Center for Cave and Karst Studies - Western Kentucky University

2. MICROGRAVITY RESEARCH PROCEDURES

2.1 Introduction
Gravity surveys are used to detect variation in the density of subsurface materials. Variations in the earth’s gravitational field higher than normal indicate underlying material of higher density while areas of low gravity indicate areas of lower density. In order to detect voids or cavities, very high precision is required. Accurate gravity readings to 10 microGals (l Gal = l cm/s2) are necessary. This is equal to l part in 100,000,000 of the earth’s normal gravity. A SCINTREX CG-3M Autograv Microgravity Meter that has a 0.5-microGal sensitivity was used for this investigation.

2.2 Microgravity Research Procedures

The SCINTREX CG-3M Autograv underwent a 48-hour stabilization period prior to field use. Field calibration was performed on the instrument and consisted of a long-term drift correction and temperature compensation adjustment. The following corrections were calculated for each gravity measurement:

Instrument Drift (short term),

Earth Tides,

Reference Ellipsoid (latitude),

Free-Air Effect (elevation), and

Bouguer Slab Density

A base station was established at the survey site, and gravity was repeatedly measured at this base station approximately every two hours in order to derive instrument drift. A base station derived instrument drift curve was interpolated to the time of each survey station reading, and
each station reading was then corrected for instrument drift by the Geosoft OASIS Montaj reduction program.

Earth tide corrections are based on latitude and longitude of the survey station and the gravitational effect of the sun and moon at any given point in time. This correction was made for each gravity reading using latitude and longitude derived from a GPS measurement made at the site and determined by recording date and time for each instrument reading (converted to UTC for calculations). The reference ellipsoid correction is necessary because the earth is an imperfect sphere with gravitational variation as a function of latitude.

Differences in elevation between each survey station and the base station were compensated for by using the free-air correction calculation. The free-air effect compensates for the decrease in gravity with elevation due to increasing distance from the center of the earth. Elevation for each microgravity survey station was measured to the nearest hundred of a foot, and instrument height was measured to the nearest 1/10 of an inch at each station.

Theoretical gravity is modified to obtain simple Bouguer gravity by applying the Bouguer slab effect correction. This correction refers to the attraction of the slab of material, which is caused by variation in density, between the station elevation and sea level. Topographic relief across the survey site did not require terrain corrections to be applied to the data set.
In most karst areas, the following average density values are assumed:
Air = 0 g/cm3 Water = 1.0 g/cm3 Sandstone = 2.35g/cm3
Regolith or cave sediments = 1.5 g/cm3 Limestone = 2.5 g/cm3
Therefore, density contrasts of 1.0 to 2.5 g/cm3 are anticipated for any subsurface cavity, depending on whether the cavity is filled with air, water or sediment.

Although microgravity subsurface investigations usually consist of measuring at stations established in a grid pattern, Crawford, Webster, and Winter (1989) have demonstrated the effectiveness of using traverses established perpendicular to linear subsurface features and groundwater flow paths for the detection of caves.

2.3 Detection of Subsurface Features in Karst Terrain

Bouguer gravity can identify locations on the earth’s surface that have relatively higher or lower gravity caused by lateral variations in subsurface density. Crawford (1999) has used microgravity extensively to locate bedrock caves from the ground surface (Appendix II). The lower densities of the air, water or mud within a cave compared to the surrounding carbonate rock results in a low- gravity anomaly. Crawford has also used microgravity to locate voids in the regolith (unconsolidated material above bedrock) that are potential sinkhole collapses. Since regolith is less dense that limestone bedrock, Bouguer gravity can also identify variations in depth to bedrock.

2.4 Microgravity Used for Sinkhole Collapse Investigations

Crawford has used microgravity to investigate subsurface conditions in the vicinity of sinkhole collapses. Microgravity provides useful information concerning: a) depth to bedrock, b) extent and shape of a void below the surface, c) location of a crevice, or crevices, through which regolith and water are sinking, and d) additional regolith voids and low density material in the vicinity of a sinkhole collapse.

north and Traverse 11 on the south along the easternmost edge of the inside shoulder. Traverse 5, north, and Traverse 12, south, were positioned along the southbound lanes’ centerline, 8 feet from Traverse 4 and 11, respectively. Traverse 6, on the north of Interstate 24, and Traverse 13, on the south, were along the westernmost edge of the outside shoulder of the southbound lanes.

Traverse lines and measurement stations were marked by placing a labeled wood stake at each location where a microgravity measurement was to be taken. The stakes were labeled with the traverse number and the location of the stake in feet along the traverse. A compass was used to locate the stakes perpendicular to the established centerline survey stakes, and a cloth tape was used to set each station 10 feet apart. A base station was established at a central location in the study area in order to measure the changes in drift during the time microgravity measurements were being made.

2.6 Field Method

The SCINTREX CG-3M Autograv microgravity meter used for this survey provided the following on-board data corrections:

1. Continuous Tilt Correction-for instrument level.

2. Seismic Filter-for interference caused by vibration.

3. Auto-Reject-for statistical rejection of anomalous readings.

At each measuring station the instrument was manually leveled to within +/- 5 arcseconds. Instrument height was measured to the nearest 1/10 inch for each station. Measurement read-time on the SCINTREX CG-3M Autograv was programmed for 60 seconds (one reading per second for resultant average). The time of measurement (HH/MM) was accurately recorded for each measurement. Data was recorded digitally by the microgravity meter as well as in field notes maintained by the survey team.

2.7 Data Reduction

Corrections to measured field gravity were applied based on latitude and longitude, time of measurement, elevation of measurement, and instrument height data recorded by the field personnel for each survey station. Data reduction was facilitated by a computer program called Geosoft Oasis Montaj. Data reduction includes the following corrections:
1. Instrument Drift
2. Reference Ellipsoid (a function of latitude)
3. Earth Tide
4. Elevation (free-air effect)
5. Bouguer slab effect (density)

After all corrections have been calculated, the reduced data consists of a Simple Bouguer Gravity value for each measured point. Increasingly negative values for Bouguer gravity indicate greater deficits in mass below each measurement point. Graphic plotting of data produces a trend line that illustrates the relative fluctuations in gravity along each traverse.

2.8 Criteria for Interpreting Reduced Data

Reduced survey data consist of Simple Bouguer Gravity. Fluctuations in measured gravity can be attributed to changes in depth to bedrock, variations in density of competent subsurface materials, regolith voids and bedrock voids. Existing information on depth to bedrock was used to facilitate interpretation. The following criteria were used to guide interpretation of the reduced microgravity data:
Anomalies are interpreted based on disconformity between local trends in measurements. This includes data sets with essentially “flat” graphic trends as well as trends which increase or decrease with horizontal distance. A gradually increasing or decreasing trend across a data set is often representative of either change in latitude or depth to bedrock. Anomalies within a data set are described by variations within such trends.

Anomalies are interpreted based on magnitude. While neither the magnitude of the actual subsurface feature nor the depth to the feature can be concluded from survey data, greater magnitudes of disconformity within the data set indicate more probable detections of actual subsurface features, such as increased depth to bedrock or sediment-filled, water-filled or air-filled voids in the limestone bedrock or regolith.

Symmetry of an anomaly within the data set indicates a more probable detection of actual subsurface features. Data sets exhibiting a gradual decrease from local average in Bouguer microgravity followed by a gradual increase to local average (i.e. a “bowl” shape) are considered more positive indicators with less likelihood of instrument error. Single point anomalies are generally considered unreliable indicators of actual anomalies.