Echometer Company
Model – M Manual
Phone: (940) 767-4334
5001 Ditto Lane
Page
5
Fax: (940) 723-7507
Wichita Falls, Texas 76302, U.S.A.
E-Mail:2-Principles of Acoustic Measurements
Acoustic liquid level instruments were developed in the 1930's. An acoustic wellhead attachment is connected to an
opening in the casing annulus at the surface of a well as shown in Figure 1 in the appendix. The acoustic wellhead
attachment consists of an acoustic pulse generator, a microphone and optionally a pressure gage. Throughout the
years, acoustic pulse generators have included a dynamite cap, 45-caliber blank, 10 gauge black powder blank, a
compression gas pulse and a rarefaction gas pulse. The explosive dynamite caps and blanks are a safety hazard and
have resulted in damage to wells and environment. While these explosive sources should not create a problem if the
casing annulus contains only hydrocarbon gas, major explosions have occurred when oxygen was allowed to enter the
casing annulus during work-overs or when special conditions resulted in a vacuum in the casing annulus
The versatility, economy and convenience of gas guns have resulted in widespread use of this type of acoustic pulse
generator. The expansion of gas from a volume chamber into the well generates the acoustic pulse. In most cases,
compressed CO
2
or N
2
gas is loaded into the volume chamber, which is charged to a pressure greater than the well
pressure. A valve in the wellhead attachment is opened rapidly, either manually or electrically, resulting in a pressure
pulse being generated in the casing annulus gas. The acoustic pulse travels through the gas in the casing annulus and
is partially reflected by changes in cross sectional area such as tubing collars, tubing anchors, casing perforations, etc.
The remaining pulse energy is then reflected by the gas/liquid interface at the depth of the liquid level. The reflected
signals travel back to the surface of the well where they are detected by the microphone.
The microphone within the wellhead attachment converts the reflected acoustic signal into an electrical signal
consisting of a series of pulses, which correspond to the sequence of reflections. The microphone must operate over
a wide pressure range from a vacuum to the maximum pressure that exists in the wells being tested. The microphone
should be designed to cancel the mechanical vibrations of the wellhead while remaining sensitive to the acoustic
signal reflections.
Recording and Interpretation of Signals
An amplifier/recorder filters and amplifies the electrical signal from the microphone and records the enhanced
signals on a strip chart. Modern instruments use analog to digital converters and microprocessors to improve the
signal quality and print the chart. The frequency content of the reflected acoustic signals varies depending on the
characteristics of the initial pulse, the pressure in the gas, the distance traveled and the type of cross sectional area
change. In general, as the pulse travels in a gas, the amplitude of the signal decays. The high frequency energy
decays more rapidly than the low frequency energy. Thus, the acoustic response from the collars at the top of the
well contains high frequency energy, the response from deep collars contains medium frequency and the signal
from the liquid level is mostly low frequency energy. This is especially apparent in deep wells with low casing
pressure. Fluid level instruments are designed to include various filters, which can be used to accent signals that
correspond to these frequency ranges. The Model M records the signal on the dual channels. One channel is tuned
to higher frequencies from the collars while the second channel is tuned to low frequencies from the liquid level.
Single channel instruments can be operated in any of these modes and it is possible to switch from one frequency
response to another while the instrument is recording. Initially, the single channel instrument is operated in the
collar mode (high or medium frequency), which is then switched to the liquid level mode (low frequency) when the
collar signal fades. Switching may be manual or automatic.