7. Log Plot
LasViewer software can be used for viewing and plotting oilfield well-log data and waveform data that is stored in either LAS, CSV, LDF or SEG-Y format files.
Some basic processing functionality for both Log data and VSP data is included.
LasViewer is a 64 bit application for computers running Windows-10. It also requires the Windows-10 version 1809 or later. Version 1809 is the Sept-2018 update (10.0.17763).
It can be downloaded from either of the two locations given below:
- the Microsoft Store for Windows-10, at
- or from the download link on this webpage http://www.starseis.com/publish_LasViewer (this also requires a certificate to be installed. see below)
The Microsoft store is the preferable link, but it may not be available to everyone, in which case try the second link.
If downloading from the webpage, a certificate has to be installed first before Windows will let the installation start. The certificate can be downloaded from the Additional Links on the download page. Once downloaded, click on the certificate and install it to the Local Machine under the folder called "Third Party Root Certification Authority".
LAS files are ASCII files that are used to store and share oilfield well-log data.
CSV or comma separated value files, are simplified ASCII files that contain only log data, and no other information. The format required for CSV files is specific to this application and discussed in a later section.
LDF and SEG-Y files are used to store seismic waveform data (VSP waveform data).
Loading Log Data
Log data can be loaded either from LAS or CSV format files.
In most cases, log data will be loaded from LAS format files, and this can be done by either selecting the File->Load LAS File option under the main menu. Alternatively, just drag and drop the file onto the data-list area. Drag and drop supports loading multiple files with one action.
Once a LAS file has been loaded, all the curves in the file will be shown in the data-list on the left side. The hierarchy of the data-list is shown below.
- Company Name
- Well Name
- Dataset Name (LAS file)
All LAS files from the same well will be grouped together. To view the data from another LAS file, click on the LAS filename in the data-list.
Clicking on either of the Company Name or Well Name will display the information about the well that was loaded from the LAS file.
The current version supports LAS version 2.0 files. A valid LAS version 2.0 file contains five sections that include the ~Version, ~Well, ~Curves, ~Parameters and ~Ascii sections. These are also referred to as the ~V, ~W, ~C, ~P and ~A sections. The ~P section optional, all the other sections are mandatory.
Right clicking on the Dataset (LAS filename) also gives options for viewing the raw data from the ~V, ~W, ~C and ~P sections of the LAS file header. Note that the "Right Click" menu is not available until a normal click has first been performed on the Dataset.
Log data can also be loaded from CSV files (comma separated value files). These can be made or edited with either Notepad or Excel. A common use for CSV files would be for loading deviation data, which would typically not be available in LAS files. The format for the CSV must be as follows:
- The first lines contains the Curve names.
- The second line contains the Units of each curve.
- The third and subsequent lines contain the curve data.
- There must be an equal number of values on each line.
- Spaces can be included, and will be ignored during loading.
An example showing the first 7 lines of a CSV is given below.
... etc ...
CSV files do not contain any information about the well name or company name, and this has to be specified upon load. As such it may be easier to load other Log data first via a LAS file; this way, the company and well-name structure will be created automatically from the LAS file data, and does not have to be re-entered manully when subsequently loading a CSV file.
Log Data Direction
The Log data in LAS and CSV files will either be written in the file as a "down-log", where the depth values are increasing, or as an "up-log" where the depth values are decreasing. Most processing options will work with the data in either direction, but some processing may require the data to be in a specific direction; for example Minimum Curvature TVD computations requires the data to be a down-log.
The log data direction can be flipped in the "Copy-a-Dataset" process.
By default, the data is loaded "as-is", but there is an option to convert the data to down-log, upon loading. If there is no reason to keep the data in the original logging direction, it is probably best to change this preference to "convert to down-log upon load". It will make some later processing options quicker, and avoid the need to flip data at a later date if required by a specific processing option.
Log Sample rate
Log data can either be regularly sampled, irregularly sampled, or mixed up-down sample rate. The latter should never exist, but is encountered from time to time. The log data is scanned upon load to determine what it is.
- If regularly sample, the file name is in Blue colour.
- If irregularly sampled, the file name is in Purple colour.
- If mixed up-down or with repeated depth levels, it will be coloured Purple and labelled as Mixed.
Some processing options require regularly sampled data, such as the moving average and median filtering.
The window directly to the left of the main form is the QuickView plot. After a LAS file is loaded, the first curve in the list which should be the Depth curve, will be displayed here. Clicking on a curve name in the data-list list will cause that curve to be displayed in the QuickView plot.
If the "Alt" key is depressed while clicking on a curve name, the curve data will be displayed instead.
The curves are displayed on the QuickView plot with a fixed format. The depth is scaled so that the whole data set fits on the page. Log curves are displayed with a pre-defined format. Log curves can wrap once, when they exceed the plot scale. A curve will change color to pink, when it is wrapped.
For log curves that don't have a format that is pre-defined, these are displayed with a linear scale and scaled so that the minimum and maximum values fit into the window. They are also plotted with a gray curve color.
Right clicking on the QuickView plot gives an option for copying the current image, so it can then be pasted into another document.
The figure above shows two features available after loading the data. On the left is a data view that is obtained by holding the "Alt" key down when clicking on a log channel name. The drop-down menu that appears when right-clicking on either the Company, Well and Dataset name is shown on the right.
Right clicking on the data-list give options for deleting a Dataset, for deleting all the data from a Well, and for deleting all the data from a Company. There must always be at least one Dataset left in a well, and always one Well left in a Company. To delete these items, simply select item that has a higher heirarchy.
Individual channels in a LAS can be deleted the same way. To delete multiple channels, select the first channel as normal, then hold down the control key to select subsequent channels. Finally, right click on the highlighted channel and select "delete".
Log Data Units
The depth units are specified inside the LAS file, and the file will be loaded referenced to those depths units. The depth unit can be changed after loading by right clicking on the data-set, and selecting the "change depth unit" option.
Log Curve Units.
It is a simple matter to convert the units of a log curve by right clicking on the curve name, select the "Change Unit" option, then choose the desired unit.
The unit of the depth channel, which will be the first curve in the list, cannot be changed with this method. The depth unit is changed by changing the depth unit of the entire data-set as described previously.
Saving all the data to a Project file
All the data that has been loaded into LasViewer can be saved to project file.
The option to do this can be found in the Main menu under the File option. This saves the current internal database to a disk file, which can then be re-loaded for further analysis, re-processing and plotting at a later date. Only the data is saved; the plots are not saved, and will need to be re-created.
The "Log Plot" panel is on the right side of the main form is for making composite log plots and cross-plots.
The Log Plot panel is used for making multi-column plots and allows user defined formatting. As with the QuickView plot, the curves are initially displayed using pre-defined formatting options, but these can be easily changed later.
Right clicking on a specific track of the plot shows a drop-down menu. Some of these menu options are track dependent, while others apply to the entire plot.
Drop down menu options from the Log Plot panel
- Add track - adds a new log track to the right of the last track.
- Insert track - inserts a new track immediately to the right of the currently selected track.
- Delete track - deletes the currently selected track.
- Add Log Curve - presents a selection of log curves to be added to the currently selected track.
- Clear Log Curves - deletes all log curves from the currently selected track.
- Change Track width - by default, a log track is 200 pixels wide. This menu offers alternatives values for the track width.
- Change Log Plot parameters - to change the plotting scale and other parameters for all the logs displayed on the currently selected track.
- Depth axis scale - by default, the depth scale is chosen to fit all the data in the display window. The menu option allows change the depth scale.
- Copy Image - copies the current plot to the clipboard, so it can be pasted into another document.
- Print - prints the current image. Printing to a PDF file is a good option, to save the log plot to a disk file.
- Reset (delete all) - deletes all the tracks and plots reverts back to initial conditions.
On the right side of the Log Plot panel is the Cross-plot button. Clicking this will bring up a new window that allows cross-plotting curves of the currently selected data set.
The crossplot thta is displayed is linked to a dataset, so if another dataset is selected, then the crossplot associated with that dataset will be displayed. Reverting back to the earlier dataset will bring the associated crossplot back into the crossplot window.
Some basic log processing functionality is available. To use, first click on a well-name, log data-set or a log curve name in the data-list, then the Log Process tab on the far right will become enabled. Click on this tab, and the Log Processing window will appear.
The Log Processing window will be focused to the current Well and only datasets with-in this well are accessable. To access data from other wells, close the Log Processing window, select another well in the data-list, then re-open the Log Processing window.
Next, click on the processing option required, select the data-set and/or log curve name. Choose parameters if required, and click Compute. The processing result can then be found in the data-list.
Converting Logs to a TVD depth Index
TVD depth conversion is available in the Log Processing menu.
Converting logs from a Measured Depth index to a TVD depth index is a two step process. The first step is to take a series of Deviation and Azimuth measurements which may be sampled sparsely and irregularly along along the wellbore trajectory and compute a TVD value at each of these Deviation/Azimuth depth measurements.
Ideally, these Devi/Azim measurements should extend all the way up to the surface; this way, the (X, Y, Z) tie-in will be (0, 0, 0). If the Devi/Azim measurements only start half way down the well, then a TVD tie-in value is required, together with the X and Y coords of the top measuremnts of the deviation survey.
The Step-1 process is shown below. The left panel shows the deviation survey, the central panel shows the processing options for the TVD computation. The third panel shows the results from the computation, which are appended to same file as the input data. The sampling rate of the input data in this example was approximately 100 metres, and the results are computed at the same sampling rate.
The second step is to take the TVD result of the deviation survey computed above, and use this to re-index an MD indexed log dataset to a TVD depth index. This is shown in the figure below. First the "Re-index a dataset to TVD" process is selected. The deviation survey dataset is selected in the first drop down list and the dataset to converted to TVD index is selected in the second drop-down list. Finally the TVD curve from the deviation survey is selected, and the Compute button then clicked.
There are two conversion options. The first options simply comnverts the dataset to TVD without any re-sampling or interpolation; that is, it will map the MD values direct to TVD depth. If the input logs are at 6 inch sample rate and the well deviation is 45 degrees, then the resulting TVD sample rate will be 6 * Cos(45) = 4.24". However, since the well devaition is never constant, then the effective sampling rate will be variable. Since no interpolation is done, the log curve values will be unchanged.
The second option (default) will re-sample the data to a TVD sample rate that has the same value as the MD sample rate. For example, a 6" MD rate goes to a 6" TVD rate. An un-avoidable consequence of this method, is that there will be a slight loss in resolution of the TVD converted data.
Loading VSP waveform data
VSP waveform data can be loaded from files in either LDF format or SEGY format. The process of loading and displaying LDF and SEGY files is similar to LAS files discussed above.
One minor complication with LDF files and SEGY files, is that these files do not contain any information on the Company name or Well name inside the file. Therefore, this information has to be specified upon load, and a window will appear before loading to capture this information. If other data has already been loaded, then the existing Company and Well names will be offered as choices.
And another complication with SEGY files, is that this format was not designed to support VSP data and a standard method of saving VSP data has never been determined. So different companies have adopted different ways to use SEGY format with VSP data. In most cases, the waveform traces will be loaded correctly, but the waveform headers which contain the depths, coordinates and other pertinent information may not be loaded correctly. Some common types of SEGY VSP files known to the author are listed as options, and the SEGY file type needs to be selected prior to the file being loaded. Extra types of SEGY files can be considered upon notification to the author.
An option to preview the SEGY file headers prior to loading the data is available, to assist in determining the best method to load the SEGY data.
The QuickView plot will only show a depth-time plot. The waveforms can be viewed from the Log Plot panel.
When an LDF or SEGY file is loaded, an attempt is made to determine what type of waveform data it is. Common types are X, Y or Z component waveform.
The "waveform type" information cannot be found inside an LDF or SEG-Y file, and it is guessed from information found in the filename. For example, if the filename ends with a "Z", or has "_Z" anywhere in the filename, it will classified as a Z component. This classification can be seen after the data-list listing.
If the classification has not been determined, or is incorrect, then it should be corrected. This can be done by right-clicking on the filename, select "waveform type", then select the appropriate value as shown below.
Confirming the correct waveform type is important for some later processing steps, since it will assist in quickly finding the correct waveform files for certain processing options.
VSP Waveform Processing
Similar to the Log Processing described above; when a waveform file has been selected, the VSP Processing tab on the far right will become enabled. Click on this tab to bring up the VSP Processing window.
Waveform processing option that have been implemented include, Butterworth filter, FIR filter, Hilbert Transform, Frequency Spectrum, Q-Analysis, 3-C rotations and transit time picking. The transit time picking algorithm will be expanded to include extra options in a future release.
26-May-2018. First release
18-Sept-2019. Second release