<PAGE>


                                                                    Exhibit 10.3

                                 PROMISSORY NOTE
                                 ---------------

$2,500,000.00                    Houston, Texas                 January 15, 2002


         FOR VALUE RECEIVED, the undersigned, CONCORD TECHNOLOGIES, LP, a Texas
limited partnership, GEOSPACE ENGINEERING RESOURCES INTERNATIONAL, LP, a Texas
limited partnership, GEO SPACE, LP, a Texas limited partnership, OYO
INSTRUMENTS, LP, a Texas limited partnership and OYOG OPERATIONS, LP, a Texas
limited partnership, jointly and severally ("Maker"), hereby promise to pay to
the order of SOUTHWEST BANK OF TEXAS, N.A., a national banking association
("Payee"), at its offices at Five Post Oak Park, 4400 Post Oak Parkway, Houston,
Harris County, Texas, or such other address as may be designated by Payee, in
lawful money of the United States of America, the principal sum of TWO MILLION
FIVE HUNDRED THOUSAND AND NO/100 DOLLARS ($2,500,000.00), or so much thereof as
may be advanced and outstanding hereunder, together with interest on the
outstanding principal balance from day to day remaining, at a rate equal to
eight percent (8.0%) per annum. If an Event of Default (hereinafter defined) has
occurred and is existing, the principal hereof and any past due interest hereon
shall bear interest at the Default Rate (hereinafter defined).

         Principal of and interest on this Note shall be due and payable as
follows:

                  (a) Accrued and unpaid interest on this Note shall be payable
         monthly, on the fifteenth (15th) day of each month commencing on
         February 15, 2002 and upon the maturity of this Note, however such
         maturity may be brought about; and

                  (b) All outstanding principal of this Note and all accrued
         interest hereon shall be due and payable on July 15, 2002.

         Principal of this Note shall be subject to mandatory prepayment at the
times described in the Agreement (hereinafter defined).

         Interest on the indebtedness evidenced by this Note shall be computed
on the basis of a year of 365 days, and the actual number of days elapsed
(including the first day but excluding the last day) unless such calculation
would result in a usurious rate in which case interest shall be calculated on
the basis of a year of 365 or 366 days, as the case may be.

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         As used in this Note, the following terms shall have the respective
meanings indicated below:

                  "Agreement" means that certain Loan Agreement dated as of
         February 16, 2001 between Maker and Payee, as amended by First
         Amendment to Loan Agreement dated as of February 17, 2001, and Second
         Amendment to Loan Agreement dated as of January 15, 2002, and as the
         same may be further amended or modified from time to time.

                  "Default Rate" means a per annum rate of interest equal to ten
         percent (10%) per annum.

                  "Event of Default" shall have the meaning given to such term
         in the Agreement.

                  "Maximum Rate" means the maximum rate of nonusurious interest
         permitted from day to day by applicable law, including Chapter 303 of
         the Texas Finance Code (the "Code") (and as the same may be
         incorporated by reference in other Texas statutes). To the extent that
         Chapter 303 of the Code is relevant to any holder of this Note for the
         purposes of determining the Maximum Rate, each such holder elects to
         determine such applicable legal rate pursuant to the "weekly ceiling,"
         from time to time in effect, as referred to and defined in Chapter 303
         of the Code; subject, however, to the limitations on such applicable
         ceiling referred to and defined in the Code, and further subject to any
         right such holder may have subsequently, under applicable law, to
         change the method of determining the Maximum Rate.

                  "Prime Rate" shall mean that variable rate of interest per
         annum established by Payee from time to time as its prime rate which
         shall vary from time to time. Such rate is set by Payee as a general
         reference rate of interest, taking into account such factors as Payee
         may deem appropriate, it being understood that many of Payee's
         commercial or other loans are priced in relation to such rate, that it
         is not necessarily the lowest or best rate charged to any customer and
         that Payee may make various commercial or other loans at rates of
         interest having no relationship to such rate.

         This Note (a) is Note-B provided for in the Agreement and (b) is
secured as provided in the Agreement. Maker may prepay the principal of this
Note upon the terms and conditions specified in the Agreement. Maker may borrow,
repay, and reborrow hereunder upon the terms and conditions specified in the
Agreement.

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         Notwithstanding anything to the contrary contained herein, no
provisions of this Note shall require the payment or permit the collection of
interest in excess of the Maximum Rate. If any excess of interest in such
respect is herein provided for, or shall be adjudicated to be so provided, in
this Note or otherwise in connection with this loan transaction, the provisions
of this paragraph shall govern and prevail, and neither Maker nor the sureties,
guarantors, successors or assigns of Maker shall be obligated to pay the excess
amount of such interest, or any other excess sum paid for the use, forbearance
or detention of sums loaned pursuant hereto. If for any reason interest in
excess of the Maximum Rate shall be deemed charged, required or permitted by any
court of competent jurisdiction, any such excess shall be applied as a payment
and reduction of the principal of indebtedness evidenced by this Note; and, if
the principal amount hereof has been paid in full, any remaining excess shall
forthwith be paid to Maker. In determining whether or not the interest paid or
payable exceeds the Maximum Rate, Maker and Payee shall, to the extent permitted
by applicable law, (a) characterize any non-principal payment as an expense,
fee, or premium rather than as interest, (b) exclude voluntary prepayments and
the effects thereof, and (c) amortize, prorate, allocate, and spread in equal or
unequal parts the total amount of interest throughout the entire contemplated
term of the indebtedness evidenced by this Note so that the interest for the
entire term does not exceed the Maximum Rate.

         If default occurs in the payment of principal or interest under this
Note, or upon the occurrence of any other Event of Default, as such term is
defined in the Agreement, the holder hereof may, at its option, (a) declare the
entire unpaid principal of and accrued interest on this Note immediately due and
payable without notice, demand or presentment, all of which are hereby waived,
and upon such declaration, the same shall become and shall be immediately due
and payable, (b) foreclose or otherwise enforce all liens or security interests
securing payment hereof, or any part hereof, (c) offset against this Note any
sum or sums owed by the holder hereof to Maker and (d) take any and all other
actions available to Payee under this Note, the Agreement, the Loan Documents
(as such term is defined in the Agreement) at law, in equity or otherwise.
Failure of the holder hereof to exercise any of the foregoing options shall not
constitute a waiver of the right to exercise the same upon the occurrence of a
subsequent Event of Default.

         If the holder hereof expends any effort in any attempt to enforce
payment of all or any part or installment of any sum due the holder hereunder,
or if this Note is placed in the hands of an attorney for collection, or if it
is collected through any legal proceedings, Maker agrees to pay all costs,
expenses, and fees incurred by the holder, including all reasonable attorneys'
fees.

                                       -3-

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         THIS NOTE SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE
LAWS OF THE STATE OF TEXAS AND THE APPLICABLE LAWS OF THE UNITED STATES OF
AMERICA. THIS NOTE IS PERFORMABLE IN HARRIS COUNTY, TEXAS.

         Maker and each surety, guarantor, endorser, and other party ever liable
for payment of any sums of money payable on this Note jointly and severally
waive notice, presentment, demand for payment, protest, notice of protest and
non-payment or dishonor, notice of acceleration, notice of intent to accelerate,
notice of intent to demand, diligence in collecting, grace, and all other
formalities of any kind, and consent to all extensions without notice for any
period or periods of time and partial payments, before or after maturity, and
any impairment of any collateral securing this Note, all without prejudice to
the holder. The holder shall similarly have the right to deal in any way, at any
time, with one or more of the foregoing parties without notice to any other
party, and to grant any such party any extensions of time for payment of any of
said indebtedness, or to release or substitute part or all of the collateral
securing this Note, or to grant any other indulgences or forbearances
whatsoever, without notice to any other party and without in any way affecting
the personal liability of any party hereunder.

                                       CONCORD TECHNOLOGIES, LP

                                       By: OYOG, LLC,
                                           its general partner


                                           By: /s/ THOMAS T. MCENTIRE
                                               ---------------------------------
                                               Thomas T. McEntire
                                               Vice President and
                                               Chief Financial Officer



                                       GEOSPACE ENGINEERING RESOURCES
                                       INTERNATIONAL, LP

                                       By: OYOG, LLC,
                                           its general partner


                                           By: /s/ THOMAS T. MCENTIRE
                                               ---------------------------------
                                               Thomas T. McEntire
                                               Vice President and
                                               Chief Financial Officer

                                       -4-

<PAGE>


                                       GEO SPACE, LP

                                       By: OYOG, LLC,
                                           its general partner


                                           By: /s/ THOMAS T. MCENTIRE
                                               ---------------------------------
                                               Thomas T. McEntire
                                               Vice President and
                                               Chief Financial Officer



                                       OYO INSTRUMENTS, LP

                                       By: OYOG, LLC,
                                           its general partner


                                           By: /s/ THOMAS T. MCENTIRE
                                               ---------------------------------
                                               Thomas T. McEntire
                                               Vice President and
                                               Chief Financial Officer



                                       OYOG OPERATIONS, LP

                                       By: OYOG, LLC,
                                           its general partner


                                           By: /s/ THOMAS T. MCENTIRE
                                               ---------------------------------
                                               Thomas T. McEntire
                                               Vice President and
                                               Chief Financial Officer


                                       -5-