One of the main factors in the oil and gas industry that influences safe drilling and operation is depth uncertainty. Depth uncertainty simply means that there’s no way to know for sure what’s below the surface of the earth until you start drilling and see what you find. The natural result of this is that oil and gas companies have to do extensive geotechnical studies on their drilling sites in order to get as much information as possible about where, exactly, they should be drilling in order to reach their targeted resources.

Surface Based Methods

While seismic reflection, refraction, and scattering surveys provide important subsurface information; depth uncertainty of these data always exists. An additional method can be used to help reduce depth uncertainty when making 3D interpretations from 2D seismic data. This method uses seismic impedance (as measured by either VSP or DST) as a means to correlate multiple adjacent 2D sections into an equivalent 3D dataset that captures more of vertical space. The technique is known as Seismic Inversion.

Gravity-Based Methods

In a so-called gravity survey, data is recorded by setting GPS receivers at various elevations above sea level and walking away. The gravity map generated for any given area will show areas with higher density of mass (above sea level) as being lighter on a computer display. For example, below-sea-level salt domes will appear brighter because they are denser than surrounding rocks or water.

Seismic Reflection Method

The seismic reflection method is a relatively old technique of depth determination. In one way, seismic reflection depth conversion is quite straightforward: use seismic waves to determine how deep features are at different points. However, because most geological structures are not simple or uniform, there are complexities to applying depth determination using reflections that must be accounted for. Two common strategies for depth determination using reflections are known as ray-tracing and scaling methods.

Direct Sound Phase Method

Here we’ll explain what depth uncertainty means to people who work in seismic data processing. For petroleum geologists, however, understanding depth uncertainty might mean avoiding a little bit of confusion when interpreting geophysical sections. So before we get into how exactly direct sound phase relates to depth conversion, let’s take a look at how geophysicists measure seismic data. Because unlike reservoir engineers, they don’t have outcrops on which to tie their interpretations.

Inferred Measurements

Seismic measurements are used to identify objects on or below Earth’s surface, but every time seismic data is interpreted, there exists a depth conversion error. When converting from surface seismic measurements to a subsurface value of interest (such as thickness), a process called depth uncertainty must be considered. In an attempt to address problems arising from seismic interpretation, seismic inversion has been employed for many years by geophysicists.

Combined Methods – Seismic and Inferred Measurements

Seismic Inversion Seismic measurements give us a way to explore for new oil and gas reserves. In principle, each layer of rock has a distinct seismic signature because different types of rocks can have very different densities, acoustic properties, and porosities. The seismic velocity (Vp) of sound through rock layers ranges from around 1 km/s to 9 km/s or greater; these values depend on many factors such as density, acoustic impedance, pores within that rock.