data, or the processing, change the horizon. By this definition, a flat spot (a flattish reflection from a fluid contact) is a horizon, even though it's not stratigraphic. This type of horizon would be one of the inputs to instantaneous attribute analysis. The geophysics-centric horizon is still, in many ways, devoid of geology. It does not match your geological tops at the wells; it's not supposed to.

A surface picked on a 2D or 3D seismic survey is this type of horizon. If picked on a 2D grid, there is a lot of uncertainty in between lines. On a 3D, there uncertainty is generally about the same everywhere, though it is especially high away from well data points, or where seismic quality is poor.

Geology-centric view
A surface picked on well logs or core, then interpolated between data points, can be thought of as a horizon. Although often not stated, the surface is highly uncertain between data points.

A horizon is a layer, a surface, an interface, in the earth, and its manifestation in the seismic data. It is the goal of seismic interpretation. In its purest form, it is unattainable: you can never know exactly where the horizon is in the subsurface. We do our best to construct it from wells, seismic, and imagination. Interestingly, because it is, to some degree, not consistent with the seismic reflections, it would not be possible to use the geology-centric horizon for instantaneous seismic attributes. It would match your well tops, if you could build it. But you can't.

Reconciliation


Exactly how seismic horizons relate to geological surfaces could be one of the unsolved problems of exploration geophysics. It is another manifestation of the integration gap.

A four well model can help us illustrate the nuance between the geological and geophysical 'definitions'. Geological tops have been correlated across these wells, and used as input to a seismic model to study the changes in thickness of the Bakken Formation (green to blue) interval.

The synthetic model shows how the seismic character changes from well to well. Notice that a stratigraphic surface is not the same thing as a seismic event. The top Bakken (BKKN) pick is a peak-to-trough zero-crossing in the middle, and pinches out and tunes at either end. The top Torquay (TRQY), transitions from a trough, to a zero-crossing, and then to another trough.

This uncertainty is part of the integration gap. It is why building a predictive geologic model is so difficult to do. The word horizon can be a catch-all term; reckless to throw around. Instead, clearly communicate the definition of your horizon pick, it will prevent confusion for yourself and for other people who come in contact with it.