The borehole gravity meter (BHGM) can be described simply
as a deep-investigating, density logging tool. Application
range beyond this simple description to include detection of
oil and gas-filled porosity, detection and definition remote
structures (e.g. salt domes, faults, reefs, etc.) and
non-Newtonian gravity experimentation.

One of the great advantages of the BHGM as a density
logging tool is that it is practically unaffected by nearhole
influences which are the scourge of nuclear tools: casing,
poor cement bonding, rugosity, washouts, and fluid invasion.
Another advantage is the fundamental simplicity of the
relationships between gravity, mass, rock volume and density.
Complex geology can be easily modeled so that the response of
a range of hypothetical models can be studied and understood
before undertaking a survey.

What is actually measured is referred to as BHGM **apparent
density*** *which is a simple function of
the measured vertical gradient of gravity. To obtain an **apparent
density** measurement, gravity is measured at two
depths. The accuracy of the computed density depends on the
accuracy of both measured differences: gravity and depth.
Operationally, BHGM surveys resemble VSP (vertical seismic
profiling) surveys. The BHGM is stopped at each planned
survey level and a five-to-ten-minute reading is taken. The
blocky appearance of the log reflects the station interval,
Figure 1.

The log is not continuous. BHGM measurements are taken at
discrete depths usually at intervals of 10 to 50 feet,
depending on the vertical and density resolution required.
While the BHGM has remarkable resolution in the measurement
of density over intervals of 10 feet or more (less than 0.01
g/cm^{3}), surveys requiring closer vertical
resolution must sacrifice density resolution.

__Figure
1. Example BHGM Log__

**The
sharp difference in density between 6330 and 6370 is
caused by porosity not detected by the gamma-gamma
density log. The Broader difference anomaly observed over
the length of the logged interval is explained by the
structural influence of the reef complex.**

__APPLICATIONS__

The spectrum of BHGM applications is defined
on one extreme by density logging and on the other by remote
sensing of structure. The first extreme sometimes focuses
strictly on formation and reservoir evaluation questions, the
other extends to basic exploration. The example shown in
Figure 1 can be used as an example of both applications. In
fact, the purpose of the survey was to detect carbonate
porosity in a reef environment that was missed by the other
logs. For this objective, the useful radius of investigation
of the measurement is on the order of 50 feet. The sharp
negative density anomaly observed between 6330 and 6370
suggests porosity obscured by near borehole effects or poor
volume sampling. On the other hand, the broad departure of
the BHGM and gamma-gamma logs over the depth range of the
logged section is typical of a structural effect, in this
case the edge of the reef complex which is within a few
hundred feet.

**Density Logging**

Borehole gravity density measurements are
unhindered by casing, poor hole conditions, and all but the
deepest fluid invasion. The BHGM measurement samples a large
volume of rock which provides a density-porosity value that
is more representative of the formation. This is especially
beneficial in carbonate and fractured reservoirs. BHGM
surveys have been used to find hydrocarbon-filled porosity
missed by other logs in both open and cased holes.
Gas-saturated sands are a particularly easy target.

The wide radius of investigation has also
been successfully used to determine gas-oil and oil-water
contacts in reservoirs where other measurements have been
ineffective. BHGM density measurements have been used to
calculate hydrocarbon saturations: the larger the fluid
density contrast, the larger the measured effect. Gas
saturations are therefore the easiest to measure. The
difference in densities measured by the gamma-gamma log and
the BHGM can be used to calculate the difference in oil
saturation between the invaded and undisturbed zones, which
can in turn give an estimate of movable hydrocarbons.

**Remote Sensing**

A useful and practical rule of thumb for BHGM
remote sensing applications is that a remote body with
sufficient density contrast can be detected by the BHGM no
farther from the well bore than one or two times the height
of the body. A salt dome with 15,000 feet of vertical relief
would have a definitive signature a few miles away. A channel
sand 20 feet thick would be detectable no more than 40 feet
away. Local geology, and in particular the thickness of local
density units, defines the effective radius of investigation
of the BHGM.

Computer modeling of BHGM measurments can be
used to develop relatively detailed salt-dome-flank or
reef-flank model interpretations. Modeling is particularly
effective where seismic data can be integrated into the
modeling process; a model is sought that is consistent with
both data sets. In one case, the presence of an imbricate
thrust sheet was confirmed by the BHGM and led to a
sidetracked hole and an economic discovery.

__THEORY__

Measurements of gravity
differences repeatable to about 3 microgals have been
achieved using a LaCoste and Romberg borehole gravity meter.
One gal (after Galileo) is 1 cm/sec^{2}. The earth's
gravity field varies from the equator to the pole from about
one part in 10^{9} of the earth's field. The BHGM is
clearly a remarkably sensitive instrument.

The underlying assumption in
computing **apparent density** is that
of an earth model made up of a layer cake of horizontal
infinite slabs. For such a model, the density of any slab is
exactly given by the gravity gradient through that slab; the
gradient measured at any point within the slab is constant;
and the slabs above and below it have no effect on the
gradient within it. The derivation of the density of an
infinite slab is shown in Figure 2. This simple assumption
serves effectively in a majority of cases. Modeling of more
complex geometries is not difficult and is routinely used in
computing structural corrections to **apparent
density**.

__Figure
2. Density of an Infinite Slab from Borehole Gravity__

**G is the
Universal Gravitational Constant.**

The formula for apparent
density is given approximately by the following (there are
small corrections for latitude and elevation):

**r =
3.6824 - .03913 D****g/****D****z**

where **r** is in
g/cm^{3}, **D****z** is in feet, and **D****g** is in microgals. The constant
density term compensates for the earth's normal vertical
gravity gradient.

__OPERATING
LIMITATIONS__

The gravity meter itself is a
delicate spring balance which measures changes in weight of a
small proof mass. The meter must be leveled at each station,
and it is accurately thermostatted at a temperature of about
126? C. The present LaCoste and
Romberg meter will not operate in a well bore deviated from
vertical by more than 14? . For higher
operating temperatures the meter is operated inside a Dewar
flask. Using the flask, operating temperatures up to 260? C are possible. BHGM surveys have been carried out
from floating platforms, but usually at depths no shallower
than 4000 feet.

**Operating Conditions
Summary**

(LaCoste and Romberg through Number 14)

Gournay, Luke S. and Maute, Robert, E., 1982, Detection of
bypassed gas using borehole gravimeter and pulsed neutron
capture logs: The Log Analyst, v. 23, No. 3, p. 27-32.

**Case
history demonstrates quantitative analysis of BHGM data
in combination with other logs.**

LaFehr, T. R., 1983, Rock density from borehole gravity
surveys: Geophysics, v. 48, No. 3, p. 341-356.

**Theoretical
development examines the response of the BHGM in the
presence of anomalous masses.**

Lines, L. R., Schultz, A. K., and Trietel S., 1988,
Cooperative inversion of geophysical data: Geophysics, v. 53,
No. 1, p. 8-20.

**BHGM
data figure prominently in this integrated interpretation
study that also utilizes seismic and surface gravity
data.**

Rasmussen, Noel F., 1975, The successful use of the
borehole gravity meter in Northern Michigan: The Log Analyst,
v. 16, No. 5, p. 3-10.

**Case
history study also includes review of theory,
interpretation and operations.**

Robbins, Stephen L., 1980, Bibliography with abridged
abstracts of subsurface gravimetry (especially borehole) and
corresponding in-situ rock density determinations: USGS Open
File Report 80-710.

Smith, Neal J., 1950, The case for gravity data from
boreholes: Geophysics, v. 15, No. 4.

**Imaginative
classic paper provides a review of basic theory as well
as a still useful source of ideas for applications and
interpretive approaches.**