This software simulates the air consumption and air storage volumes while shooting VSP and Walkaway VSP surveys. Two computations are done. The first computation shows the results on a shot-by-shot basis, so the time interval for each computation step is the time between shots. The second computation display pressure's and volume's computed at a regular time interval of 1 sec. For every one second of elapsed time, the gun usage and air compressor recharge is computed.
An example is shown below. The two plots on the left are plotted by “shot number” and shows instantaneous values of pressure and volume at the time of each shot. The two plots on the right are plotted by elapsed time and show continuous curves of pressure and volume with time.
VSPair is a 64 bit application, and is available for download from the Microsoft Store for Windows-10.
Example of the air storage volume and pressure plots.
- The two plots on the left are storage volume and the two plots on the right are storage & airgun pressures.
- The red curves/points are the storage volume and pressure, while the blue points represent the instantaneous regulated airgun pressure at the time of the airgun firing.
- Individual plots can be selected by clicking on the tab immediately above the plots.
Toggling on the "hamburger" button in the upper left corner exposes the parameter menu as shown below. Links to this webpage are also found in the lower left corner.
SURFACE HARDWARE CONFIGURATION
Three menu panels are available for setting the parameters of the air compressor, external air tanks and the air guns.
A number of permutations of hardware configurations are possible and include:
- Air Compressor + Airgun Cluster.
- External Air Tank + Airgun Cluster.
- Air Compressor + External Air Tank + Airgun Cluster.
- Air Compressor only.
- Air Compressor + External Air Tank.
The last two items in the list above don’t have an airgun cluster included so are not valid survey options, but could be used to determine how long it would take to re-charge the air tanks from empty.
AIR COMPRESSOR MENU
Two pre-configured air compressors are selectable from the drop down list, and the parameters for these are automatically updated when either is selected. If a parameter is later changed to a non-standard value, the compressor type becomes "Other compressor".
The units for discussing air storage and air pressure are often mixed between various unit systems. For example, volume can be referred to as units of cu.ft, cu.m or litres. When changing the values of a parameter, select the box corresponding to the desired unit and change the value. The parameter values in the other unit systems will be automatically updated at the same time.
If an external tank has been selected and the pressure rating of the external tank is less than maximum pressure of the air compressor, the air compressor is down-rated.
If the parameters have a beige background color, then means these are fixed values and cannot be changed. They can only be changed, when "Other Compressor" is selected.
EXTERNAL TANK MENU
A number of external air storage tanks are selectable from the drop down list, and the parameters for these are automatically updated when either is selected.
When both an air compressor and an external tank are selected together, the “pressure” box in the External Tank menu, will always show the same value as the pressure box in the Air Compressor menu.
If both an air compressor and an external tank is declared and the pressure rating of the external tank is less than maximum pressure of the air compressor, the air compressor is down-rated.
If the pressure rating of the external tank is greater than that of the air compressor, the pressure from the air compressor is used.
If an air compressor is not used, the pressure is entered via the “Fill Pressure” boxes in the External Tank menu. The pressure and fill pressure boxes are automatically allowed/disallowed depending on whether an air compressor has been selected or not.
There are also two options for external Nitrogen tanks. If either of these is selected, then if an Air Compressor has also been selected, it will be automatically de-selected.
If the parameters have a beige background color, then means these are fixed values and cannot be changed. They can only be changed, when "Other Tank Rack" is selected.
Both 150 and 250 cu.in airguns in various clustering configurations are selectable, and the parameters for these airguns clusters are automatically updated when one is selected.
If the parameters have a beige background color, then means these are fixed values and cannot be changed. They can only be changed, when "Other Cluster" is selected.
The Airgun Factor (AGF) takes into account that the airgun may not expel an air volume equivalent to the chamber volume. For example, the shuttle may close before all the air is expelled (AGF < 1.0), or the airgun may blow extra air through the hoses and out the gun ports before the shuttle closes (AGF > 1.0). This factor is dependent on the gun design and manufacturer.
As an example, for a single 150 cu.in G-gun at 2000 psi pressure, the theoretical air volume per shot is given by 150*0.0005787*(2000/14.504) = 11.97 cu.ft.
However, the G-Gun manual gives the formula SCFM = 2.39*P*(V+13)/(1000*C), which can be re-written as SCFpershot = 2.39*P*(V+13)/(1000*60). This gives 2.39*2000*163/60000 = 12.98 cu.ft.
To compensate, for a 150 cu.in G-Gun at 2000 psi in various clustering configurations, we can apply an Airgun Factor (AGF) of 12.98/11.97 = 1.09.
By similar reasoning, the AGF for the single/double/triple 250 cu.in airgun clusters at 2000 psi is 1.07.
The airgun factor is selectable and allowable values for Airgun Factor in the software are from 1.0 to 1.2. The AGF can be adjusted by the user. A reason for adjusting this would be to calibrate the computed air usage per shot to an already known value of air usage.
Survey modeling can be done for either VSP or Walkaway VSP survey configurations by selecting the appropriate tab.
The parameters for VSP are for a single survey acquired at a constant acquisition rate.
- The “Number of Levels” refers to the number of “array settings”, in the case of a multi-shuttle downhole seismic tool.
- The “Firing rate” is the time between shots while shooting a sequence of shots at one level.
- The “Time per level”, is the total time required to anchor the tool, shoot the shots, and move to the next depth.
WALKAWAY VSP SURVEY
The parameters for Boat Speed, Shot Spacing and Fire rate are inter-related. Changing the Shot Spacing will change the Fire Rate, whilst changing the Fire Rate will change the Boat Speed and vice versa.
- If the Line length is not equal to an integral number times the Shot spacing, then the Line length is adjusted to fit. This check is done when setting either the line length or the shot spacing.
- Multi-line Walkaway VSP's of equal line length are supported.
- The “Idle time between lines” is used to re-charge the air tanks when shooting multiple Walkaway VSP lines.
- If “No. of Lines” >=2, then the "Edit Lines" button becomes available.
- By default, all lines are of equal length of 6000 metres.
- Clicking the "Edit Lines" will bring up a form for defining individual line lengths and idle times.
Options are available to start the survey with empty tanks, and also to recharge the tanks at the end of the survey, for estimating the time for each of these operations. Note that both these options only pertain to the two time-indexed graphs. They will have no effect on the shot-indexed graphs.
Volume: as measured at atmospheric pressure. It would be the amount of water that fits into a tank.
- Pressure: as measured by a pressure gauge. Atmospheric pressure is 14.7 psi, but a pressure gauge will measure it as 0 psi, which is also known as Gauge Pressure. The pressure’s referred to in the VSPair software are Gauge Pressure’s
- Storage (or Storage Volume): is the air available for recharging the airguns and is equal to (volume*pressure)/14.503, where the volume and pressure are as defined above. This equals zero when the tank pressure is at atmospheric pressure. 1 bar = 14.50377 psi.