McPhar provides comprehensive survey solutions for oil and gas exploration offering our clients a wide range of options, from land gravity and GPS surveys to airborne surveys using a fixed-wing aircraft equipped with multiple geophysical sensors.   

The surveys that McPhar undertake have many applications, including but not limited to:

Airborne Geophysical Surveys

Typically, airborne geophysics for oil and gas exploration is conducted using one or more of the following techniques deployed on a moving platform (usually a fixed-wing airplane):

Airborne Gravity

An airborne gravity/magnetic survey involves the use of a stabilized gravity meter and a high-sensitivity magnetometer, installed on a fixed-wing aircraft, to acquire data over a given area concerning the earth’s gravity and magnetic fields.   Airborne gravity surveys are usually conducted in association with a magnetic survey, which provides a very reliable and relatively precise (typically 5 percent or less of the depth below the flight level) method of determining the depth of the sedimentary basin.

The data acquired by such a system may be used to:

The use of gravity and magnetic methods in oil and gas exploration are based on the fact that the pull of gravity and the strength of the earth's magnetic field vary from place to place across the world.   The variations are caused by a number of factors including the size and depth of the sedimentary basin and the density and structure of the geological formations of the area under the survey, such as domes, anticlines, and faults, which are associated with petroleum accumulations.

McPhar uses the Canadian Micro Gravity GT-1A airborne gravity meter for all its surveys.   The GT-1A gravity meter may be flown installed on a fixed-wing aircraft or installed on a small to medium-sized helicopter.    On a fixed-wing aircraft, flown at 95 knots, anomaly resolution of a GT-1A survey is about 2 km.    Daily production rates as high as 700 to 800 line-km / day are quite feasible.

GT-1A Airborne Gravity Meter

The GT-1A is an airborne, single vertical sensor, GPS-INS scalar gravimeter with a Schuler-tuned three-axis platform. The gravity sensor is a custom-designed accelerometer mounted inside a gyro- stabilised unit. Inputs from fibre optic gyro, inclinometers, angle sensors and dual frequency GPS are used to drive servo motors which maintain the sensor in a vertical position. This virtually eliminates the effects of horizontal accelerations in the measured signal. The entire assembly is mounted on a rotation table, maintaining the sensor orientation at the same heading.

Cessna C208B Grand Caravan aircraft outfitted for airborne gravity surveys

High-Resolution Aeromagnetics

Intra-sedimentary mapping (ISMAP) for oil and gas exploration involves the use of high-sensitivity aeromagnetics to resolve very low amplitude anomalies (1nT to 5nT typically) originating from structures within the essentially non-magnetic sedimentary column, as well as high amplitude features from the crystalline basement.  

ISMAP anomalies are more commonly caused by faults, joints and fractures and detrital minerals.

The benefits of ISMAP are realized by the state-of-the-art data acquisition and processing techniques we use.   These include 3DNAV drape flying, tie-line leveling, micro-leveling, equivalent source corrections where appropriate, and signal enhancement filtering.

Aircraft maneuver noise, of necessity, is very small.   Using the “Figure-of-Merit” (FOM) technique to measure maneuver noise, all our survey aircraft have an FOM of less than 1.0 nT.   In addition, the use of differential GPS for navigation and positioning allows micro-magnetic anomalies to be determined to a positional accuracy of about +/- 1 meter.

Piper PA-31 Navajo survey aircraft featuring an extended tail-stinger for a high-resolution cesium vapour magnetometer.

Basement Magnetic Mapping -   We assume that the basement beneath the sediments of interest is generally of crystalline rock.   These crystalline rocks have varying amounts of magnetite, generally in greater concentrations than the overlying sediments.   This high concentration of magnetite allows us to map the basement topography with good accuracy using the magnetic method.   Also, because the basement is generally of higher density than the sediments (the exception to this is limestones, marble, some shales and slates, and particularly dolomite) it may be successfully mapped using the combined magnetics and gravity method (Elementary Gravity and Magnetics for Geologists and Seismologists, by L.L. Nettleton).

Radiometrics (GammaSense)

The objective of surveying with a multi-channel, gamma-ray spectrometer system and a large volume gamma-ray sensor (GammaSense) is to detect subtle characteristic radiation patterns as indicators of subsurface hydrocarbon accumulations over petroliferous terrane.    ISMAP and GammaSense techniques may be applied independently of each other, however, it is practical and cost effective to combine them in one multi-sensor, multi-method survey.

Hydrocarbon anomalies can be qualitatively and directly identified from airborne radiometric measurements.   It has been repeatedly observed that the subtle anomalous patterns of radiation flux detected over petroleum basins exists over subsurface hydrocarbon accumulations.   (This we have also determined from GammaSense surveys we have conducted in Latin America , in both tropical and desert areas, and in various locations in the USA .)

Since 1986, Saunders et al. have made proprietary studies of NURE radiometric data covering approximately 77,000 km2 in Texas , Arkansas , Louisiana , Mississippi , and Florida .   They observed low K and eU values over several oil fields in the Palestine quadrangle in east Texas , and high eU values over 90% of the oil fields in San Angelo/Big Springs quadrangles in west Texas .   They reported that they found an average of 27 untested anomalies within each 2600 km2 covered by their study of NURE data, and that 72.7% of 706 oil and gas fields investigated showed characteristic K and eU anomalies relative to eTh.

Large volume gammaray sensors installed in a survey aircraft.

Drape Flying

To obtain high resolution geophysical data, fixed-wing survey airplanes should be flown at a consistent height above the ground, maintaining a consistent and safe altitude (the drape surface) on the two orthogonal survey line directions.   When and where appropriate, we operate a computer-assisted system, 3DNAV 19 , to enable our flight crews to maintain an optimal flight altitude (drape surface) during surveying while at the same time ensuring that primary and control lines intersect at the same altitude. The result can be a considerable improvement in the quality of the high resolution data acquired, particularly in hilly or mountainous terrain.

Quality Control

McPhar ensures Quality Control by using a team concept.   The instrumentation onboard the survey aircraft permits basic quality control procedures.   The team concept is continued at the Survey Base where a McPhar Geophysicist undertakes a more comprehensive QC analysis of the data, and performs preliminary data processing.   The data is then given a second, and more complete review, wherein all the systems onboard the aircraft are tested for compliance to the survey’s specifications.   Any problematic or unacceptable data is identified and flagged for reflying by the survey crew.   On a daily basis, this preliminary processed data is sent to McPhar’s data processing centre, where other geophysicists commence the Final Data Processing work.

McPhar almost always undertakes Quality Control and preliminary data processing in the field.   For this purpose all our airborne systems are mobilized with a geophysicist and a PC-based data processing system to support them.   The Field Data Verification Workstation (FWS), as this system is known, can process airborne geophysical data of all kinds, and produce plots and maps in full-colour of the survey data, often within hours of the survey flight ending.   The FWS software, which is the core of this system, permits the Q.C. geophysicist to differentially correct the GPS navigation data; carry out flight path recovery; perform magnetic compensation and leveling; undertake radiometric corrections and preliminary processing; EM leveling and processing; and generally to perform filtering, gridding and contouring of data, imaging of selected data and plotting to any map scale and layout.

In-field QC of airborne data in a forward field camp and in the friendlier environment of a field office

Data Processing

 After the infield QC of the data has been completed, the data are then sent to McPhar’s data processing center in Markham , Canada where the final processing mapping are completed.   Colour contour maps, sometimes black-line contour maps, and offset and stacked profiles are then produced.   A survey report is provided.   A full range of products is routinely derived from the data.   Our data processing center features a network of powerful PENTIUM microcomputers and state-of-the-art software for data reduction and compilation.   Image and map products are produced on HP colour plotters.

McPhar’s data processing center in Markham, Ontario, Canada

A. Total Magnetic Field (TMF) Colour Image
B. Reduction-to-the-Pole (RTP) of the TMF Colour Image
C. First Vertical Derivative of the TMF Colour Image
D. Second Vertical Derivative of the TMF Color Image
E. Total Count (Radiometrics) Colour Image
F. Thorium/Uranium (Th/U) Ratio (Radiometrics) Colour Image

Interpretation & Modeling

The interpretation/modeling of geophysical results into meaningful geological parameters is the prime function of any interpreter.   The manipulation of geophysical data is only a means to an end, and the final product of the interpretation is the compilation of a series of maps showing interpreted geological parameters.   The data processing routines and mathematical operators applied to the data by McPhar are not the end product of the interpretation; they help delineate geologic and economic targets to be discussed in the final report.   Many techniques are available to apply to an interpretation project; to determine depths to causative sources, to delineate discontinuities and boundaries, and to draw conclusions regarding geological structure beneath the survey.   A wide variety of contour and interpretation maps, profiles, cross-sections and models, and a written report are usually the result of the interpretation.

Ground Geophysical Surveys

Land Gravity & GPS Surveys

In a land gravity survey 23, the Earth's gravitation field and the location and elevation of each station are precisely measured. These measurements at different locations are used to map the variations in rock density over the selected survey area, which is normally covered with a systematic grid.   After appropriate processing and interpretation, the subsurface geological structures can be determined.   All conventional gravity meters measure the relative gravity with respect to a base or known location.    McPhar uses both LaCoste & Romberg model-G gravity meters, as well as the Scintrex CG-3 and CG-5 gravity meters.

These gravity meters can be transported manually while walking between close stations, and with more widely spaced stations by vehicle or helicopter.

LaCoste & Romberg Gravity Meters

LaCoste & Romberg manufactures two basic land gravity meters.   The model “G” geodetic meter and the model “D” microgal meter.

LaCoste & Romberg model-G gravity meter

A zero length spring suspension is used to attain high sensitivity.   A lever system is used to null the meter.   The lever system acts on the main spring rather than on a weak “measuring spring”, thus reducing hysteresis errors and stabilizing the calibration.  

The model “G” has a repeatability of 0.01 milligal and accuracy better than 0.04 milligal.

The model “D” has a repeatability of 0.005 milligal and accuracy better than 0.01 milligal.

Scintrex CG-3 and CG-5 “ AutoGrav” Gravity Meters

The Scintrex CG-3 and CG-5 Automated Gravity Meter have reading resolutions of 0.005 milligal.   Operator error is reduced through the automatic taking of readings, which are continuously sampled for real time signal enhancement and statistical analysis, and then stored in solid-state memory.  It is rugged and can be transported without clamping and carried freely in the hand or on the back.  The CG-3 /CG-5 can be operated for 8 hours without the requirement of repeated loop closure for drift and tidal corrections, although prudently, the loops are usually closed after several hours.

The CG-3 features automated corrections for tidal effects, instrument tilt, and noise reading rejection.
Scintrex CG-3 AutoGRAV gravity meter

Twelve McPhar gravity crews, with drivers and helpers, in Bolivia 2003.   A very large high-resolution Gravity & GPS survey was carried-out for a US oil company using a mix of L & R G-meters and Scintrex CG-3 AutoGRAV gravity meters

Methodology

A land gravity / GPS survey is comprised of the following tasks:

The purpose of a survey usually decides the gravity station grid, while the access and topography, as well s the technology available, determines the equipment to be used and the procedures to be followed. A standard gravity survey involves the measurements at each station of the gravitation field with either a Scintrex CG-3 (-3M) or a L & R model-G meter to an accuracy of 0.01 milligal, and differential GPS elevation to an accuracy of a few centimeters and the location to better than 20 cm.

Depending on the access, topography and spacing between each station, the crew may walk carrying the equipment, or drive in a 4WD vehicle, or land by helicopter. With regional surveys, post-processing of the GPS data is undertaken. On some surveys, such as a mine site, a levelled grid may be already established, and only gravity readings are taken. Also, when the station spacing is close and the terrain is relatively flat, optical levelling can be economically undertaken. All gravity surveys require survey planning as to the gravity grid, the accuracy of the elevation, location and gravity reading.

Gravity & GPS measurements using a Scintrex CG-3 and a geodetic quality GPS system