Well tie — g-Space
Well tie

Well tie software

Put the well and the seismic on the same time axis. g-Space builds the synthetic from your sonic and density curves, gives you four ways to arrive at the wavelet, and scores every adjustment against the seismic trace at the well — then saves the calibrated time-depth relationship back onto the well as ordinary log curves.

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Four wavelets
Theoretical to log-derived
Anchor & auto-shift
Tie points, best correlation
Drift & residual
Two independent QC curves
Well tie view in g-Space with sonic and density tracks, a reflectivity curve, the synthetic overlaid on the seismic gather, the wavelet and its spectrum, and the cross-correlation function
Well tie workspace in g-Space with the Well tie ribbon bar, a location map, the well tie view and the checkshot table showing base and result time-depth values
In view
Well tie view & cross-correlation
A tie is an argument between two measurements. The logs know depth precisely and time only through the checkshot; the seismic knows time precisely and depth not at all. g-Space puts the two side by side in one view — logs, reflectivity, synthetic and the real seismic gather at the well, on a shared time axis — and every anchor you move is answered immediately by the cross-correlation function, the drift curve and the residual trace. When the argument is settled, the calibration is written back onto the well as log curves, so the rest of the project inherits it rather than re-deriving it.
Building the synthetic

From logs to a trace you can compare.

The synthetic is generated from the curves you nominate and displayed against the seismic nearest the well — with the reflectivity series, the interval velocities and the time-depth table all on screen while you work.

Sonic, density, impedance
Choose the inputs the well actually has — sonic alone, sonic with density, or an impedance curve — then pick the named curves from the well's own LAS logs. An anti-aliasing filter is applied by default.
No checkshot? Build one
A checkshot can be created from the sonic log by integrating its interval transit times. Because the sonic rarely reaches the surface, you set a first velocity value for the section above its shallowest sample.
Reflectivity as a curve
The reflectivity series computed for the synthetic is written back to the well as its own LAS curve on a regular depth step, so it can be displayed, exported and reasoned about like any other log.
The seismic nearest the well
Set a search distance around the well head and g-Space offers the lines and volumes inside it, reporting how far the nearest one lies; a trace count decides how much seismic is drawn either side of the well.
Tracks you arrange
Two-way time, measured and true vertical depth, interval velocity, the logs, the reflectivity, the synthetic and the seismic each get a track, and the view opens scaled to the interval the synthetic actually covers.
Horizons on the tie
Time-domain horizons are drawn across the seismic track, following the reflections trace by trace in their own colours rather than as a flat line at the well — so the tie is judged against the interpretation.
Residual trace
The synthetic minus the seismic at the well is generated automatically alongside the synthetic. A flat, low-amplitude residual is the quickest visual statement that the two agree.
Settings stay with the well
The seismic selection, the log choices and the correlation window are remembered per well and saved with the project, and the whole display arrangement can be stored as a well-tie template.
Wavelet analysis

Four routes to the wavelet, one window to judge it.

The wavelet is where most ties are won or lost, so it gets its own non-modal window: the wavelet and its controls on one side, the amplitude and phase spectra on the other, and the rest of g-Space still usable while it is open.

Theoretical
Ricker, Gaussian, Ormsby

An analytic wavelet built from the parameters you type — the starting point when you want a clean, known shape before letting the data have a say. A Ricker wavelet is what the tie uses until you change it.

Statistical
Estimated from the seismic

Estimated from the dataset you select: from the trace at the current well, or from every trace inside an inline and crossline range on a 3D volume, or a CDP range on a 2D line.

Extracted
Taken from chosen traces

Extracted with a single constant phase or with the phase left as the data has it, from the trace at the well head, an average of the traces within a radius, the composite trace along the wellbore, or one inline and crossline you name.

From well log
Derived at the well

Built from the current well's own sonic and density logs together with the seismic trace at the well, so the wavelet and the reflectivity it will be convolved with come from the same place.

Shape
Length, phase, spectrum

Set the wavelet length, rotate the phase by hand or estimate it from the data, restrict the time interval the estimate is taken from, and read the amplitude spectrum with the phase curve overlaid.

Reuse
Saved and re-run

Update refreshes the preview, Apply refreshes the synthetic, and a wavelet can be saved by name. The same inputs and parameters can instead be stored as a job in a project workflow and run later.

Workflow

Six steps from loaded logs to a calibrated well.

1
Check the data and the datum
Load the seismic and the well — checkshot, sonic, density, impedance if you have it — and confirm the seismic reference datum and replacement velocity before anything is compared.
2
Open the tie workspace
One click on the Well tie bar arranges the well tie view and its cross-correlation view; add a location map so the well being tied is the one you meant.
3
Generate the synthetic
Nominate the sonic and density curves, set how much seismic to show around the well and how wide the synthetic should be, and lay the tracks out the way you read them.
4
Settle the wavelet
Try a theoretical shape, an estimate from the data and one derived from the logs, compare their spectra, and set a correlation window of at least two to three wavelet lengths.
5
Anchor the reflections
Double-click a reference reflection to drop an anchor, drag it into place, and use auto-shift to find the position that maximises the cross-correlation coefficient. Several anchors over a long interval beat one.
6
Save the calibration
Write the calibrated time-depth model back onto the well, keep the synthetic and the reflectivity as LAS curves, and let horizon picking, velocity modelling and depth conversion work from them.
Tie & outputs

Numbers to judge it by, curves to keep.

The tie is not finished when it looks right — it is finished when the correlation, the drift and the residual all agree, and the result is stored where the rest of the project can read it.

Judging the tie
What the workspace reports back while you edit
Cross-correlation
A dedicated view plots the correlation function between the synthetic and the real trace; its zero-lag coefficient is the single number the tie is argued over.
Correlation window
Start, end and lag are set on the bar. The manual's guidance is a window of at least two to three wavelet lengths for a meaningful result.
Auto-shift
Adjusts the tie automatically to the position of maximum cross-correlation, either over the whole interval or inside a shorter one you have narrowed down.
Drift curve
The difference in two-way time between your edited checkshot and the base checkshot, recomputed on every edit — how far the calibration has moved from where it started.
Residual trace
Synthetic minus seismic at the well, over the synthetic's own footprint. Flat and low amplitude is a good tie; structure in the residual is where to look next.
Time-depth table
Base and result depth and time columns side by side, so each anchor's effect on the time-depth relationship is visible as a number, not just as a shift on screen.
What the tie writes
Everything the finished tie leaves on the well
Calibrated T–D model
Saved from the edited anchors as two LAS curves on the well: a calibrated two-way time curve in milliseconds and a calibrated interval velocity curve in metres per second.
Drift curve
Stored as a LAS curve in milliseconds under the well, so it can be reviewed or exported alongside the logs it was derived from.
Reflectivity curve
Written as a per-well LAS curve indexed by measured depth. Running the action again overwrites it in place rather than accumulating versions.
Synthetic to LAS
The synthetic trace itself saves out for use and comparison outside the tie workspace.
Checkshots in and out
Tab-delimited checkshot tables and Petrel checkshot files import; export writes a well's checkshot at a constant step, at markers or at depths you pick. Currently on the KB datum.
Seismic onto the well
Extract seismic trace resamples the nearest trace along the wellbore into a LAS curve — for one well, or for every well under a folder in a single run.
Views and tools in this group
The parts of g-Space this topic is built from
Well tie bar Well tie view Cross-correlation view Wavelet analysis Wavelets Checkshots Reflectivity curve Drift curve Residual seismogram Extract seismic trace
More g-Space capabilities

The tie is what makes the rest trustworthy.

Every horizon named after a formation, every depth-converted map and every velocity model leans on a well that was tied properly. These are the other topics in the g-Space workflow.

g-Space overview Geological interpretation Horizon picking Fault picking Seismic mis-tie Velocity modeling Petrophysics Attribute analysis & AVO Geological modeling Reservoir characterization
FAQ

Questions, answered.

What data does a well tie need?
Seismic data — a 2D line or a 3D volume near the well — and the well itself: a checkshot for the time-depth relationship, a sonic log, a density log, and optionally an impedance curve. A well with a sonic log but no checkshot is not stuck: g-Space builds one by integrating the interval transit times, with a starting velocity you supply for the section above the first sonic sample. Checkshots also import from tab-delimited tables and from Petrel checkshot files, currently on the KB datum.
Where does the wavelet come from?
Four routes, chosen in the Wavelet analysis window. Theoretical builds a Ricker, Gaussian or Ormsby wavelet from parameters. Statistical estimates one from the selected seismic — the trace at the well, or every trace inside an inline and crossline range you set. Extracted takes it from the data at a chosen location, with constant or variable phase. From well log derives it from the well's own sonic and density curves together with the seismic trace at the well. Length and phase are adjustable, the amplitude and phase spectra are plotted beside the wavelet, and any wavelet can be saved by name for reuse.
How do you tell a good tie from a bad one?
Three independent readings. The cross-correlation view plots the correlation function between the synthetic and the seismic trace, and its zero-lag coefficient is the headline number — the manual recommends a correlation window of at least two to three wavelet lengths. The drift curve records the difference in two-way time between your edited checkshot and the original one, so you can see how far the calibration has moved. The residual trace is the synthetic minus the seismic at the well: flat and low in amplitude means the two match.
What does a finished tie leave in the project?
A calibrated time-depth model, saved from the edited anchors as two LAS curves on the well — a calibrated two-way time curve in milliseconds and a calibrated interval velocity curve in metres per second. The drift curve is stored as a LAS curve in milliseconds as well, the reflectivity series can be written as its own log curve indexed by measured depth, and the synthetic itself saves to LAS. Because they are ordinary well curves, everything downstream — horizon picking, velocity modelling, depth conversion — reads them like any other log.
Get started

Tie your own wells to your own seismic.

Load a checkshot, a sonic and a density curve, build the synthetic and watch the correlation move as you anchor it — take g-Space for a trial run, or talk to Geomage about a demo on your data.

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